Cellulosic fibres, crystallinity

It is more difficult to control the enzymatic processing of wool. Hence there is a greater danger of fibre damage compared with cellulosic fibres. Since cellulose is a highly crystalline... 

As water swells cellulose in an intercrystalline way (i.e. only within the non-crystalline amorphous regions), a relationship would be expected between accessibility and moisture uptake, and this is indeed found (Figure 5.5). Refining causes cellulosic fibres to swell and it would therefore be expected to cause a change in the water adsorption isotherm. This is indeed observed (Figure 5.6). 

A similar theory was published by Frey-Wyssling [37] who suggested a scheme composed of two projections outlined in Fig. 79, where the crystalline phases of the cellulose fibre are marked as dotted line rectangles. 

At the present moment it is difficult to decide which of the two hypotheses concerning the structure of cellulose is correct the idea of an amorpho-crystalline structure, or that postulating solely an amorphous texture. Nikitin assumes that the first hypothesis is the more probable, more especially as it is well in line with the most recent work of Zaydes and Sinitskaya [45] who conclude on the basis of electron diffraction investigations that in the natural cellulose fibre of Chinese nettle, there exist phases having a distinct microcrystalline structure. This suggests that structures shown in Figs. 78, 79 and 80 are the most probable. 

Immedial Pure Blue (25) forms a crystalline complex with sodium bisulfite and the thiazine ring structure has been clearly established (48AG141). The compound is isosteric with a number of vat dyes, e.g. indanthrone, and thus its hue and good affinity for cellulosic fibres might reasonably be expected. 

Deactivation of cellulases after the desired effects have been achieved is very important. If the enzyme is not completely removed from the fabric, or is not effectively deactivated, the hydrolysis reaction will continue, although at a slower rate. As very large molecules, cellulases cannot diffuse into the crystalline parts of the cellulose fibres. They react on the fibre surface, so fibre damage takes time. But eventually enough hydrolysis will have taken place to weaken the affected fabrics or garments, leading to customer complaints and returns. 

Polymer crystallization may also complicate the transition from the amorphous state to the intermediate liquid crystalhne state. Even small crystalline fractions in a polymer prevent free movement of macromolecules This is the case, for example, with cellulose. Cellulose fibres, obtained via cellulose xanthate (viscose), in the process of forming partially crystallize. Although the amorphous fraction in these fibres is large (up to 70-75 %), at a short-term heating of the fibres above the glass-transition temperature (240-260 °C) only slight self-elongation of fibres is observed, which can be attributed to the transition to the liquid crystalline state at the expense of the amorphous fraction... 

Hermans, P.H. Weidinger, A. Quantitative X-ray investigation on the crystallinity of cellulose fibres. A background analysis. J. Appl. Phys. 1948,19, 491-506. 

In the crystalline part, the cellobiose units are closely packed to form Cellulose I in native cellulose fibres and Cellulose II in regenerated cellulose fibres. In Cellulose I the chain molecules are parallel to one another [16]. The folded chain occurs at Cellulose II, in the crystalline regions the chain molecules are antiparallel. Thus, the basis for helical structure for Cellulose I is preferably extended to the structure of Cellulose II [17]. 

Figure 9-4. Influence of NaOH concentration on the crystalline structure of cellulose fibres [27].

Viscose rayon is inherently a weak fibre, particularly when wet, therefore it is highly susceptible to damage if enzymatic hydrolysis is not controlled. The enzymatic hydrolysis of viscose fibres causes a decrease of the intrinsic viscosity from 250 to 140 ml/g and an increase in crystallinity from 29 to 39% after 44 h [34]. Strong changes of the structure, however, are not typical for the enzymatic hydrolysis of cellulosic materials. Neither cotton nor wood pulp show an essential decrease of the DP during enzymatic hydrolysis [35-37]. The kinetics of the enzymatic hydrolysis of regenerated cellulose fibres before and after acid prehydrolysis changes the kinetics from a monophasic to a biphasic first order reaction [38]. 

As a result of these investigations it is generally agreed that naturally-occurring cellulosic fibres contain of the order of 60 to 70 per cent of molecules orientated in crystalline structure. The regenerated celluloses contain 30 to 40 per cent, Terylene 50 per cent, and nylon between 50 and 60 per cent. 

Dyeings of very good fastness can be obtained in a one-bath method. The polyamide fibre is dyed with a procinyl dye and the cellulosic fibre with a monochlorotriazinyl dyestuff. The dyeing is carried out forone hour at a temperature not exceeding 85°C (185°F) in a liquor containing 100 g/1 of common salt. After one hour trisodium phosphate (crystalline) equivalent to 15 g/1 is added to bring about the fixation of the reactive dyes. 

There is in fact no single compound holocellulose, since the structures and crystallinity of the cellulose fibres vary with the source of the wood. This is part of the reason for the variation in properties of pulps. 

Cellulose fibres are crystalline and very strong materials when they are dry. However, they are hydrophilic and in the presence of moisture they absorb water, becoming permeable by microorganisms. For this reason paper became much less important as a food packaging material when the cheap hydrophobic synthetic polymers emerged in the second half of the 20th century. 

The mercerizing is based on the action of cold strong alkali (usually NaOH) solution and concomitant application of a stretching force, followed by neutralization. As is shown in Figure 9.6.4, this allows cellulose fibre to pass from the meta-stable natural crystalline structure I to the stable cellulose II form. The mercerization produces effluents with very high pH, requiring further acidic treatment for discarding. 

On the other hand, Hess and coworkers have succeeded in destroying the crystalline order in cellulose fibres by mechanical treatment. (Complete disappearance of the characteristic X-ray interferences). As a result of the treatment, sorptive power and integral heat of sorption increase considerably . 

Applied to cellulose fibres of different kind, it appeared, however, that equation (40) still held, but that equation (41) fails. The reason of this must be that the composition of the various fibres, though all consisting of cellulose, is not the same Native fibres have a greater percentage of crystalline cellulose than regenerated ones (about 60%... 

In systems like cellulose fibres, extended rubber, polyamide fibres etc, the double refraction may be considered as being due to a superposition of the birefringence of the amorphous and that of the crystalline components. 

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