CELLULOSE
fiber, main construction material flora, forming the cell walls of trees and other higher plants. The cleanest natural form cellulose - hairs of cotton seeds.
Purification and isolation. Currently, only two sources of cellulose are of industrial importance - cotton and wood pulp. Cotton is almost pure cellulose and does not require complex processing to become a starting material for man-made fibers and non-fiber plastics. After the long fibers used to make cotton fabrics are separated from the cotton seed, short hairs, or “lint” (cotton fluff), 10-15 mm long, remain. The lint is separated from the seed, heated under pressure with a 2.5-3% sodium hydroxide solution for 2-6 hours, then washed, bleached with chlorine, washed again and dried. The resulting product is 99% pure cellulose. The yield is 80% (wt.) lint, the rest being lignin, fats, waxes, pectates and seed husks. Wood pulp is usually made from the wood of coniferous trees. It contains 50-60% cellulose, 25-35% lignin and 10-15% hemicelluloses and non-cellulosic hydrocarbons. In the sulfite process, wood chips are boiled under pressure (about 0.5 MPa) at 140° C with sulfur dioxide and calcium bisulfite. In this case, lignins and hydrocarbons go into solution and cellulose remains. After washing and bleaching, the purified mass is cast into loose paper, similar to blotting paper, and dried. This mass consists of 88-97% cellulose and is quite suitable for chemical processing into viscose fiber and cellophane, as well as cellulose derivatives - esters and ethers. The process of regeneration of cellulose from a solution by adding acid to its concentrated copper-ammonia (i.e. containing copper sulfate and ammonium hydroxide) aqueous solution was described by the Englishman J. Mercer around 1844. But the first industrial application This method, which laid the foundation for the copper-ammonia fiber industry, is attributed to E. Schweitzer (1857), and its further development is the merit of M. Kramer and I. Schlossberger (1858). And only in 1892 Cross, Bevin and Beadle in England invented a process for producing viscose fiber: a viscous (hence the name viscose) aqueous solution of cellulose was obtained after treating the cellulose first with a strong solution caustic soda, which gave "soda cellulose", and then carbon disulfide (CS2), resulting in soluble cellulose xanthate. By squeezing a stream of this "spinning" solution through a spinneret with a small round hole into an acid bath, the cellulose was regenerated in the form of rayon fiber. When squeezing the solution into the same bath through a die with narrow gap The resulting film was called cellophane. J. Brandenberger, who worked in France with this technology from 1908 to 1912, was the first to patent continuous process cellophane production.
Chemical structure. Despite the widespread industrial use of cellulose and its derivatives, the currently accepted chemical structural formula cellulose was proposed (by W. Howworth) only in 1934. However, since 1913 its empirical formula C6H10O5, determined from data, has been known quantitative analysis well washed and dried samples: 44.4% C, 6.2% H and 49.4% O. Thanks to the work of G. Staudinger and K. Freudenberg, it was also known that this is a long-chain polymer molecule consisting of those shown in Fig. 1 repeating glucosidic residues. Each unit has three hydroxyl groups - one primary (- CH2CHOH) and two secondary (>CHCHOH). By 1920, E. Fisher had established the structure of simple sugars, and in the same year, X-ray studies of cellulose first showed a clear diffraction pattern of its fibers. The X-ray diffraction pattern of cotton fiber shows a clear crystalline orientation, but flax fiber is even more ordered. When cellulose is regenerated into fiber form, crystallinity is largely lost. How easy it is to see in the light of achievements modern science, the structural chemistry of cellulose practically stood still from 1860 to 1920 for the reason that all this time the auxiliary scientific disciplines necessary to solve the problem.
REGENERATED CELLULOSE
Viscose fiber and cellophane. Both viscose fiber and cellophane are regenerated (from solution) cellulose. Purified natural cellulose is treated with an excess of concentrated sodium hydroxide; After removing the excess, the lumps are ground and the resulting mass is kept under carefully controlled conditions. With this “aging,” the length of the polymer chains decreases, which promotes subsequent dissolution. Then the crushed cellulose is mixed with carbon disulfide and the resulting xanthate is dissolved in a solution of sodium hydroxide to obtain “viscose” - a viscous solution. When viscose enters an aqueous acid solution, cellulose is regenerated from it. The simplified total reactions are:
Viscose fiber, obtained by squeezing viscose through small holes of a spinneret into an acid solution, is widely used for the manufacture of clothing, drapery and upholstery fabrics, as well as in technology. Significant quantities of viscose fiber are used for technical belts, tapes, filters and tire cord.
