Alkali cellulose reactivity

Alkali celluloses are extremely important intermediates because they exhibit a markedly enhanced reactivity compared with original cellulose. The reagents can penetrate more easily into the swollen cellulose structure and thus react with the hydroxyl groups. For instance, preparation of alkali cellulose, named mercerization after its inventor John Mercer (1844), is an important step when producing cellulose xanthate, from which viscose fibers and cellophane are prepared. 

A purified form of cellulose is reacted with sodium hydroxide to produce a swollen alkali cellulose, which is chemically more reactive than untreated cellulose. The alkali cellulose is then reacted with ethylene oxide to produce a series of hydroxyethyl cellulose ethers. 

A purified form of cellulose is reacted with sodium hydroxide to produce a swollen alkali cellulose that is chemically more reactive than untreated cellulose. The alkali cellulose is then reacted with propylene oxide at elevated temperature and pressure. The propylene oxide can be substituted on the cellulose through an ether linkage at the three reactive hydroxyls present on each anhydroglucose monomer unit of the cellulose chain. Etherification takes place in such a way that hydroxypropyl substituent groups contain almost entirely secondary hydroxyls. The secondary hydroxyl present in the side chain is available for further reaction with the propylene oxide, and chaining-out may take place. This results in the... 

We have studied the relative reactivity of cellulose and mixed polysaccharides (III) and (IV) in reactions with aqueous and alcoholic solutions of NaOH. The data on the composition of alkali compounds of polysaccharides are given in Table 6. As seen from the data presented in Table 6, the amount of bound alkali in preparations of alkaline compounds of mixed polysaccharides is less than in alkali cellulose obtained under the same conditions, the DS with respect to NaOH decreasing with increasing content of altrose units in polysaccharide (III) and of 3,6-anhydroglucose units in polysaccharide (IV). 

When the nonaqueous cellulose solvent was used as a medium, the order of reactivities was 6-OH>2-OH 3-OH. This order is similar to observations in the case of simple alcohols. Thus, although the primary hydroxyl group 6-OH has the highest reactivity, and the secondary hydroj l groups 2-OH and 3-OH have almost equal reactivities, the difference of reactivities between 6-OH, 2-OH and 3-OH is small. On the other hand, when heterogeneous alkali cellulose systems were used, where cellulose was swollen in aqueous alkali rather than being in solution, the order of reactivity was... 

OH>6-OH>>3-OH at all concentrations of aqueous alkali (22). This order is not consistent with the pattern for lower molecular weight compounds and suggests consideration of effects of intra- and intermolecular hydrogen bonds which may remain even in swollen alkali cellulose. The secondary hydroj l group 2-OH has a higher reactivity than the primary alcohol 6-OH, and there is a remarkable difference in reactivities between the secondary alcohols 2-OH and... 

Other typical effects of the reactivity additives are that the alkali celluloses are more easily shredded and become fluffier after shredding, the dissolving pulps have a more open structure and will be more reactive towards lye, and homification of dried and stored pulp becomes less serious a problem. 

The C-3 OH is always the least reactive regardlessofthe type of reaction. However, in some cases the preferred OH group depends on the reactants and on the D.S. Thus esterification with toysl chloride occurs at the primary alcohol but benzoyl chloride gives the C-2 and C-6 derivatives. For the reaction of alkali cellulose with chloroacetic acid the reaction occurs at the C-6 OH at low D.S. values but as the D.S. increases the C-2 OH reactivity increases. 

Etherification. The accessible, available hydroxyl groups on the 2, 3, and 6 positions of the anhydroglucose residue are quite reactive (96) and provide sites for much of the current modification of cotton cellulose to impart special or value-added properties. The two most common classes into which modifications fall, include etherification and esterification of the cotton cellulose hydroxyls as well as addition reactions with certain unsaturated compounds to produce cellulose ethers (see Cellulose Ethers). One large class of cellulose-reactive dyestuffs in commercial use attaches to the cellulose through an alkali-catalyzed etherification by nucleophilic attack of the chlorotriazine moiety of the dyestuff ... 

Hydroxypropylmethylcellulose (HPMC) is one of the many mixed ethers of cellulose. It is prepared by reactions of alkali cellulose with methyl chloride and propylene oxide in a slurry process. Reaction conditions may be varied to control compositions despite the greater reactivity of methyl chloride. HPMC is an extremely effective viscosifier compared to conventional cellulose ethers. Its mi-croheterogeneous nature, phase behavior, and interaction with surfactants allow use in food, pharmaceutical, and coatings applications (72,87). 

The purified form of cellulose, obtained from cotton linters or wood pulp, is treated with sodium hydroxide solution to produce swollen alkali cellulose that is chemically more reactive than untreated cellulose. Reaction of the alkali cellulose with chloro-methane and propylene oxide produces methyl hydroxypropyl ethers of cellulose. Further purification of the fibrous reaction product is done and is grounded to fine, uniform powder or granules. 

Cellulose Ethers. They are prepared by reaction of alkyl chlorides or their analogues with alkali-celluloses that are much more reactive substrates than native cellulose itself. The reaction pathway to obtain carboxymethyl cellulose (CMC) is given below as an example ... 

The paracrystalline layers weaken the crystallites and increase their accessibility to reagents. Therefore, increasing of the fraction of the paracrystalline layers promotes reactivity of cellulose. The example of such effect is alkalization process of cellulose that causes transformation of Cl into ciystalline polymorph of alkali-cellulose (Fig. 7.36). 

The most important discovery in dyeing cellulose with reactive dyes was the appHcation of Schotten-Baumaun principles. Reaction of alcohols proceeds more readily and completely in the presence of dilute alkali, and the cellulose anion (cell- O ) is considerably more nucleophilic than is the hydroxide ion. Thus the fixation reaction (eq. 1) competes favorably with hydrolysis of the dye (eq. 2). 

Approach (a) is normally the easiest to control, and is used in the application of levelling acid and 1 1 metal-complex dyes to wool or nylon, and of the reactive, sulphur or vat dyes to cellulosic fibres. The agents traditionally used are the stronger acids and alkalis such as sulphuric, hydrochloric and formic acids, sodium carbonate and sodium hydroxide. In... 

Recent years have seen considerable research into the modification of cellulose and reactive dyes, specifically to overcome some of the drawbacks of this dye-fibre system, including the limited degree of fixation in full depths, the need for alkali and relatively high concentrations of electrolyte. This research, which is driven by environmental considerations, was discussed in sections 7.10 and 10.9.1. Thus it need not be considered further here. 

Ionisation of the hydroxy groups in cellulose is essential for the nucleophilic substitution reaction to take place. At neutral pH virtually no nucleophilic ionised groups are present and dye-fibre reaction does not occur. When satisfactory exhaustion of the reactive dye has taken place, alkali is added to raise the pH to 10-11, causing adequate ionisation of the cellulose hydroxy groups. The attacking nucleophile ( X ) can be either a cellulosate anion or a hydroxide ion (Scheme 7.8), the former resulting in fixation to the fibre and the latter in hydrolysis of the reactive dye. The fact that the cellulosic substrate competes effectively with water for the reactive dye can be attributed to three features of the reactive dye/ cellulosic fibre system ... 

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