Mercerization:
Mercerization is a finishing treatment of cotton with a strong caustic alkaline solution in order to improve the luster, increase affinity for dyes. John mercer 1st use this system, so this process is called mercerization. It is done in a concentrated solution of caustic soda (300 g/l), under tension and ambient temperature. Flow Chart of Yarn Mercerizing Process is as follows:
Flow Chart of Yarn Mercerizing Process:
Yarn
↓
Singeing
↓
Reeling (Hank making)
↓
Mercerizing
↓
Bleaching
↓
Dyeing
↓
Hydroextraction
↓
Drying
↓
Hank yarn
↓
Hank to cone
If we specify only mercerizing process then we can arrange the process in the following way:
Feed the hank
↓
Caustic impregnation for 3 min 45 sec (With no stretch)
↓
Hot wash at 80ºC for 60 sec (With stretch)
↓
Hot wash at 60ºC for 45 sec (With stretch)
↓
Cold wash for 60 sec (With stretch)
↓
Pull up
- To improve the lusture.
- To-improve the strength.
- To improve the dye uptake and moisture regain.
Process of mercerizing:
Hydroxyl groups on long cellulose chains attract the water molecules. When cellulose takes up H2O2 (water), the structure expands its transversily as some of the mutual secondary valency linkages. It’s replaced by H2O2 (water) in hydroxyl linkages. Now in alkali solution, some of the hydroxyl hydrogen atoms will replace by sodium atoms. A system of high ionic concentration will be established.
Owing to osmosis, H2O2 (water) tends to enter this system and more secondary linkages are broken. It’s replaced by linkages with alkali in water. When the alkali cellulaose is washed the sodium ions and the hydroxyl ions are diffused away. And the osmotic pressure falls, the cellulose gel contracts by virtue of its elasticity.
During this contraction, hydroxyl–hydroxyl linkages are reform. But not in such a great number as before. The orientation of the micells is more random. The greater proportions of free hydroxyl groups accounts for the increase in the reactivity and absorptive capacity. The decrease in this high absorptive capacity on drying, particularly at high temperature. It due to the formation of new secondary linkages on account of greater amplitude of thermal vibrations of hydroxyl groups as suggested by Urqhart.
Author of this Article: Rana Sohel Executive Engineer Sunman Textile Mills Ltd. Chittagong Cell: +8801912-420118 Email: rsohel07tex@gmail.com