Structure application and chemical composition of cellulose. Cellulose formula

dextrinum starch

Video experience “Acid hydrolysis of cellulose”

Esterification reactions

Cellulose is a polyhydric alcohol; three hydroxyl groups fall on the unit cell of the polymer. In this regard, esterification reactions (formation of esters) are characteristic of cellulose. Of greatest practical importance are reactions with nitric acid and acetic anhydride. Cellulose does not produce a silver mirror reaction.

1. Nitration:

(C 6 H 7 O 2 (OH) 3) n + 3 nHNO 3 H 2 SO 4 (conc.) → (C 6 H 7 O 2 (ONO 2) 3) n + 3 nH 2 O

pyroxylin

Video experience "Obtaining and properties of nitrocellulose"

Fully esterified fiber is known as pyroxylin, which, after appropriate treatment, turns into smokeless powder. Depending on the nitration conditions, cellulose dinitrate can be obtained, which is called colloxylin in the art. It is also used in the manufacture of gunpowder and solid rocket fuels. In addition, celluloid is made based on colloxylin.

2. Interaction with acetic acid:

(C 6 H 7 O 2 (OH) 3) n + 3nCH 3 COOH H2SO4 ( conc .)→ (C 6 H 7 O 2 (OCOCH 3) 3) n + 3nH 2 O

When cellulose reacts with acetic anhydride in the presence of acetic and sulfuric acids, triacetyl cellulose is formed.

Cellulose Triacetyl (or cellulose acetate) is a valuable product for the manufacture of non-combustible film andacetate silk. For this, cellulose acetate is dissolved in a mixture of dichloromethane and ethanol, and this solution is forced through dies into a stream of warm air.

And the die itself looks schematically like this:

1 - dope solution
2 - die,
3 - fiber.

The solvent evaporates and the trickles of the solution turn into the thinnest threads of acetate silk.

Speaking about the use of pulp, one can not say that a large amount of pulp is consumed for the manufacture of various paper. Paper- This is a thin layer of fiber, glued and pressed on a special paper machine.

Cellulose is a polysaccharide built from elementary units of anhydroD glucose and representing poly-1, 4-β - D glucopyranosylD glucopyranose. Cellulose macromolecule along with anhydroglucose units may contain residues of other monosaccharides (hexoses and pentoses), as well as uronic acids (see. Fig.). The nature and amount of such residues is determined by the conditions of biochemical synthesis.

Cellulose is the main component of the cell walls of higher plants. Together with the substances accompanying it, it plays the role of a skeleton that carries the main mechanical load. Cellulose is mainly found in the hairs of seeds of some plants, for example, cotton (97-98% of cellulose), wood (40-50% based on dry matter), bast fibers, inner layers of plant bark (flax and ramie - 80-90% , jute - 75% and others), the stems of annual plants (30-40%), for example, reeds, corn, cereals, sunflowers.

The separation of cellulose from natural materials is based on the action of reagents that destroy or dissolve non-cellulosic components. The nature of the treatment depends on the composition and structure of the plant material. For cotton fiber (non-cellulose impurities - 2, 0-2, 5% of nitrogen-containing substances; about 1% of pentosans and pectin substances; 0, 3-1, 0% of fats and waxes; 0, 1-0, 2% of mineral salts) use relatively mild selection methods.

The cotton fluff is subjected to a park (3-6 hours, 3-10 atmospheres) with a 1, 5-3% sodium hydroxide solution, followed by washing and bleaching with various oxidizing agents - chlorine dioxide, sodium hypochlorite, hydrogen peroxide. Some polysaccharides with a low molar weight (pentosans, partially hexosans), uronic acids, and part of fats and waxes pass into the solution. Contentα cellulose (fraction insoluble in 17.5% solutionN aON at 20 ° for 1 hour) can be brought up to 99, 8-99, 9%. As a result of the partial destruction of the morphological structure of the fiber during cooking, the reactivity of cellulose increases (a characteristic that determines the solubility of the esters obtained during the subsequent chemical processing of cellulose and the filterability of the dope solutions of these esters).

