Mercerization, cellulose

Conversion to cellulose II and cellulose III via caustic mercerization and Hquid ammonia treatment are commercial textile processes that are discussed later. Figure 7 shows the characteristic diffractograms (CuKa radiation) of native cellulose, cellulose mercerized with sodium hydroxide, and cellulose treated with Hquid ammonia. 

Mercerized Cellulose (Mercerized Cotton). Mercerizatlon is a process of treating cotton with 25% Na hydroxide which causes it to shrink and become heavier, stronger, denser, and acquire a milky luster it will not shrink and is more easily dyed. It is more active and easier to nitrate than the original cellulose, but the resulting prod is not very stable... 

Strong basic solutions, such as sodium hydroxide, penetrate the crystalline lattice of a-cellulose producing an alkoxide called alkali or soda cellulose. Mercerized cotton is produced by aqueous extraction of the sodium hydroxide. 

Under similar acetylation conditions [153], the acetyl content of cotton (4.3%) was more than twice that of ramie (2.1%) or linen (1.3%) cellulose. Mercerization [8,154,155] greatly enhances the reactivity (or accessibility) of cellulose if it is maintained in a never-dried state. Drying of mercerized cellulose considerably reduced its reactivity to even less than half that of the unmercerized samples [8]. However, its impact, can be alleviated by dehydration through a solvent exchange process. The reactivity of mercerized samples expressed by acetyl content increased from 1.7% to 17.7% if water-washed sample was solvent-exchanged with pyridine and to 29% if washing directly with absolute ethanol and a subsequent pyridine exchange. 

In attaining a high alpha-cellulose with simultaneous removal of pentosan, there is a considerable loss of other material, presumably hexosan in nature. Although higher purity can be attained by treatment with cold, concentrated sodium hydroxide, this results in mercerization of the cellulose. Mercerized cellulose is of no use in the acetylation process unless it is specially treated in order to avoid inactivation by drying. ... 

Strong alkali solutions acting on cellulose (at room temperatures) produce alkali cellulose. The studies on the structure of alkali cellulose [43] obtained with 20-40% NaOH solutions indicated that the substance is not a true alcoholate but an addition complex, RceiiOH NaOH. A true alcoholate can be obtained, for example, from dry cellulose and Na in liquid ammonia. Alkali cellulose has a large range of applications as an intermediate product in the preparation of cellulose ethers, and xanthate (dithiocarbonate), as well as in cellulose mercerization. 

The effect and action of enzymes seems to be very limited because ol the stronger conditions of alkali of mercerizing strength. Enzymatic hydrolysis is accelerated when mercerization is carried out without tension [44]. The greater accessibility and lower crystallinity of cellulose mercerized without tension is a decisive factor in the enzymatic hydrolysis process. Mercerized cotton is generally more prone to enzymatic modification than untreated cotton. 

When re-crystallized (for example, from base or CS2), cellulose I gives the thermodynamically more stable Cellulose II structure with an antiparallel arrangement of the strands and some inter-sheet hydrogen-bonding. Cellulose II contains two different types of anhydroglucose (A and B) with different backbone structures the chains consisting of -A-A- or -B-B-repeat units. Cellulose III is formed from cellulose mercerized in ammonia and is similar to cellulose II but with the chains parallel, as in cellulose la and cellulose Ip. 

The radical sites are capable of initiating polymerizations of monomers. A similar approach can be taken with cellulose. " Mercerized cotton and sodium salt of carboxymethyl cellulose will react with p-aminophenacyl chloride ... 

Fig. 16. Integrated fraction of the downfield component of the C4 resonance line versus degree of crystallinity, /, determined by X-ray analysis., Native cellulose O, regenerated cellulose , mercerized cotton and ramie. (The data for regenerated cellulose samples, except for cupra rayon fibres, were reproduced from ref. 45).

Grafting to cellulose in the absence of solvent (87) is initiated by y-rays by means of a °Co source. The polymer-to-monomer ratio has only a minor effect on the PVF content, which is in the range of 2-5 wt%. In the presence of solvents that cause swelling of cellulose (mercerization), the PVF content increases dramatically. At a radiation of 30 kGy (3 Mrad) in the presence of dimethylformamide, the PVF content of the graft copolymer is 26.6% in its absence it is 4.1%. 

An important chemical finishing process for cotton fabrics is that of mercerization, which improves strength, luster, and dye receptivity. Mercerization iavolves brief exposure of the fabric under tension to concentrated (20—25 wt %) NaOH solution (14). In this treatment, the cotton fibers become more circular ia cross-section and smoother ia surface appearance, which iacreases their luster. At the molecular level, mercerization causes a decrease ia the degree of crystallinity and a transformation of the cellulose crystal form. These fine stmctural changes iacrease the moisture and dye absorption properties of the fiber. Biopolishing is a relatively new treatment of cotton fabrics, involving ceUulase enzymes, to produce special surface effects (15). 