Cellophane. Cellophane, obtained by squeezing viscose into an acid bath through a spinneret with a narrow slot, then passes through washing, bleaching and plasticizing baths, is passed through drying drums and wound into a roll. The surface of cellophane film is almost always coated with nitrocellulose, resin, some kind of wax or varnish to reduce the transmission of water vapor and provide the possibility of thermal sealing, since uncoated cellophane does not have the property of thermoplasticity. On modern production For this purpose, polymer coatings of the polyvinylidene chloride type are used, since they are less moisture permeable and provide a more durable connection during heat sealing. Cellophane is widely used mainly in the packaging industry as a wrapping material for dry goods, food products, tobacco products, and also as a basis for self-adhesive packaging tape.
Viscose sponge. As well as forming a fiber or film, viscose can be blended with suitable fibrous and finely crystalline materials; After acid treatment and water leaching, this mixture is converted into a viscose sponge material (Fig. 2), which is used for packaging and thermal insulation.
Copper-ammonia fiber. Regenerated cellulose fiber is also produced on an industrial scale by dissolving cellulose in a concentrated copper-ammonia solution (CuSO4 in NH4OH) and spinning the resulting solution into fiber in an acid precipitation bath. This fiber is called copper-ammonia fiber.
PROPERTIES OF CELLULOSE
Chemical properties. As shown in Fig. 1, cellulose is a highly polymeric carbohydrate consisting of glucosidic residues C6H10O5 connected by ether bridges at position 1,4. The three hydroxyl groups in each glucopyranose unit can be esterified with organic agents such as a mixture of acids and acid anhydrides with a suitable catalyst such as sulfuric acid. Ethers can be formed by the action of concentrated sodium hydroxide leading to the formation of soda cellulose and subsequent reaction with an alkyl halide:
Reaction with ethylene or propylene oxide produces hydroxylated ethers:
The presence of these hydroxyl groups and the geometry of the macromolecule determine the strong polar mutual attraction of neighboring units. The attractive forces are so strong that ordinary solvents are not able to break the chain and dissolve cellulose. These free hydroxyl groups are also responsible for the greater hygroscopicity of cellulose (Fig. 3). Esterification and etherization reduce hygroscopicity and increase solubility in common solvents.
Under the influence aqueous solution acids break oxygen bridges at the 1,4- position. Complete breakage of the chain produces glucose, a monosaccharide. The initial chain length depends on the origin of the cellulose. It is maximum in natural state and decreases during isolation, purification and conversion to derivative compounds (see table).
DEGREE OF CELLULOSE POLYMERIZATION
Material Number of glucoside residues
Raw cotton 2500-3000
Purified cotton lint 900-1000
Refined wood pulp 800-1000
Regenerated cellulose 200-400
Industrial cellulose acetate 150-270
Even mechanical shear, for example during abrasive grinding, leads to a decrease in chain length. When the polymer chain length decreases below a certain minimum value the macroscopic physical properties of cellulose change. Oxidizing agents affect cellulose without causing cleavage of the glucopyranose ring (Fig. 4). Subsequent action (in the presence of moisture, such as in climate testing) typically results in chain scission and an increase in the number of aldehyde-like end groups. Because the aldehyde groups are easily oxidized to carboxyl, the content of carboxyl, which is practically absent in natural cellulose, increases sharply under conditions of atmospheric influences and oxidation.
Like all polymers, cellulose is destroyed under the influence of atmospheric factors as a result joint action oxygen, moisture, acidic components of air and sunlight. Important has the ultraviolet component of sunlight, and many good UV protection agents increase the life of cellulose derivative products. Acidic air components, such as nitrogen and sulfur oxides (and they are always present in atmospheric air industrial areas), speed up decomposition, often with greater effects than sunlight. Thus, in England, it was noted that cotton samples tested for exposure to atmospheric conditions in winter, when there was practically no bright sunlight, degraded faster than in summer. The fact is that burning large quantities of coal and gas in winter led to an increase in the concentration of nitrogen and sulfur oxides in the air. Acid scavengers, antioxidants, and UV absorbers reduce the weathering sensitivity of cellulose. Substitution of free hydroxyl groups leads to a change in this sensitivity: cellulose nitrate degrades faster, and acetate and propionate - more slowly.