To isolate cellulose from wood containing 40-55% cellulose, 5-10% of other hexosanes, 10-20% of pentosans, 20-30% of lignin, 2-5% of resins and a number of other impurities and having a complex morphological structure, more rigid processing conditions; most often use sulphite or sulphate cooking of wood chips.

When sulphite cooking, wood is treated with a solution containing 3-6% freeSO 2 and about 2% SO 2 bound in the form of calcium, magnesium, sodium or ammonium bisulfite. Cooking is carried out under pressure at 135-150 ° for 4-12 hours; cooking solutions for acid bisulfite cooking have a pH from 1, 5 to 2, 5. When sulfite cooking, lignin is sulfonated, followed by its transition into a solution. At the same time, part of the hemicelluloses is hydrolyzed, the oligo- and monosaccharides formed, as well as part of the resinous substances, are dissolved in the cooking alkali. When using cellulose (sulphite cellulose) emitted by this method for chemical processing (mainly in the production of viscose fiber), cellulose is subjected to refinement, the main task of which is to increase the chemical purity and uniformity of cellulose (removal of lignin, hemicellulose, reduction in ash and resin content, change in colloidal chemical and physical properties). The most common refinement methods are to treat bleached pulp with a 4-10% solution.N aON at 20 ° (cold upgrading) or 1% solutionNaOH at 95-100 ° (hot refinement). Refined sulfite cellulose for chemical processing has the following indicators: 95-98%α cellulose; 0, 15--0, 25% of lignin; 1, 8-4, 0% pentosans; 0.07-0.14% resin; 0, 06-0, 13% ash. Sulphite pulp is also used for the manufacture of high quality paper and paperboard.

Wood chips can also be cooked with 4-6% solution N aON (sodium boiling) or its mixture with sodium sulfide (sulfate cooking) at 170-175 ° under pressure for 5-6 hours. In this case, lignin dissolves, a part of the hemicelluloses (mainly hexosanes) is converted and hydrolyzed, and the resulting sugars are converted to organic hydroxy acids (lactic, saccharin and others) and acids (formic). Tar and higher fatty acids gradually pass into the cooking alkali in the form of sodium salts (the so-called“Sulfate soap”). Alkaline cooking is applicable for processing both spruce, and pine and deciduous wood. When using the cellulose (sulfate cellulose) released by this method for chemical processing, the wood is subjected to prehydrolysis before cooking (treatment with dilute sulfuric acid at elevated temperature). Prehydrolysis sulfate cellulose used for chemical processing, after enrichment and bleaching, has the following average composition (%):α cellulose - 94, 5-96, 9; pentosans 2-2, 5; resins and fats - 0, 01-0, 06; ash - 0, 02-0, 06. Sulphate pulp is also used for the production of sack and wrapping papers, paper cords, technical papers (bobbin, emery, condenser), writing, printing and bleached strong papers (drawing, cartographic, for documents).

Sulphate cooking is used to produce high yield cellulose used to produce corrugated cardboard and sack paper (in this case, the yield of cellulose from wood is 50-60% against~ 35% for prehydrolysis sulphate pulp for chemical processing). High yield cellulose contains significant amounts of lignin (12-18%) and retains the shape of wood chips. Therefore, after cooking, it is subjected to mechanical grinding. Soda and sulphate cooking can be used in the separation of cellulose from straw containing large quantitiesSiO 2 removed by alkali.

Cellulose is also isolated from deciduous wood and annual plants by hydrotropic cooking - processing of raw materials with concentrated (40-50%) solutions of alkali metal salts and aromatic carboxylic and sulfonic acids (e.g. benzoic, cimol and xylene sulfonic acids) at 150-180 ° for 5-10 hours. Other methods for the isolation of cellulose (nitric acid, chlor-alkali and others) are not widely used.