Mercerized cellulose fibers have improved luster and do not shrink further. One of the main reasons for mercerizing textiles is to improve their receptivity to dyes. This improvement may result more from the dismption of the crystalline regions rather than the partial conversion to a new crystal stmcture. A good example of the fundamental importance of the particular crystal form is the difference in rate of digestion by bacteria. Bacteria from cattle mmen rapidly digest Cellulose I but degrade Cellulose II very slowly (69). Thus aHomorphic form can be an important factor in biochemical reactions of cellulose as well as in some conventional chemical reactions. 

Cellulose III. Cellulose III results from treatment of cellulose with Hquid ammonia (ammonia mercerization) or amines. Cellulose III can be made from either Cellulose I or II. When treated with water. Cellulose III can revert to its parent stmcture. Some cellulose III preparations are much more stable than other preparations. The intensities on diffraction patterns from Cellulose III differ slightly depending on whether the Cellulose III was made from Cellulose I or II, and thus these allomorphs are called IIIj or IHjj- Workers studying III concluded, based partiy on the results of I and II, that the packings of IIIj and IIIjj are parallel and antiparallel, respectively (67). IIIjj also is thought to have hydrogen bonds between the corner and center chains. 

In one process to produce highly activated cellulose for acetylation, cellulose is treated with NaOH (mercerization) followed by a hydroxyalkylating agent, eg, ethylene oxide or propylene oxide, to give a cellulose hydroxyalkyl ether with a DS of 0.05—0.3 (76). The resulting water-insoluble material is highly reactive to conventional acetic anhydride—sulfuric acid acetylation. 

An older method of cellulose fiber modification is mercerization [22,33-36], which has been widely used on cotton textiles. Mercerization is an alkali treatment of cellulose fibers. It depends on the type and concentration of the alkalic solution, its temperature, time of treatment, tension of the material, and the additives used [33,36]. At present there is a tendency to use mercerization for natural fibers as well. Optimal conditions of mercerization ensure the improvement of the tensile properties [33-35,37] and absorption characteristics [33-35], which are important in the composing process. 

Those containing less than 7.5% N show a fiber character and give no X-ray diffraction indicating nitration, but only the pattern of mercerized cellulose... 

N are more or less disintegrated, yielding very diffuse diffractions, apparently due in part to very small crystalline elements of mercerized cellulose... 

Acid anhydrides have been employed with, and without the use of a base catalyst. For example, acetates, propionates, butyrates, and their mixed esters, DS of 1 to ca. 3, have been obtained by reaction of activated cellulose with the corresponding anhydride, or two anhydrides, starting with the one with the smaller volume. In all cases, the distribution of both ester groups was almost statistic. Activation has been carried out by partial solvent distillation, and later by heat activation, under reduced pressure, of the native cellulose (bagasse, sisal), or the mercerized one (cotton linters). No catalyst has been employed the anhydride/AGU ratio was stoichiometric for microcrystalhne cellulose. Alternatively, 50% excess of anhydride (relative to targeted DS) has been employed for fibrous celluloses. In all cases, polymer degradation was minimum, and functionalization occurs preferentially at Ce ( C NMR spectroscopic analysis [52,56,57]). 

Since bacterial cellulose from all suitable carbohydrate substrates i8 identical with natural cellulose, its industrial importance20 is obvious. Relatively large amounts of bacterial cellulose were produced in Germany during the first World War. More recently products similar to parchment, mercerized cotton, cellulose nitrate,21 acetate14 and viscose rayons have been produced from bacterial cellulose. 

Viscose Also known as the Cross-Bevan-Beadle process. A process for making regenerated cellulose fibers. The product has been known by the generic name rayon since 1924. Cellulose, from cotton or wood, is first reacted with sodium hydroxide ( mercerization), yielding alkali cellulose. This is dissolved in carbon disulfide, yielding cellulose xanthate, which is dissolved in sodium hydroxide solution. Injection of this solution (known as viscose... 

Sisson has traced the evolution of current concepts of the crystalline part of cellulose structures. The fiber diagram obtained by X-ray diffraction is now known to be produced by a series of elementary crystals, called crystallites, which have a definite arrangement with respect to the fiber axis. It is also known that the crystallites in regenerated cellulose may be oriented to varying degrees with respect to the fiber axis and that the crystallites in regenerated cellulose and mercerized cotton differ from those in native fibers. These hydrate type crystallites appear to be more reactive chemically than the native type. 

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