Physical properties. Cellulose polymer chains are packed into long bundles, or fibers, in which, along with ordered, crystalline ones, there are also less ordered, amorphous sections (Fig. 5). The measured percentage of crystallinity depends on the type of cellulose as well as the method of measurement. According to X-ray data, it ranges from 70% (cotton) to 38-40% (viscose fiber). X-ray structural analysis provides information not only about the quantitative relationship between crystalline and amorphous material in the polymer, but also about the degree of fiber orientation caused by stretching or normal growth processes. The sharpness of diffraction rings characterizes the degree of crystallinity, and diffraction spots and their sharpness characterize the presence and degree of preferred orientation of crystallites. In a sample of recycled cellulose acetate produced by the dry-spinning process, both the degree of crystallinity and orientation are very small. In the triacetate sample, the degree of crystallinity is higher, but there is no preferred orientation. Heat treatment of triacetate at a temperature of 180-240 ° C significantly increases the degree of crystallinity, and orientation (by stretching) in combination with heat treatment gives the most ordered material. Flax exhibits a high degree of both crystallinity and orientation.
see also
ORGANIC CHEMISTRY;
PAPER AND OTHER WRITING MATERIALS;
PLASTICS.
Rice. 5. MOLECULAR STRUCTURE of cellulose. Molecular chains pass through several micelles (crystalline regions) of length L. Here A, A" and B" are the ends of the chains lying in the crystallized region; B is the end of the chain outside the crystallized region.
LITERATURE
Bushmelev V.A., Volman N.S. Processes and apparatus for pulp and paper production. M., 1974 Cellulose and its derivatives. M., 1974 Akim E.L. and others. Technology of processing and processing of cellulose, paper and cardboard. L., 1977
Collier's Encyclopedia. - Open Society. 2000 .
Cellulose (C 6 H 10 O 5) n – natural polymer, a polysaccharide consisting of β-glucose residues, the molecules have a linear structure. Each residue of a glucose molecule contains three hydroxyl groups, so it exhibits the properties of a polyhydric alcohol.
Physical properties
Cellulose is a fibrous substance, insoluble either in water or in ordinary organic solvents, and is hygroscopic. Has great mechanical and chemical strength.
1. Cellulose, or fiber, is part of plants, forming cell walls in them.
2. This is where its name comes from (from the Latin “cellulum” - cell).
3. Cellulose gives plants the necessary strength and elasticity and is, as it were, their skeleton.
4. Cotton fibers contain up to 98% cellulose.
5. Flax and hemp fibers are also mainly composed of cellulose; in wood it is about 50%.
6. Paper and cotton fabrics are products made from cellulose.
7. Particularly pure examples of cellulose are cotton wool obtained from purified cotton and filter (un-glued) paper.
8. Selected from natural materials Cellulose is a solid fibrous substance that is insoluble in either water or common organic solvents.
Chemical properties
1. Cellulose is a polysaccharide that undergoes hydrolysis to form glucose:
(C 6 H 10 O 5) n + nH 2 O → nC 6 H 12 O 6
2. Cellulose is a polyhydric alcohol that undergoes esterification reactions to form esters
(C 6 H 7 O 2 (OH) 3) n + 3nCH 3 COOH → 3nH 2 O + (C 6 H 7 O 2 (OCOCH 3) 3) n
cellulose triacetate
Cellulose acetates are artificial polymers used in the production of silk acetate, film (film), and varnishes.
Application
The uses of cellulose are very diverse. It is used to produce paper, fabrics, varnishes, films, explosives, artificial silk (acetate, viscose), plastics (celluloid), glucose and much more.
Finding cellulose in nature.
1. In natural fibers, cellulose macromolecules are located in one direction: they are oriented along the fiber axis.
2. The numerous hydrogen bonds that arise between the hydroxyl groups of macromolecules determine the high strength of these fibers.
3. In the process of spinning cotton, flax, etc., these elementary fibers are woven into longer threads.
4. This is explained by the fact that the macromolecules in it, although they have a linear structure, are located more randomly and are not oriented in one direction.
Construction of macromolecules of starch and cellulose from different cyclic forms glucose significantly affects their properties:
1) starch is an important human food product; cellulose cannot be used for this purpose;
2) the reason is that enzymes that promote starch hydrolysis do not act on the bonds between cellulose residues.
Cellulose– one of the most common natural polysaccharides, the main component and main structural material of plant cell walls. The cellulose content in cotton seed fibers is 95-99.5%, in bast fibers (flax, jute, ramie) 60-85%, in wood tissue (depending on the type of tree, its age, growing conditions) 30-55%, in green leaves , grass, lower plants 10-25%. Almost in an individual state, cellulose is found in bacteria of the genus Acetobacter. Companions to cellulose in the cell walls of most plants are other structural polysaccharides that differ in structure and are called hemicelluloses– xylan, mannan, galactan, araban, etc. (see section “Hemicelluloses”), as well as non-carbohydrate substances (lignin - a spatial polymer of an aromatic structure, silicon dioxide, resinous substances, etc.).
