Cellulose

2.1.8 Cellulose

Cellulose represents one of the most widely distributed and abundantly available organic matter on this planet. It is, in-fact, the most important structural element of higher-plant-cell walls. In nature, wood (40-50% cellulose) caters as the major source of cellulose for industrial utilities, whereas cotton (98% cellulose) provides the balance requirement globally.

Geographical Source It has been observed that nearly thirty billion MT of carbon is transformed annually into organic compounds by higher plants and out of this approximately 1/3rd is converted into cellulose. As cellulose is profusely utilized in the form of wood to build houses, paper industry and textile industry, a considerable amount of research has been duly conducted on this well-known polysaccharide.

Preparation The scientific and large-scale methods for preparing cellulose essentially involves the removal of excess of the non-cellulose substances e.g. Lignin. In fact, there are three well defined and established procedures whereby the undesired ‘lignin content’ present in wood shavings

are removed exhaustively, namely:

(a) Treatment with Sodium Bisulphite [Sulphonate Process]: The small wood chips are boiled with a solution of sodium bisulphite whereupon the lignin is removed as lignosulphonate,

(b) Treatment with Sodium Hydroxide [Soda Process]: The wood chips on being boiled with sodium hydroxide solution removes the lignin content as soluble products, and

(c) Treatment with NaOH and Na₂SO₄ [Sulphate Process]: The sodium sulphide (Na₂S) obtained by the interaction of NaOH and Na₂SO₄ will remove most of the lignin component from the wood shavings.

However, the traces of lignin may be removed by bleaching with chlorine. The remaining mixture of hemicellulose and cellulose are subsequently extracted by subjecting it to alkaline treatment. The readily soluble hemicellulose are removed by treatment with higher concentration of NaOH solution, whereas the cellusans (Xylans) may be removed by treatment with a 5% solution of NaOH.

Description Cellulose has molecular weights ranging from 250,000 to 1,000,000 or even more. It is assumed that at least 1500 glucose units may be present in each molecule. Based on the findings by X-ray analysis and electron microscopy it is revealed that these long chains lie side by-side in bundles, held together by H-bonds available between the huge number of adjoining –OH moieties.

Further, these bundles are twisted together to give rise to rope-like structures, that ultimately are clubbed together to yield the normal apparently visible fibers. Interestingly, in the case of wood

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* HPLC: High performance liquid chromatography.

these cellulose “ropes” are meticulously embedded in lignin to afford a structure that resembles to concrete reinforced structures used for making buildings.

Chemical Constituents Cellulose is comprised of chains of D-glucose units, whereby each unit is joined by a glycosidic linkage to C-4 of the next unit.

Cellulose Cellulose derived from various sources and also from different modes of preparations usually display great differences not only in their mean chain length but also in their degree of homogenity. Generally, the cellulose that are distinctly more homogenous are the most suitable for industrial utilities.

Uses

1. The viscose when forced through a spinnerette into an acid-bath, it gives rise to the generation of cellulose as fine filaments that yield threads of a substance termed as RAYON.

2. Cellulose undergoes an analogous reaction to produce cellulose xanthate, that is made to dissolve in alkali to yield a viscous colloidal dispersion known as VISCOSE.

3. Methyl, ethyl and benzyl ethers of cellulose are proved to be important in the commercial production of films, textiles and various types plastic materials.

Source: Pharmacognosy And Pharmacobiotechnology By Ashutosh Kar