To determine the molar weight of cellulose, a viscometric one is usually used [for the viscosity of cellulose solutions in a copper-ammonia solution, in solutions of a quaternary ammonium base, cadmium ethylene diamine hydroxide (the so-called cadoxene), in an alkaline solution of sodium yellow iron complex and others, or for the viscosity of mainly cellulose ethers acetates and nitrates obtained under conditions excluding destruction] and osmotic (for cellulose ethers) methods. The degree of polymerization, determined using these methods, is different for different pulp preparations: 10-12 thousand for cotton pulp and bast fiber pulp; 2, 5-3 thousand for wood pulp (as determined in an ultracentrifuge) and 0, 3-0, 5 thousand for cellulose of viscose silk.

Cellulose is characterized by a significant polydispersity in molar weight. Cellulose is fractionated by fractional dissolution or precipitation from a copper-ammonia solution, from a solution in cupriethylenediamine, cadmium ethylene diamine or in an alkaline solution of sodium yellow iron complex, and also by fractional precipitation from solutions of cellulose nitrates in acetone or ethyl acetate. Cotton cellulose, bast fiber, and softwood pulp are characterized by molar distribution curves with two maxima; the curves for hardwood pulp have one maximum.

Cellulose has a complex supramolecular structure. Based on X-ray, electron diffraction, and spectroscopic data, it is generally accepted that cellulose is a crystalline polymer. Cellulose has a number of structural modifications, the main of which are natural cellulose and hydrated cellulose. Natural cellulose turns into hydrated cellulose upon dissolution and subsequent precipitation from solution, under the action of concentrated alkali solutions and subsequent decomposition of alkaline cellulose and others. The reverse transition can be carried out by heating cellulose hydrate in a solvent, causing its intense swelling (glycerin, water). Both structural modifications have different X-ray diffraction patterns and differ greatly in reactivity, solubility (not only of cellulose itself, but also of its esters), adsorption ability, and others. Cellulose hydrate preparations have increased hygroscopicity and colorability, as well as a higher rate of hydrolysis.

The presence of acetal (glucoside) bonds between the elementary units in the cellulose macromolecule causes its low resistance to the action of acids, in the presence of which cellulose hydrolysis proceeds (see Fig.). The speed of the process depends on a number of factors, of which the decisive factor, especially when carrying out the reaction in a heterogeneous medium, is the structure of the preparations, which determines the intensity of the intermolecular interaction. In the initial stage of hydrolysis, the rate may be higher, which is associated with the possibility of the existence of a small number of bonds in the macromolecule that are less resistant to the action of hydrolyzing reagents than ordinary glucoside bonds. Partial hydrolysis products of cellulose are called hydrocellulose.

As a result of hydrolysis, the properties of the cellulosic material significantly change - the mechanical strength of the fibers decreases (due to a decrease in the degree of polymerization), the content of aldehyde groups and the solubility in alkalis increase. Partial hydrolysis does not alter the resistance of the cellulose preparation to alkaline treatments. The product of the complete hydrolysis of cellulose is glucose. Industrial methods for the hydrolysis of cellulose-containing plant materials are treated with diluted solutionsHCl and H 2 SO 4 (0, 2-0, 3%) at 150-180 °; sugar output during step hydrolysis - up to 50%.

By chemical nature, cellulose is a polyhydric alcohol. Due to the presence of hydroxyl groups in the elementary macromolecule, the cellulose reacts with alkali metals and bases. When processing dried cellulose with a solution of sodium metal in liquid ammonia at minus 25-50 ° C for 24 hours, trisodium alcoholate of cellulose is formed:

n + 3nNa → n + 1, 5nH 2.

Under the action of concentrated alkali solutions on cellulose, along with the chemical reaction, physicochemical processes occur - cellulose swelling and partial dissolution of its low molecular weight fractions, structural transformations. The interaction of alkali metal hydroxide with cellulose can proceed in two ways:

n + n NaOH ↔ n + nH 2 O

[C 6 H 7 O 2 (OH) 3] n + n NaOH ↔ n.

The reactivity of the primary and secondary hydroxyl groups of cellulose in an alkaline medium is different. The acid properties are most pronounced for hydroxyl groups located at the second carbon atom of the elementary unit of cellulose, which are part of the glycol group and are inα -position to the acetal bond. The formation of cellulose alcoholate, apparently, occurs precisely due to these hydroxyl groups, while a molecular compound is formed upon interaction with other OH groups.