Cellulose determines the mechanical strength of the cell membrane and plant tissue as a whole. The distribution and orientation of cellulose fibers relative to the axis of the plant cell using wood as an example are shown in Fig. 1. The submicron organization of the cell wall is also presented there.
The wall of a mature wood cell, as a rule, includes a primary and secondary cell wall (Fig. 1). The latter contains three layers - outer, middle and inner.
In the primary shell, natural cellulose fibers are arranged randomly and form a network structure ( dispersed texture). The cellulose fibers in the secondary casing are oriented generally parallel to each other, which gives the plant material a high tensile strength. The degree of polymerization and crystallinity of cellulose in the secondary shell is higher than in the primary shell.
In layer S 1 secondary shell (Fig. 1, 3 ) the direction of the cellulose fibers is almost perpendicular to the axis of the cell, in the layer S 2 (Fig. 1, 4 ) they form an acute (5-30) angle with the cell axis. Fiber orientation in the layer S 3 varies greatly and can differ even in adjacent tracheids. Thus, in spruce tracheids, the angle between the predominant orientation of cellulose fibers and the cell axis ranges from 30-60, and in the fibers of most hardwoods it is 50-80. Between layers R And S 1 , S 1 and S 2 , S 2 and S 3 observed transition areas(lamellas) with a different microorientation of fibers than in the main layers of the secondary shell.
Technical cellulose is a semi-finished fibrous product obtained by cleaning plant fibers from non-cellulose components. Cellulose is usually called by the type of raw material ( wood, cotton), method of extraction from wood ( sulfite, sulfate), as well as for its intended purpose ( viscose, acetate, etc.).
Receipt
1.Wood pulp production technology includes the following operations: removing bark from wood (barking); obtaining wood chips; cooking wood chips (in industry, cooking is carried out using the sulfate or sulfite method); sorting; bleaching; drying; cutting
Sulfite method. Spruce wood is treated with an aqueous solution of calcium, magnesium, sodium or ammonium bisulfite, then the temperature is raised to 105-110°C for 1.5-4 hours, and boiled at this temperature for 1-2 hours. Next, increase the temperature to 135-150°C and cook for 1-4 hours. In this case, all non-cellulosic components of wood (mainly lignin and hemicelluloses) become soluble, and de-lignified cellulose remains.
Sulfate method. Chips of any type of wood (as well as reed) are treated with cooking liquor, which is an aqueous solution of caustic soda and sodium sulfide (NaOH + Na 2 S). Within 2-3 hours, increase the temperature to 165-180°C and cook at this temperature for 1-4 hours. The non-cellulose components, converted into a soluble state, are removed from the reaction mixture, and cellulose purified from impurities remains.
2.Cotton pulp obtained from cotton linters. Receiving technology includes mechanical cleaning, alkaline cooking (in 1-4% aqueous NaOH solution at a temperature of 130-170°C) and bleaching. Electron micrographs of cotton cellulose fibers are shown in Fig. 2.
3. Bacterial cellulose synthesized by bacteria of the genus Acetobacter. The resulting bacterial cellulose has a high molecular weight and a narrow molecular weight distribution.
The narrow molecular weight distribution is explained as follows. Since carbohydrate enters the bacterial cell evenly, the average length of the resulting cellulose fibers increases proportionally over time. In this case, there is no noticeable increase in the transverse dimensions of microfibers (microfibrils). The average growth rate of bacterial cellulose fibers is ~0.1 μm/min, which corresponds to the polymerization of 10 7 -10 8 glucose residues per hour per bacterial cell. Therefore, on average, in each bacterial cell, 10 3 glucopyranose units are attached to the growing ends of insoluble cellulose fibers per second.
Microfibers of bacterial cellulose grow from both ends of the fibril to both at the same speed. Macromolecular chains inside microfibrils are arranged antiparallel. For other types of celluloses such data have not been obtained. An electron micrograph of bacterial cellulose fibers is shown in Fig. 3. It can be seen that the fibers have approximately the same length and cross-sectional area.
First of all, it is necessary to explain what exactly cellulose is and what its general outline its properties.
Cellulose(from Latin cellula - letters, room, here - cell) - cellulose, the substance of plant cell walls, is a polymer of the carbohydrate class - a polysaccharide, the molecules of which are built from the remains of glucose monosaccharide molecules (see diagram 1).
