CHEMICAL PROPERTIES OF COTTON

CHEMICAL PROPERTIES OF COTTON

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The cotton fibre is an elongated cell, constructed from millions of cellulose molecules. Small amount of moisture, fatty materials, min are other constituents of cotton. So the chemical properties of c mostly influenced by the chemical characteristics of cellulose.

ACTION OF HEAT

Cotton fibre ignites easily and burns with an odour similar to that of It burns with a bright flame, which continues even after the fibre is removed from fire. After the flame has been extinguished, the fibre continues to smolder and smoke. This is a typical test of cellulose.

Cotton can be heated in a dry state to 150°C without any decomposition. But if heating continues, a slight brown discoloration can occur at temperatures lower than 150°C, which does not deteriorate the fibre, However, it is sufficient to spoil the effects of bleaching. So care should be taken to control the temperature of drying.

The temperature should not exceed more than 93°C. Prolonged own colour on cotton develops gradually. A exposure at high temperature to an atmosphere containing tendering due to the formation of oxycellulose causes

 At about 170°C, cotton begins to scorch even in a short time. If cotton is heated out of contact with air, the cotton cellulose molecules break down to form gaseous hydrocarbons, methyl alcohol, acetic acid and carbon dioxide .The mechanism of thermal degradation of cellulose may be assumed to include two main reactions.

One reaction consists of dehydration and other, scissions of C-O bond in the chain i.e., either in the rings or between the rings. The C-O bond is weaker than the C-C bond and so are more likely to be ruptured. Scission of C-O bond in the ring results in the disintegration of the ring as per the scheme Scissioning of the external C-O bond degrades the chain molecule with the formation of levoglucosan unit and another glucose unit with hydroxyl end.

ACTION OF LIGHT

Exposure to air in presence of sunlight for a long period will have an effect on cotton like that of heat. Oxycellulose is gradually formed accompanied by tendering because of atmospheric oxygen. The tendering effect by light and air is accelerated by traces of metals like copper.

ACTION OF WATER

Raw cotton is very hard to wet because the wax present on the surface of the fibre i.e., cuticle is difficult to wet. Wax can be removed by' scouring. So unscoured cotton will not absorb water so easily as scoured cotton Cold water swells cotton without any chemical damage. The swelling is accompanied by the disappearance of the natural twist i.e., deconvolution.

The irregular cross-section becomes more circular, which reappears on drying. Structurally, swelling is due to the intercrystalline areas, which means only amorphous regions are affected by swelling. Sea-water can sometimes degrade cellulose and form hydrocellulose.

ACTION OF ACIDS

1. Cold dilute solutions of mineral acids at boil have no effect on cotton cellulose, provided the acid are neutralized or washed out completely before drying. However, if traces of mineral acid like 0.01 % be allowed to dry in , tendering soon becomes apparent due to formation of hydrocellulose.

2. Boiling with dilute acids will ultimately hydrolyse cellulose to glucose. At low temperature, the action by acid is mild hydrocellulose forms

3. Cold concentrated sulphuric acid dissolves cellulose and forms cellulose hydrate. If this solution is poured in cold water, the cellulose hydrate is precipitated in a gelatinous form. This principle is used for parchmentising paper to give a transparency effect with hi Hydrochloric acid affects cotton much more severely than sulphüric Degradation is more rapid and severe in presence of hydhochilgric abid than sulphuric acid.

4. Nitric acid, on account of its oxidising action, differs from other acids in its behavior towards cellulose. Immersion for a short tintein concentrated nitric acid results partial shrinkage with higher tensile strength and affinity for dyestuffs. Prolonged action oxidizes cellulose to oxycellulose and ill breaks it down to oxalic acid. The reaction rate is higher at higher temperature. If nitric acid is allowed to dry in cotton, the material will tender on storage in a similar manner like that of other mineral acids.

ACTION OF ALKALIS

1. One of the main advantages of cotton is its resistance to alkali solutions.  Mild alkalis like sodium carbonate have no action on cotton in the absence of air either at low temperature or at high temperature. However, in presence of oxygen or air, oxycellulose is formed with gradual tendering of cotton.

2. On the other hand, the action of strong alkalis on cotton fibre is very interesting. Dilute solution of strong alkalis like sodium hydroxide with concentration of2 % -7 % can be boiled without least tendering in absence of air. Generally, dilute solution-of sodium hydroxide is used for scouring i.e., removal of waxy and other impurities from cotton fibre. The scouring process purifies cellulose and imparts hydrophillic character and permeability to cotton fibre.

In this range, the fibre will have moderate swelling depending upon concentration of alkali used

3. Strong alkalis with higher concentration induce structural and physical angers in cotton fibre. Sodium hydroxide as well as potassium hydroxide form different hydrated forms in association with water as shown in table 4.2. The diameter of these hydrated forms depend on the concentration of the alkali used . For small concentration of alkali i.e. , less than 5 % , the diameter of hydrated ions is too large. So it cannot penetrate into the structure of cotton. As the concentration of alkali increases, the number of water molecules per molecule of alkali decreases for the formation of smaller hydrates. Thus the diameter of the hydrated form of alkali decreases.