The composition of alkaline cellulose depends on the conditions for its production - alkali concentration; temperature, the nature of the cellulosic material and others. Due to the reversibility of the alkaline cellulose formation reaction, an increase in the alkali concentration in the solution leads to an increaseγ (the number of substituted hydroxyl groups per 100 elementary units of the cellulose macromolecule) of alkaline cellulose, and a decrease in the temperature of mercerization increasesγ alkaline cellulose obtained by the action of equiconcentrated alkali solutions, which is explained by the difference in temperature coefficients of the forward and reverse reactions. The different intensity of interaction with alkalis of different cellulosic materials is apparently associated with the physical structure of these materials.

An important part of the process of interaction of cellulose with alkalis is the swelling of cellulose and the dissolution of its low molecular weight fractions. These processes facilitate the removal of low molecular weight fractions (hemicelluloses) from cellulose and the diffusion of esterifying reagents into the fiber during subsequent esterification processes (for example, during xanthogenation). With decreasing temperature, the degree of swelling increases significantly. For example, at 18 °, an increase in the diameter of the cotton fiber with 12%NaOH is 10%, and at -10 ° it reaches 66%. With an increase in alkali concentration, an increase occurs first, and then (over 12%) the degree of swelling decreases. The maximum degree of swelling is observed at those alkali concentrations at which an x-ray of alkaline cellulose occurs. These concentrations are different for different cellulosic materials: for cotton 18% (at 25 °), for ramie 14-15%, for sulphite cellulose 9, 5-10%. The interaction of cellulose with concentrated solutionsN aON is widely used in the textile industry, in the production of artificial fibers and cellulose ethers.

The interaction of cellulose with other alkali metal hydroxides proceeds similarly to the reaction with sodium hydroxide. An X-ray pattern of alkaline cellulose appears when natural cellulose preparations are exposed to approximately equimolar (3, 5-4, 0 mol / L) solutions of alkali metal hydroxides. Strong organic bases — some tetraalkyl (aryl) ammonium hydroxides appear to form molecular compounds with cellulose.

A special place in the series of reactions of cellulose with bases is occupied by its interaction with cupriamine hydrate [Cu (NH 3) 4] (OH) 2 and also with a number of other complex compounds of copper, nickel, cadmium, zinc - cupriethylenediamine [Cu (en) 2] (OH) 2 (en - ethylenediamine molecule), nioxane [Ni (NH 3) 6] (OH) 2, Nioxene [Ni (en) 3] (OH) 2, Cadoxene [Cd (en) 3] (OH) 2 and others. Cellulose dissolves in these products. The deposition of cellulose from a copper-ammonia solution is carried out under the action of water, alkali or acid solutions.

Under the action of oxidizing agents, partial oxidation of cellulose occurs - a process successfully used in technology (bleaching of cellulose and cotton fabrics, alkali cellulose pre-ripening). Cellulose oxidation is a side process in the refinement of cellulose, the preparation of copper-ammonia dope, the operation of products from cellulose materials. Partial oxidation products of cellulose are called oxycelluloses. Depending on the nature of the oxidizing agent, cellulose oxidation can be selective or non-selective. The most selective oxidizing agents include iodic acid and its salts, which oxidize the glycol group of the elementary unit of cellulose with a break in the pyran cycle (formation of dialdehyde cellulose) (see. Fig.). Under the action of iodic acid and periods, an insignificant number of primary hydroxyl groups are also oxidized (to carboxyl or aldehyde). According to a similar scheme, cellulose is oxidized by the action of lead tetraacetate in an environment of organic solvents (acetic acid, chloroform).