SCHEME 1 Structure of the cellulose molecule
Each residue of the glucose molecule - or, for short, the glucose residue - is rotated relative to the neighboring one by 180° and is connected to it by an oxygen bridge -O-, or, as is commonly said in in this case, glucosidic bond through an oxygen atom. The entire cellulose molecule is thus like a giant chain. The individual links of this chain have the shape of hexagons, or - in chemistry terms - 6-membered cycles. In the glucose molecule (and its residue), this 6-membered cycle is built from five carbon atoms C and one oxygen atom O. Such cycles are called pyran cycles. Of the six atoms of the 6-membered pyran ring in Scheme 1 shown above, only the oxygen atom O is shown at the vertex of one of the corners - a heteroatom (from the Greek heteroatom; - another, different from the rest). At the vertices of the remaining five corners there is a carbon atom C (these “usual” carbon atoms for organics, unlike the heteroatom, are not usually depicted in the formulas of cyclic compounds).
Each 6-membered cycle has the shape not of a flat hexagon, but of a curved one in space, like an armchair (see Scheme 2), hence the name of this shape, or spatial conformation, which is the most stable for a cellulose molecule.
DIAGRAM 2 Chair shape
In diagrams 1 and 2, the sides of the hexagons located closer to us are highlighted with a bold line. Scheme 1 also shows that each glucose residue contains 3 hydroxyl groups -OH (they are called hydroxy groups or simply hydroxyls). For clarity, these -OH groups are enclosed in a dotted frame.
Hydroxyl groups are capable of forming strong intermolecular hydrogen bonds with the hydrogen atom H as a bridge, therefore the bond energy between cellulose molecules is high and cellulose as a material has significant strength and rigidity. In addition, -OH groups promote the absorption of water vapor and give cellulose its properties polyhydric alcohols(this is the name given to alcohols containing several -OH groups). When cellulose swells, the hydrogen bonds between its molecules are destroyed, the chains of molecules are pulled apart by water molecules (or molecules of an absorbed reagent), and new bonds are formed between the molecules of cellulose and water (or reagent).
IN normal conditions cellulose is a solid substance with a density of 1.54-1.56 g/cm3, insoluble in common solvents - water, alcohol, diethyl ether, benzene, chloroform, etc. In natural fibers, cellulose has an amorphous-crystalline structure with a degree of crystallinity of about 70%.
Chemical reactions with cellulose usually involve three -OH groups. The remaining elements from which the cellulose molecule is built react at more strong influences- at elevated temperatures, during action concentrated acids, alkalis, oxidizing agents.
For example, when heated to a temperature of 130°C, the properties of cellulose change only slightly. But at 150-160°C, the process of slow destruction begins - the destruction of cellulose, and at temperatures above 160°C this process occurs quickly and is accompanied by the rupture of glucosidic bonds (at the oxygen atom), deeper decomposition of molecules and charring of cellulose.
Acids have different effects on cellulose. When cotton cellulose is treated with a mixture of concentrated nitric and sulfuric acids, hydroxyl groups -OH react, and as a result, cellulose nitrates are obtained - the so-called nitrocellulose, which, depending on the content of nitro groups in the molecule, has different properties. The most famous of the nitrocelluloses are pyroxylin, used for the production of gunpowder, and celluloid - plastics based on nitrocellulose with some additives.
Other type chemical interaction occurs when cellulose is treated with hydrochloric or sulfuric acid. Under the influence of these mineral acids, a gradual destruction of cellulose molecules occurs with the rupture of glucosidic bonds, accompanied by hydrolysis, i.e. exchange reaction involving water molecules (see Scheme 3).
SCHEME 3 Hydrolysis of cellulose
This diagram shows the same three links in the cellulose polymer chain, i.e. the same three residues of cellulose molecules as in Scheme 1, only the 6-membered pyran rings are presented not in the form of “armchairs”, but in the form of flat hexagons. This symbol cyclic structures are also generally accepted in chemistry.
Complete hydrolysis carried out by boiling with mineral acids, leads to the production of glucose. The product of partial hydrolysis of cellulose is the so-called hydrocellulose; it has lower mechanical strength compared to conventional cellulose, since mechanical strength indicators decrease with decreasing chain length of the polymer molecule.
A completely different effect is observed if the cellulose is treated for a short time with concentrated sulfur or hydrochloric acid. Parchmentation occurs: the surface of the paper or cotton fabric swells, and this surface layer, which is partially destroyed and hydrolyzed cellulose, gives the paper or fabric a special shine and increased strength after drying. This phenomenon was first noticed in 1846 by French researchers J. Pumaru and L. Fipoye.
Weak (0.5%) solutions of mineral and organic acids at temperatures up to approximately 70°C, if their application is followed by washing, they do not have a destructive effect on cellulose.
