Mercerization transformations

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). 

Dinsmore and Mercer further investigated this reaction using DBU as a base and n-Bu3P/DBAD (di-tert-butyl azodicarboxylate) as Mitsunobu s reactants, and found an unexpected steroselectivity in the Mitsunobu transformation [75b], In fact, the stereochemical course of the Mitsunobu reaction (Scheme 6.11) depended on whether the carbamic acid intermediate was N-substituted with hydrogen (retention) or with carbon (inversion). 

When cellulose fibers are mercerized in 12-18% sodium hydroxide solution, the original cellulose (cellulose I) is transformed into cellulose II and the unit cell dimensions are changed. This transformation, taking place at somewhat different concentrations depending on the origin of the sample, can be followed by X-ray measurements (Fig. 9-3). 

The most important alternative crystalline form is cellulose II. This form can result from treatment of cellulose in concentrated alkali, such as 23% NaOH, followed by rinsing in water. This is also the main form that results from crystallization of dissolved cellulose, such as regeneration of rayon. Supercritical water can also effect the transformation [216]. The treatment of cotton in milder alkali, for industrial mercerization, amounts mainly to disruption and decrystallization rather than transformation to crystalline II. Cellulose II can occur as the native state when the normal biosynthesis and subsequent crystallization is disrupted [217-219]. 

The same phenomena were observed for soda celluloses. Na-oellulose Ij prepared from the I family under conditions of low swelling (high temperature or with stretching) could be converted to cellulose I with hot water. Na-cellulose Ijj prepared from cellulose II under the same conditions was converted to cellulose II (7-10). In ordinary mercerization of native cellulose a mixture of Na-cellulose Ij and Ijj was obtained both decomposed to cellulose II with cold water. Na-cellulose Ij was transformed irreversibly into Na-cellulose Ijj. 

Figures 1 and 2 show x-ray diffractograms of members of the cellulose I and II families, respectively. Diffractograms of each were typical, and Indicated complete transformation and uniplanar orientation of (110) relative to the membrane surface. It was remarkable to retain this orientation of the mercerized bacterial cellulose and of the lllu and IVjj prepared from it. The crystallinity of members of the cellulose II family were not high. But their IR spectra showed enough resolution for detailed discussion.

On treatment with NaOH solution of mercerizing strength, the cellulose I pattern of purified flax completely transforms to the cellulose II pattern. In other natural cellulosic fibers (except ramie), this transformation is only partial [124]. The degree of crystallinity of flax is estimated to be 70% [113]. 

X-ray diffraction photographs of ramie show the sharp characteristic patterns of cellulose I. The degree of crystallinity of ramie (native cellulose) by x-ray methods is generally estimated to be about 70% [19,113]. In other studies, the estimates range from 74% in the dry state to 54% in the moist state [124]. The treatment with NaOH solution of mercerizing strength completely transforms the cellulose I pattern to cellulose II [169] and reduces the degree of crystallinity to 50% [113]. On the other hand, treatment with dilute NaOH (about 5% solution) increases the crystallinity [185] the same is true for acid hydrolysis [19]. 

DarVeniZa M, Mercer DR Lightning protection of pole mounted transformer, IEEE Trans. Power Del. 1989 (2) 1087-95. 

The transformation of cellulose I to cellulose II during mercerization has been suggested to be the result of a progressive shift of the sheets of cellulose chains within the crystallites of a microfibril from the quarter-staggered relationship in cellulose I to the complete correspondence found in cellulose II. Observed changes in lateral discnrder, cell dimensions, swellii, and AT-ray diffraction reflections of cellulose fibres support this theory. Such a shift may occur in the transformation of native celluloses with antiparallel structures as well as those with parallel polarity. 

The transformations now described correspond in part to the technical process of mercerization, which is expected to produce a cellulose lattice more accessible to water and other polar solvents than that of the cellulose I. The details of the intra- and intermolecular hydrogen bonding mechanism are, as in the case of cellulose I, essentially unknown. No theory has, to our knowledge, been able to give a satisfactory explanation of the cause for the irreversible chain tilting occurring during the allotropic transformation. 

Other processes of structural modification of cellulose, e.g., mercerization, treatment with amines or liquid ammonia, and regeneration from solutions, lead to a decrease in crystallinity, length, and lateral size of crystallites these treatments cause transformation of the initial cellulose structure into another structure in which small crystallites are surrounded by non-crystalline matrix [Fig. 7.28]. 

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