Mercerization
The
process of mercerisation was developed by John Mercer (UK). He found that when
a cotton fabric is treated with 25% NaOH solution then there occurs shrinkage
in the fabric thereby becoming denser or fuller. Later on it was found that
when done in stretched form, it imparts high lusture to the fabric. Also it
adds to the tensile strength of the fabric. Ideally for mercerisation we use
25% solution of NaOH for 40-45 seconds. Furthermore, we generally express the
concentration of a solution in terms of °Tw (Twedell). The °Tw is measured by
the twedell meter. 25% NaOH ~ 50-52°Tw
and 17% NaOH ~ 35°Tw
 |
| Mercerized Yarn |
Caustisization: Caustisization is the treatment the cotton fabric with 15-17% NaOH
(35°Tw) solution. This process increases the dyeability of the fabric but does
not add to its lusture.
Process of mercerization: Cotton fabric in stretched
form is passed through 25% NaOH solution and after a time of 40-45 seconds
fabric is washed and neutralised. The washed off NaOH is then passed to the CRP
(Caustic Recovery Plant) for recycling of caustic.
Stages of
Mercerization:
The process of mercerization can be done at any
stage of process sequence. Some of them are:
A) Grey stage: when the grey fabric is mercerized some desizing and scouring takes place
but we have some major disadvantages:
1. The
mercerization is not optimum reason being the grey fabric lacks in the
hydrophilicity. Hence along with caustic we have to add some wetting agent. But
all wetting agents are not stable to 25% NaOH. Hence we usually go for alkali
stable wetting agent.
2. Also
the mercerization is not even due to differential take up of warp and weft
yarns due to sizing.
3. The
recovery of caustic is not easy on the grey fabric as it is full of impurities
which go in to NaOH solution.
B) Desized
stage: This stage of mercerization also has similar type of disadvantages as
grey stage due to same reasons. Some advantages of this mercerization over grey
stage are:
1.
The
recovery of caustic is some easier as impurities to some extent are removed by
desizing.
2.
The
residual alkali may directly used for scouring
C)
Scoured/Bleached stage: These two stages give very
good lusture property and mercerized effect as the degree of hydophilicity is
high. Hence this is generally preferred.
Effect of Mercerization
The following are the
effect of the mercerization.
Swelling: During the process of scouring there occurs a lateral swelling of the
fibres. The fibres get swelled up due to the preferential uptake of the NaOH.
The cross-section before mercerization is elliptical while it becomes
relatively circular when mercerised. |
| Swelling of Fiber |
Lusture Improvement: Since by
mercerization the fibre cross- section becomes more circular. Hence the fibre
surface becomes more even and circular which gives a better reflection of light.
Hence a gain in lusture.
Increase in tensile strength: The lumen is the
hollow part of fibre. When mercerized the walls of the lumen come closed to
such an extent that sometimes the lumen may even disappears. Due to thickening
of the fibre lumen the strength increases. Hence the increase in strength takes
place due to internal swelling.
Test for efficiency of Mercerization
1. Barium activity number: this test depends
upon the amount of Ba(OH)2 absorbed by the mercerised fabric. The
absorption of Ba(OH)2 by mercerized fabric is more than the
unmercerized fabric.
Barium Activity
No. = Amount of Ba(OH)2
absorbed by Mercerized fabric ×100
Amount of
Ba(OH)2 absorbed by unmercerized fabric
Ideally
barium activity number should be 120-130 for a bleached fabric.
2. Contrast Ratio (Lusture): In this test light is
incident on the mercerised sample at 45° and 90° and the amount of the light
reflected is then compared. The angle of reflection will be 45°, only if the
surface is smooth i.e properly mercerized.
Contrast Ratio
= Light reflected at 45°
Light reflected at
90°
Ideally,
it should be between 1.2-2.4 for fabric and should be more than 2 for yarn.
3. Axial Ratio: We know that an
unmercerized cotton fibre have an elliptical cross-section, while it becomes
relatively circular in mercerized fibre. The axial ratio decrease when a cotton
sample is mercerised. This test is done by using a microscope.
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