Cellulose is resistant to alkalis (diluted solutions). Solutions of caustic soda in a concentration of 2-3.5% are used for alkaline cooking of rags used for paper production. In this case, not only contaminants are removed from cellulose, but also products of destruction of cellulose polymer molecules that have shorter chains. Unlike cellulose, these degradation products are soluble in alkaline solutions.
Concentrated solutions of alkalis have a unique effect on cellulose in the cold - at room temperature or more low temperatures. This process, discovered in 1844 by the English researcher J. Mercer and called mercerization, is widely used for refining cotton fabrics. The fibers are treated under tension at a temperature of 20°C with a 17.5% solution of sodium hydroxide. Cellulose molecules attach to alkali, so-called alkali cellulose is formed, and this process is accompanied by strong swelling of cellulose. After washing, the alkali is removed, and the fibers acquire softness, a silky shine, become more durable and receptive to dyes and moisture.
At high temperatures in the presence of atmospheric oxygen, concentrated solutions of alkalis cause the destruction of cellulose with the rupture of glucosidic bonds.
Oxidizing agents used for bleaching cellulose fibers in textile production, as well as for producing papers with high degree whiteness have a destructive effect on cellulose, oxidizing hydroxyl groups and breaking glucosidic bonds. Therefore in production conditions All parameters of the bleaching process are strictly controlled.
When we talked about the structure of the cellulose molecule, we had in mind its ideal model, consisting only of numerous residues of the glucose molecule. We did not specify how many of these glucose residues are contained in the chain of the cellulose molecule (or, as giant molecules are commonly called, in the macromolecule). But in reality, i.e. in any natural plant material, there are greater or lesser deviations from the described ideal model. The cellulose macromolecule may contain a certain amount of residues of molecules of other monosaccharides - hexoses (i.e. containing 6 carbon atoms, like glucose, which also belongs to hexoses) and pentoses (monosaccharides with 5 carbon atoms in the molecule). The macromolecule of natural cellulose may also contain residues of uronic acids - so called carboxylic acids class of monosaccharides, the glucuronic acid residue, for example, differs from the glucose residue in that instead of the -CH 2 OH group it contains a carboxyl group -COOH, characteristic of carboxylic acids.
The number of glucose residues contained in a cellulose macromolecule, or the so-called degree of polymerization, denoted by the index n, is also different for different types cellulose raw materials and varies widely. Thus, in cotton n averages 5,000 - 12,000, and in flax, hemp and ramie 20,000 - 30,000. Thus, the molecular weight of cellulose can reach 5 million oxygen units. The higher n, the stronger the cellulose. For cellulose obtained from wood, n is much lower - in the range of 2500 - 3000, which also causes lower strength of wood cellulose fibers.
However, if we consider cellulose as a material obtained from any one type of plant raw material - cotton, flax, hemp or wood, etc., then in this case the cellulose molecules will have unequal length, unequal degree of polymerization, i.e. in this cellulose there will be longer and shorter molecules present. The high molecular weight part of any technical cellulose is usually called a-cellulose - this is how the part of cellulose that consists of molecules containing 200 or more glucose residues is conventionally designated. A special feature of this part of cellulose is its insolubility in a 17.5% sodium hydroxide solution at 20°C (these, as already mentioned, are the parameters of the mercerization process - the first stage of viscose fiber production).
The part of technical cellulose that is soluble under these conditions is called hemicellulose. It, in turn, consists of a fraction of b-cellulose, containing from 200 to 50 glucose residues, and y-cellulose - the lowest molecular weight fraction, with n less than 50. The name “hemicellulose”, as well as “a-cellulose”, is conditional: The composition of hemicelluloses includes not only cellulose of relatively low molecular weight, but also other polysaccharides, the molecules of which are built from the remains of other hexoses and pentoses, i.e. other hexosans and pentosans (see, for example, the content of pentosans in Table 1). Their common property is a low degree of polymerization n, less than 200, and as a result, solubility in a 17.5% sodium hydroxide solution.
The quality of cellulose is determined not only by the content of a-cellulose, but also by the content of hemicelluloses. It is known that with an increased content of a-cellulose, the fibrous material is usually characterized by higher mechanical strength, chemical and thermal resistance, whiteness stability and durability. But to obtain a durable paper web, it is necessary that hemicellulose satellites are also present in technical cellulose, since pure a-cellulose is not prone to fibrillation (splitting of fibers in the longitudinal direction with the formation of the finest fibers - fibrils) and is easily chopped during the grinding process of fibers. Hemicellulose facilitates fibrillation, which in turn improves the cohesion of the fibers in the paper sheet without excessively reducing their length during milling.