In terms of acid resistance, dialdehyde cellulose differs little from the original cellulose, but is much less resistant to alkalis and even water, which is the result of hydrolysis of the semi-acetal bond in an alkaline medium. Oxidation of aldehyde groups to carboxylic acid by sodium chlorite (the formation of dicarboxyl cellulose), as well as their reduction to hydroxyl (the formation of the so-called"Alcohol" - cellulose) stabilize oxidized cellulose to the action of alkaline reagents. Solubility of nitrates and acetates of dialdehyde cellulose even of a low oxidation state (γ \u003d 6-10) is significantly lower than the solubility of the corresponding cellulose ethers, apparently due to the formation of intermolecular semi-acetal bonds during esterification. Under the action of nitrogen dioxide on cellulose, mainly primary hydroxyl groups are oxidized to carboxyl groups (formation of monocarboxyl cellulose) (see. Fig.). The reaction proceeds according to a radical mechanism with the intermediate formation of nitrous acid esters of cellulose and subsequent oxidative transformations of these esters. Up to 15% of the total content of carboxyl groups are neuronic (COOH groups are formed at the second and third carbon atoms). At the same time, the oxidation of the hydroxyl groups of these atoms to keto groups occurs (up to 15-20% of the total number of oxidized hydroxyl groups). The formation of keto groups is apparently the reason for the extremely low resistance of monocarboxyl cellulose to alkalis and even water at elevated temperatures.

When the content of 10-13% COOH groups, monocarboxyl cellulose dissolves in a dilute solutionNaOH, solutions of ammonia, pyridine with the formation of the corresponding salts. Its acetylation proceeds more slowly than cellulose; acetates do not completely dissolve in methylene chloride. Monocarboxyl cellulose nitrates do not dissolve in acetone even with a nitrogen content of up to 13.5%. These features of the properties of monocarboxyl cellulose esters are associated with the formation of intermolecular ether bonds in the interaction of carboxyl and hydroxyl groups. Monocarboxyl cellulose is used as a hemostatic agent, as a cation exchanger for the separation of biologically active substances (hormones). By the combined oxidation of cellulose with periodate, and then with chlorite and nitrogen dioxide, preparations of the so-called tricarboxyl cellulose containing up to 50.8% of COOH groups were synthesized.

The direction of cellulose oxidation under the action of indiscriminate oxidizing agents (chlorine dioxide, hypochlorous acid salts, hydrogen peroxide, oxygen in an alkaline environment) largely depends on the nature of the medium. In acidic and neutral environments, under the action of hypochlorite and hydrogen peroxide, the formation of products of the reducing type occurs, apparently as a result of the oxidation of the primary hydroxyl groups to aldehyde and one of the secondary OH groups to the keto group (hydrogen peroxide also oxidizes glycol groups with a broken pyran cycle ) When oxidized with hypochlorite in an alkaline medium, the aldehyde groups gradually turn into carboxylic groups, as a result of which the oxidation product has an acidic character. Hypochlorite treatment is one of the most commonly used methods for bleaching cellulose. To obtain high-quality pulp with a high degree of whiteness, it is bleached with chlorine dioxide or chlorite in an acidic or alkaline environment. In this case, lignin is oxidized, dyes are destroyed, and aldehyde groups in the cellulose macromolecule are oxidized to carboxyl; hydroxyl groups are not oxidized. Oxidation of air with oxygen in an alkaline environment, proceeding by a radical mechanism and accompanied by significant destruction of cellulose, leads to the accumulation of carbonyl and carboxyl groups in the macromolecule (alkaline cellulose pre-ripening).

The presence of hydroxyl groups in the cellulose macromolecule element allows the transition to such important classes of cellulose derivatives as ethers and esters. Due to their valuable properties, these compounds are used in various fields of technology - in the production of fibers and films (acetates, cellulose nitrates), plastics (acetates, nitrates, ethyl, benzyl ethers), varnishes and electrical insulating coatings, as stabilizers for suspensions and thickeners in oil and textile industry (low substituted carboxymethyl cellulose).

Cellulose-based fibers (natural and artificial) are a full-fledged textile material with a range of valuable properties (high strength and hygroscopicity, good tintability. The disadvantages of cellulose fibers are flammability, insufficient elasticity, easy destruction under the influence of microorganisms, etc. The tendency to directional a change (modification) of cellulosic materials was caused by the appearance of a number of new cellulose derivatives, and in some cases, new classes of cellulose derivatives.

