Properties Changes of Cellulose Due to Mercerization

Introduction:
Cellulose undergoes chemical, physico-chemical and structural modifications
on treatment with caustic soda solution of mercerizing strength. Chemical reactions lead to the formation of alkali cellulose, physical reactions, to a change in arrangement of units of cellulose. The optimum modifications in the properties of cellulose can be manipulated with the selection of proper concentration of alkali, time, tension and temperature during the mercerizing process.

Swelling and shrinkage of cellulose:
When cellulose is immersed in a solution of caustic soda of mercerizing strength, water and alkali diffuses in and the material swells. The fibre hair quickly commences to untwist from its twisted ribbon like form and tends to become cylindrical rodlike surface due to deconvolution. The cross -section of the fibre diminishes, the diameter of the fibre becomes more round. The surface of the nearly cylindrical cotton fibre after mercerizing reflects light more evenly to all sides than the kidney shaped cotton fibre and the fibre surface becomes more lustrous. As the fibre swells, the fibre shrinks in length. Swelling and shrinkage are more when there is no tension in the fibre, but the alteration in cross-section caused by swelling is more when mercerization is carried out under tension. Under optimum conditions each cotton fibre may contract nearly 9% in length and swell nearly 150%. Swelling of the cotton fibre also has a disadvantage. The fibre becomes more compact in its swollen condition. This compacting diminishes the further penetration of caustic soda into the fibre i.e. penetration slows down and mercerization in the fiber's core is lower than on its surface.
Changes of Cellulose Due to Mercerization
Changes of Cellulose Due to Mercerization
Structural modification:
Due to swelling of cellulose in caustic soda solution of mercerization strength, many hydrogen bonds are broken, the plane of molecular chains have been moved apart, molecular structure tends to become decrystallised, the chains or spaces within the cellulose structure become more uniform and the chains of glucose residues have been given a slight twist. Because of the distortion of polymer network and changes in crystalline structure, the process of mercerization is irreversible. Mercerization also affects the size of the crystallites and orientation of the crystalline region and the extent of orientation depends on the tension during mercerization.

Increased luster:
Unmercerized cotton has a general appearance of a flat ribbon with spiral twists, its surface is rough and non-uniform, its cross-section is irregular and ear-shaped while the lumen, the central canal, is broad, irregular and resembles a collapsed tube. All these factors result in less lustre. When a cotton hair is brought into an aqueous solution of sodium hydroxide of 18% (40~ cellulose begins to swell immediately, the hair is elliptical in section in a few seconds, and on further swelling becomes circular and the lumen is practically eliminated. The untwisting of the fibre takes place under effect of swelling and increased alignment and packing of the fibres in the yam also take place.

Lustre also depends on other factors. Mercerization reduces the axial ratio and increase the light scattering within the fibre (transparency) and thus increases the lustre.

Gain in strength:
Mercerization, both slack and with tension, increases the strength uniformity along the fibre length, but mercerized fibre with tension shows greater gain in strength than that of without tension. In practice, the improvements in strength are noticed mostly upon yarn treatments, with fabric the major effect is on the surface only. Mercerization increases the tensile strength of cotton fibres by eliminating the weakest points in the fibre. Mercerization increases the cohesion between individual cotton hairs and this closer embedding of the hairs in the yam not only increases the strength but makes it more uniform in strength and less in diameter.

Increased moisture absorption:
Mercerized celluloses absorb more water, have higher regains and more easily wet out than unmercerized fibres. Due to caustic soda penetration, many hydrogen bonds are broken and it is estimated that the number of available, hydroxyl groups are increased by about 25%. Mercerization, thus decreases the amount of crystalline part or increases the amorphous content of the fibre. This increase in the proportion of amorphous part is directly related to the moisture sorption. Moisture is assumed to be absorbed by suitable groups in the amorphous region and on the surface of the crystallites. When mercerization is carried out under tension, the changes in crystalline portion is comparatively lower than that without tension and hence also the moisture sorption. Standard cotton has moisture content of about 7%, mercerized cotton with tension has about 9% and that of without tension about 11%.

Increased dye adsorption:
Mercerized cotton shows increased depth of shade, increased rate of dyeing and the irregularities due to neps and unripe cotton are less prominent. Generally immature cotton with large lumen responds particularly well to increased light scattering and hence decreased dye uptake.

Mercerization thus lowers the dye costs, savings at 2% depth averaged 40%, while at 6% with the same dyes the average saving is about 60%. Cotton can be causticized or half-mercerized to increase the dye uptake and economics up to 25% may be realised.

Increased reactivity:
The reactivity of mercerized cotton is increased by about 1 I/2 times at lower temperature in comparison to that of unmercerized cotton. The increased reactivity is not so marked when mercerization is carried out under tension. The reactivity ratio is generally referred to as the ratio of copper number of the mercerized to unmercerized samples. The increased reactivity increases dye absorption, moisture absorption and chemical reaction, but at the same time it also accelerates the reaction with acids and oxidising agents and is susceptible to degradation.

Removal of immature cotton:

Mercerization has been recognized as a method for removing immature (dead) fibres to obtain level dyeing effect on cotton fabrics. The dead fibres are underdeveloped and appear as flat or slightly twisted tapes. They are non-crystalline, convolutions are sometimes absent, cell walls are extremely thin and the lumen is collapsed and hence do not contain dye to same extent as matured fibres.

Physical compactness:

Mercerization improves dimensional stability of cotton woven fabrics. When knitted fabrics are compared with respect to their relative openness, temperature increases can be said to improve mercerization because when the goods are bleached and then mercerized, the fabric becomes more dense. However, when unbleached fabrics are mercerized, the fabrics become more open. Mercerization also gives moderate improvement in crease recovery of cotton fabrics as well as some protection against the decrease in tensile strength caused by easy-care finishing.

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