When we said that the concept of “a-cellulose” is also conditional, we meant that a-cellulose is not an individual chemical compound. This term refers to the total amount of substances found in technical cellulose and insoluble in alkali during mercerization. The actual content of high molecular weight cellulose in a-cellulose is always lower, since impurities (lignin, ash, fats, waxes, as well as pentosans and pectin substances chemically bound to cellulose) are not completely dissolved during mercerization. Therefore, without parallel definition the amount of these impurities, the content of a-cellulose cannot characterize the purity of cellulose; it can only be judged if these necessary additional data are available.
Continuing the presentation of the initial information about the structure and properties of cellulose satellites, let us return to Table. 1.
In table Table 1 lists substances found along with cellulose in plant fibers. Pectin substances and pentosans are listed first after cellulose. Pectic substances are polymers of the carbohydrate class, which, like cellulose, have a chain structure, but are built from uronic acid residues, more precisely, galacturonic acid. Polygalacturonic acid is called pectic acid, and its methyl esters are called pectins (see Scheme 4).
DIAGRAM 4 Section of the pectin macromolecule chain
This, of course, is only a diagram, since pectins different plants vary in molecular weight, the content of -OCH3 groups (the so-called methoxy-, or methoxyl, groups, or simply methoxyls) and their distribution along the chain of the macromolecule. Pectins contained in cell sap plants, are soluble in water and are capable of forming dense gels in the presence of sugar and organic acids. However, pectin substances exist in plants mainly in the form of insoluble protopectin - a polymer of a branched structure in which the linear sections of the pectin macromolecule are connected by cross bridges. Protopectin is contained in the walls of plant cells and intercellular cementing material, acting as supporting elements. In general, pectin substances are a reserve material from which, through a series of transformations, cellulose is formed and cell wall. So, for example, in initial stage As cotton fiber grows, the content of pectin substances in it reaches 6%, and by the time the boll is opened, it gradually decreases to approximately 0.8%. At the same time, the cellulose content in the fiber increases, its strength increases, and the degree of cellulose polymerization increases.
Pectin substances are quite resistant to acids, but under the influence of alkalis when heated, they are destroyed, and this circumstance is used to clean cellulose from pectin substances (by cooking, for example, cotton fluff with a solution of caustic soda). Pectin substances are easily destroyed by oxidizing agents.
Pentosans are polysaccharides built from pentose residues - usually arabinose and xylose. Accordingly, these pentosans are called arabans and xylans. They have a linear (chain) or slightly branched structure and in plants usually accompany pectin substances (arabans) or are part of hemicelluloses (xylans). Pentosans are colorless and amorphous. Arabans are highly soluble in water; xylans are insoluble in water.
The next most important companion of cellulose is lignin, a branched polymer that causes lignification of plants. As can be seen from table. 1, lignin is absent in cotton fiber, but in other fibers - flax, hemp, ramie and especially jute - it is contained in smaller or large quantities. It mainly fills the spaces between plant cells, but also penetrates into the surface layers of the fibers, playing the role of an encrusting substance that holds cellulose fibers together. Wood contains especially a lot of lignin - up to 30%. By its nature, lignin no longer belongs to the class of polysaccharides (like cellulose, pectin substances and pentosans), but is a polymer based on derivatives of polyhydric phenols, i.e. refers to the so-called fatty-aromatic compounds. Its significant difference from cellulose is that the lignin macromolecule has an irregular structure, i.e. A polymer molecule is composed not of identical residues of monomer molecules, but of various structural elements. However, the latter have in common that they consist of an aromatic core (which is in turn formed by 6 carbon atoms C) and a side propane chain (of 3 carbon atoms C), this is common to all lignins structural element called a phenylpropane unit (see Scheme 5).
SCHEME 5 Phenylpropane unit
Thus, lignin belongs to the group of natural compounds that have general formula(C 6 C 3)x. Lignin is not an individual chemical compound with a strictly defined composition and properties. Lignins of various origins differ markedly from each other, and even lignins obtained from the same type of plant material, but different ways, sometimes vary greatly in elemental composition, the content of certain substituents (this is the name given to groups connected to benzene ring or propane side chain), solubility and other properties.
The high reactivity of lignin and the heterogeneity of its structure make it difficult to study its structure and properties, but nevertheless it has been established that all lignins contain phenylpropane units, which are derivatives of guaiacol (i.e., pyrocatechol monomethyl ether, see Scheme 6).