Modification of the properties and synthesis of new cellulose derivatives is carried out using two groups of methods:

1) by esterification, O-alkylation or conversion of the hydroxyl groups of an elementary unit to other functional groups (oxidation, nucleophilic substitution using some cellulose ethers - nitrates, esters withn toluene and methanesulfonic acid);

2) grafted by copolymerization or the interaction of cellulose with polyfunctional compounds (the conversion of cellulose, respectively, into a branched or crosslinked polymer).

One of the most common methods for the synthesis of various cellulose derivatives is nucleophilic substitution. The starting substances in this case are cellulose ethers with some strong acids (toluene and methanesulfonic acid, nitric and phenylphosphoric acids), as well as halogenodeoxy derivatives of cellulose. Using the nucleophilic substitution reaction, cellulose derivatives have been synthesized in which the hydroxyl groups are replaced by halogens (chlorine, fluorine, iodine), rhodane, nitrile and other groups; synthesized deoxycellulose preparations containing heterocycles (pyridine and piperidine), cellulose ethers with phenols and naphthols, a number of cellulose esters (with higher carboxylic acids,α - amino acids unsaturated acids). The intramolecular nucleophilic substitution reaction (saponification of tosyl cellulose ethers) leads to the formation of mixed polysaccharides containing 2, 3– and 3, 6-anhydrocycles.

The synthesis of grafted cellulose copolymers is of the greatest practical importance for the creation of cellulosic materials with new technically valuable properties. The most common methods for the synthesis of grafted cellulose copolymers include the use of a chain transfer reaction to cellulose, radiation-chemical copolymerization, and the use of redox systems in which cellulose plays the role of a reducing agent. In the latter case, the formation of a macroradical can occur due to the oxidation of both hydroxyl groups of cellulose (oxidation with cerium salts), and functional groups specially introduced into the macromolecule — aldehyde, amino groups (oxidation with vanadium and manganese salts), or decomposition of the diazocompound formed by diazotization introduced into cellulose aromatic amino groups. The synthesis of grafted cellulose copolymers in some cases can be carried out without the formation of a homopolymer, which reduces the consumption of monomer. The grafted cellulose copolymers obtained under ordinary copolymerization conditions consist of a mixture of the original cellulose (or its ester to be grafted) and the grafted copolymer (40-60%). The degree of polymerization of the grafted chains varies depending on the initiation method and the nature of the grafted component from 300 to 28,000.

The change in properties resulting from grafted copolymerization is determined by the nature of the grafted monomer. Inoculation of styrene, acrylamide, acrylonitrile leads to an increase in the strength of the cotton fiber in the dry state. The grafting of polystyrene, polymethyl methacrylate and polybutyl acrylate allows you to get hydrophobic materials. Grafted copolymers of cellulose with flexible chain polymers (polymethyl acrylate) with a sufficiently high content of the grafted component are thermoplastic. Grafted copolymers of cellulose with polyelectrolytes (polyacrylic acid, polymethylvinylpyridine) can be used as ion-exchange fabrics, fibers, films.

One of the disadvantages of cellulose fibers is their low elasticity and, as a result, poor shape retention and increased creasing. The elimination of this drawback is achieved by the formation of intermolecular bonds during the treatment of tissues with polyfunctional compounds (dimethylol urea, dimethylol cycloethylene urea, trimethylolmelamine, dimethylol triazone, various diepoxides, acetals) that react with OH groups of cellulose. Along with the formation of chemical bonds between cellulose macromolecules, the crosslinking reagent is polymerized to form linear and spatial polymers. The cellulose fiber fabrics are impregnated with a solution containing a crosslinking agent and a catalyst, squeezed, dried at a low temperature, and heat treated at 120-160 ° for 3-5 minutes. In the treatment of cellulose with polyfunctional crosslinking reagents, the process proceeds mainly in amorphous sections of the fiber. To achieve the same crush-proof effect, the crosslinking agent consumption during processing of viscose fibers should be significantly higher than during processing of cotton fiber, which is apparently associated with a higher degree of crystallinity of the latter.