Alkali cellulose transition

For rayon production, a controlled cleavage of the cellulose molecule is required to reduce the viscosity of the cellulose xanthate so that it can be forced through fine spinnerets. A similar reduction of viscosity is required for cellulose which will be sprayed as a lacquer. This cleavage is brought about by the air oxidation of alkali cellulose, and the process is speeded by the addition of a few parts per million of a transition metal compound which functions as an oxidation catalyst. 

It has been established that changes can occur in the cellulose physical structure, using x-ray diffraction [132-134] and infrared spectroscopy [135,136] by several workers. Nine allotropes of alkali cellulose have been identified. Sobue [134] has described some of the possible transitions outlined in Figure 10.54 that are related to the alkalinity and temperature. 

Once dehydrated, the microfibrils are practically without functionality in ordinary food processing and preparation operations, because the inert microcrystallites are difficult for water to penetrate. The polymorphs, cellulose I and II (Blackwell, 1982 Coffey el al., 1995), are differentiated by their molecular orientation, hydrogen-bonding patterns, and unit-cell structure. Cellulose I is the natural orientation cellulose II results from NaOH treatment under tension of cellulose I with 18-45% alkali (mercerization). The I—II transition is irreversible. Mercerization strengthens the fibers and improves their lustre and affinity for dyes (Sisson, 1943). Sewing thread was relatively pure mercerized cotton until the advent of synthetic polymer fibers. 

Fig. 9-3. Transition of cellulose I (/r = 0) to cellulose II (/r = 1) during cold alkali treatment of wood cellulose (W), cotton cellulose (C), bacterial cellulose (B), and animal cellulose (A) (RSnby, 1952).

The alkali-resistant material from the cyst walls of the green alga Acetahularia mediterranea was studied by X-ray and other physical methods. The predominant, structural polysaccharide was found to be cellulose I. The stalk and cap walls of the alga contain poly[(l—>4)-/3-D-Manp]. It is possible that the change from mannan to cellulose in Acetahularia is paralleled by a transition from the diploid to the haploid stage. 

The transition from cellulose I to cellulose II is irreversible cellulose II is the thermodynamically more stable arrangement. In contrast to cellulose I the chains do not point in the same direction as seen in Figure 3.4, bottom. The corner chain points upwards whereas the centre chain points downwards. This poses some difficulties in explaining the solid-state phase transition from natural cellulose to cellulose II after alkali treatment and thus an interdiffusion model was proposed for chains from adjacent natural cellulose crystallites of opposite orientation [19]. 

Uses Multifunctional prod, for pretreatment of cellulosics and its blends in exhaust applies. peroxide stabilizer to protect goods from alkali earth and transition metal catalytic damage rapid wetting agent wax/oil emulsifier disperses impurities in goods reduces scale deposition on equip. 

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