SCHEME 6 Guaiacol derivative
Some differences were also revealed in the structure and properties of lignins of annual plants and cereals, on the one hand, and wood, on the other. For example, lignins of grasses and cereals (these include flax and hemp, which we will discuss in more detail) dissolve relatively well in alkalis, while wood lignins do not. This determines more stringent parameters for the process of lignin removal (delignification) from wood using the soda boiling method (such as: more high temperatures and pressure) compared with the process of removing lignin from young shoots and herbs by boiling in lye - a method that was known in China at the beginning of the first millennium AD and which was widely used under the name of maceration or boiling in Europe when processing rags and various kinds waste (linen, hemp) into paper.
We have already talked about high reactivity lignin, i.e. about his ability to enter into numerous chemical reactions, which is explained by the presence of lignin in the macromolecule large quantity reactive functional groups, i.e. capable of undergoing certain chemical transformations inherent in a certain class chemical compounds. This especially applies to alcohol hydroxyls -OH, located at the carbon atoms in the side propane chain; these -OH groups, for example, cause sulfonation of lignin during sulfite cooking of wood - another method of its delignification.
Due to the high reactivity of lignin, its oxidation easily occurs, especially in alkaline environment, with the formation of carboxyl groups -COOH. And under the action of chlorinating and bleaching agents, lignin is easily chlorinated, and the chlorine atom Cl enters both the aromatic ring and the side propane chain; in the presence of moisture, simultaneously with chlorination, oxidation of the lignin macromolecule occurs, and the resulting chlorinated lignin also contains carboxyl groups. Chlorinated and oxidized lignin is more easily washed out of cellulose. All these reactions are widely used in the pulp and paper industry to purify cellulose materials from lignin impurities, which is a very unfavorable component of technical cellulose.
Why is the presence of lignin undesirable? First of all, because lignin has a branched, often three-dimensional, spatial structure and therefore does not have fiber-forming properties, i.e., threads cannot be obtained from it. It imparts rigidity and fragility to cellulose fibers, reduces the ability of cellulose to swell, color, and interact with reagents used in various processes fiber processing. When preparing paper pulp, lignin complicates the grinding and fibrillation of fibers and impairs their mutual adhesion. In addition, it itself is colored yellow-brown, and when the paper ages, it also increases its yellowing.
Our discussions about the structure and properties of cellulose satellites may seem, at first glance, unnecessary. Indeed, is it even appropriate here? brief descriptions structure and properties of lignin, if the graphic restorer deals not with natural fibers, but with paper, i.e. material made from lignin-free fibers? This is, of course, true, but only if we're talking about about rag paper made from cotton raw materials. There is no lignin in cotton. There is practically no it in rag paper made of flax or hemp - it was almost completely removed during the process of weaving the rags.
However, in paper made from wood, and especially in types of newsprint in which wood pulp serves as a filler, lignin is contained in fairly large quantities, and this circumstance should be kept in mind by a restorer working with a wide variety of papers, including low-grade ones. .
Which consists of the residues of a glucose molecule and is a necessary element for the formation of the shell of all plant cells. Its molecules have and contain three hydroxyl groups. Thanks to this, it exhibits properties.
Physical properties of cellulose
Pulp is white solid, which can reach temperatures of 200°C and not collapse. But when the temperature rises to 275°C, it begins to ignite, which indicates that it is a flammable substance.
If you examine cellulose under a microscope, you will notice that its structure is formed by fibers no more than 20 mm in length. Cellulose fibers are connected by many hydrogen bonds, but they do not have branches. This gives cellulose the greatest strength and ability to maintain elasticity.
Chemical properties of cellulose
The remnants of glucose molecules that make up cellulose are formed when. Sulfuric acid and iodine in the process of hydrolysis color cellulose blue, and simply iodine turns it brown.
There are many reactions with cellulose that produce new molecules. Reacting with nitric acid, cellulose is converted into nitrocellulose. And in the process acetic acid Cellulose triacetate is formed.
Cellulose does not dissolve in water. Its most effective solvent is an ionic liquid.
How is cellulose obtained?
Wood consists of 50% cellulose. By cooking wood chips for a long time in a solution of reagents, and then purifying the resulting solution, you can obtain it in its pure form.
Methods for pulping differ in the type of reagents. They can be acidic or alkaline. Acidic reagents contain sulfurous acid and are used to obtain cellulose from low-resin trees. There are two types of alkaline reagents: soda and sulfate. Thanks to soda reagents, cellulose can be obtained from deciduous trees and annual plants. But using this reagent, cellulose is very expensive, so soda reagents are rarely used or not used at all.
The most common method of production is a method based on sulfate reagents. Sodium sulfate is the basis for white liquor, which is used as a reagent and is suitable for producing cellulose from any plant material.
Applications of cellulose
Cellulose and its esters are used to create artificial fibers, viscose and acetate. Wood pulp is used to create a variety of things: paper, plastics, explosive devices, varnishes, etc.