Mercerizing | Object of Mercerizing | Mercerizing Treatment on Cellulosic Materials
A treatment of cotton yarn or fabric to increase its luster and affinity for dyes. The material is immersed under tension in a cold sodium hydroxide (caustic soda) solution in warp or skein form or in the piece, and is later neutralized in acid. The process causes a permanent swelling of the fiber and thus increases its luster.It is the process of treatment of cellulosic material with cold or hot caustic conditions under specific conditions to improve its appearance and physical as well as chemical properties.
Purpose of mercerizing
1.To improve the lusture
2.To improve the strength
3. To improve the dye uptake and moisture regain.
What is the mercerizing process
The mercerizing involves these three subsequent steps,
a. Impregnation of the material in in relaxed state,cold caustic solution of required strength and wettability..
b. Stretching while the material is still impregnated in the caustic solution.
c. Washing off the caustic soda from the material while keeping the material still in the stretch state.
Fig.The material flow chart in yarn mercerizing
            Theory of mercerizing
Neales theory in Broad
The hydroxyl groups on long cellulose chains attract the water molecules when cellulose takes up water, the structure expands transversily as some of the mutual secondary valency linkages are replaced by water hydroxyl linkages.Now in alkali solution ,some of the hydroxyl hydrogen atoms are replaced by sodium atoms and a system of high ionic concentration is established.Owing to osmosis ,water tends to enter this system and more secondary linkages are broken and are 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 and 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 ,is due to the formation of new secondary linkages on account of greater amplitude of thermal vibrations of hydroxyl groups as suggested by Urqhart.
Changes during mercerizing process
A. Fiber level
1.Swelling
2.Cross sectional morphology changes from beam shape to round shape.
3.Shrinkage along with longitudinal direction.
B. Molecular level
1.Hydrogen bond readjustment
2.Orientation (parallelization) of molecular chains in amorphous region along the direction of fiber length.
3.Orientation of the crystallinity in the direction of the fiber length.
4.Increased crystallinity
C. Chemical Changes
1.Increased rate of reaction on hydrolysis and oxidation
2.Liberation of heat during the caustic treatment.(heat of sorption and heat of reaction)
3.Increase in the alkali absorption.
4.Increase in the absorption of iodine.
Q.Why there is a swelling in cellulose?
1.Due to Swelling
Swelling is due to molecular attraction with associated hydration in this instance.
Since the alkali cellulose is more hydrated than native cellulose ,maximum swelling concentration is result of attraction of alkali cellulose in solution on one hand and free alkali on the other. Hydration of the cellulose increases with increased fixation of alkali in solution of rinsing concentration upto a certain limit, after which the free alkali exerts a dehydrating effect on alkali cellulose to a greater extent.
Dissociation of alkali ions from the alkali cellulose compound corresponds to an absorption of OH- ions ,and in this manner a negative charge results.The cellulose molecules repel each other and absorb water ,this absorption being greater, the greater is the charge. If however the dissociation of alkali cellulose salt is forced back ,then there is reduction in the charge. Further if the concentration of the electrolyte is high in swelling liquor ,then charge of cellulose particles is shielded by free ions and force of repulsion is diminished.
2.Due to osmotic phenomenon.
The fiber is surrounded by a hardened euticle which acts as a dialyzing membrane to induce osmotic action ,which is better explained with the help of Neales theory
Q.Why there is a rise in temperature of caustic solution during mercerization?
Ans ;-Because when cellulose is treated with a moderately concentrated caustic solution ,the heat is evolved due to heat of sorption and heat of reaction.
Q.Why there is increase in luster of yarn after mercerization?
Ans:-The cotton hair swells in strong caustic soda solution ,which change its cross section from squashed circular pipe shape to an oval shaped . if the fibers are placed under a tension or stretched position in the swollen state and then washed to reduce the caustic concentration below a particular limit, then there is an increase in the luster of the fiber.
Q.What should be caustic concentration for yarn mercerization?
Ans:-Although mercerization can be done with the caustic concentration ranging between 150Be to 380Be, As for as the economy of the process and optimum mercerizing effect in yarn , the concentration of the caustic used is 250Be(225 gpl) to 32 0Be(325 gpl) at room temperature.
Q. What shall be the ideal temperature for mercerization?
Ans:- The relation between the shrinkage of cotton fiber, the concentration of alkali and the processing temperature was studied by H. Flecken, and the result is shown in this Figure.
He treated cotton yarn at 0°C to 40°C, using 30°/38° Bé caustic soda and measured the shrinkage by mercerization, and found that the least shrinkage variation is obtained at 10°C and 30° Bé. Therefore, robust conditions insensitive to the concentration of alkali and temperature must be selected for stable mercerization.
Q.Whether the recovered caustic from mercerizing process is reusable?
Ans:-The caustic soda from the washing process is recoverable and can be reusable ,but it has to be free from the contaminants and must be purified to remove the undesirable impurities.
Q.What is the mercerizing cycle for yarn mercerizing?
A typical mercerizing cycle in yarn mercerizing is having following steps,
1. Loading of material on the rollers in rest position.
2. Lowering of rollers
3. Caustic tray movement
4. Pre tensioning
5. Free shrinkage
6. Lye tensioning
7. Squeezing
8. Washing tray movement
9. First wash and tensioning
10. Second wash
11. Third wash
12. Final wash
13. Squeezing
14. Lifting Of Rollers
15. Unloading the hanks.
What is the significance of different steps in mercerizing cycle?
1.Pre tensioning
This is the first step in yarn mercerizing cycle ,which runs to uniformly distribute the greig yarn hanks on the rollers in combination with reversal of direction of motion of rollers, without any entanglement. The material should become completely wet in this step.
2.Shrinking
This is the actual caustic treatment step or the mercerizing step in which the yarn is allowed to shrink freely , the yarn runs for sufficient time in relaxed state to react completely with the caustic , higher the shrinkage achieved better will be the mercerizing.
3. Lye tensioning
The lye tensioning is done to stretch the yarn back to original length.
4.Squeezing
To remove the unbound caustic solution from the material, so that the material can be washed effectively, and quickly to reduce the caustic content . This step also ensure the to minimize the wastage of excess caustic liquor during washing.
5. Washing and tensioning
The stretch applied to get the maximum luster , material is washed along with stretching beyond its original length produce better luster. The amount of stretch applied depends upon the luster required and quality of yarn. The washing temperature is kept near boil to make washing efficient and short. Time or sequence of washing is so adjusted to reach a residual caustic content below 10% within a shortest possible time.
6.Final caustic content
The final caustic content should be below 10% ,because if it is more it will be sufficient to carry out further mercerizing effect and the material will shrink back. During the storage of mercerized material , the water from the exposed areas will evaporate and got concentrated locally causing mercerizing effect ,which is called local mercerizing and will lead to patchy dyeing.
The level of residual caustic in the yarn is achieved less than 3% , if it is on the higher side ,then the washing is not effective.
