Mercerization
is a process of impregnating the textile material with a concentrated
solution of cold NaOH, keeping it in contact with this cold solution for a
given time with or without tension, and subsequently rinsing it.
MECHANISM-
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Mercerization
causes swelling in fibre
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Swelling
causes Cross-section to become rounder, loss of convolutions &
detwisting leading to more lustrous surface
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Opening
of fibre structure
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Increase
in amorphous content due to decrystallization
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Although
mercerization is accompanied by swelling, it is more closely related to
internal modification of fiber.
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Effect
is observed at 18-24 % w/w NaOH solutions.
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Higher
no of –OH groups available as compared to un-mercerized cotton
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Higher
moisture regain, dye-uptake and reactivity
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The
effect of mercerization is related to the size of the sodium hydroxide
hydrates (NaOH. xH2O). The concentration of NaOH solution
determines the value of x or the size of the hydrate. A dilute solution has
larger sized hydrates, which can cause swelling in cotton fibre by entering
the amorphous phase of the fibre. At higher concentrations, the hydrate
volume is smaller. Such hydrates cause lower swelling but have the ability to
penetrate the crystalline phase and change the crystal structure. Such
changes are capable of inducing important structural modifications in fibre
structure.
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In
the table given below, the composition of NaOH hydrate, corresponding NaOH
concentration and the type of hydrate formation are shown:
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What
are Cellulose I and Cellulose II?
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The
native form of cellulose which occurs in cotton and other natural cellulosic
fibres, is known as cellulose I. It has it unique crystal diffraction pattern
(monoclinic). It is a thermodynamically less stable form of cellulose. When
cellulose in converted to regenerated cellulosic fibres like viscose,
dissolution of cellulose is an intermediate step. During regeneration, it
gets converted to a different crystalline form, known as Cellulose II.
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During
mercerization also, some native cellulose gets converted to cellulose II. The
extent of this conversion depends on process conditions like temperature,
tension, NaOH concentration etc.
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| Mercerization |
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Structural
Modification
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Effect
of NaOH Concentration
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It
has been discussed that mercerization leads to conversion of cellulose I to
cellulose II. Native cellulose is Cellulose I and regenerated cellulose
(example – viscose) is cellulose II. However, during mercerization,
conversion of cellulose I to cellulose II is only partial. Hence mercerized
cotton is a combination of both crystalline forms. The extent of conversion
is dependent on various process parameters like concentration of NaOH, time
of treatment, temperature and tension employed in the process.
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In
the figure given below, the effect of NaOH concentration on crystallinity of
cotton is given (time 60 sec, temp 20 °C, tension- to maintain length
change=0%)
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| Mercerization | |
Effect of
temperature
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Temperature
seems to have a similar effect on conversion of cellulose I to cellulose II.
Overall crystallinity also goes down. This is shown in the figure below:
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| Mercerization | |
Enhancement
in lustre
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Cotton
owes its non lustrous appearance to its non uniform twisted, convoluted, bean
shaped fire cross section. As there is continuous change in the fibre cross
section along its length, it has poor specular reflection which is
responsible for high lustre. On mercerization, the fibre swells gradually and
the false twist and the convolutions gradually disappear. The cross section
becomes almost circular and the fibre acquires the shape of a regular uniform
cylinder. This process of gradual change in the fibre cross section is
depicted in figure below:
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| Enhasnce in lustre |
In the
above figure, stage 1-5 show swelling of cross section of fibre. It is easy
to see conversion of a bean shaped flat cross section to almost round shape
and gradual disappearance of lumen. Stage 6 and 7 are associated with removal
of NaOH from fibre on washing and drying. It is important to note that on
loss of sodium hydroxide during washing and subsequent drying, the fibre
shrinks in cross sectional area, but maintains its rounder shape. Overall,
there is very little gain in fibre volume due to mercerization, and hence the
density of the fibre remains unaffected.
Mechanical
properties
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Mercerization
has been found to increase the tensile strength of cotton fibres even though
a drop in overall crystallinity is recorded. The main reason for this
phenomenon to occur is swelling of fibres. The presence of many convolutions
and false twists in the fibres cause weak spots to be present. These weak
spots act as zones of stress concentrations due to lower cross sectional area
on application of a tensile stress. Swelling, which is an integral part of
mercerization process, removes convolutions and false twists and the fibres
acquire a more uniform cross section along its length. With weak spots gone,
the fibres exhibit enhanced tensile strength as compared to a non-mercerized
fibre.
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Increase
in tensile strength can be summarized as:
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- Removal
of convolutions results in removal of weak spots at the point of reversal
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- Fibres
have more uniform, circular and smoother cross section after
mercerization
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- The
fibrills are arranged parallel along the fibre axis
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- Fibre
alignment along fibre axis is better in case of tension mercerization as
compared to slack mercerization
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Effect
of tension
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Mercerization
has a significant effect on mechanical properties of cotton. In the figure
given below, tenacity, breaking elongation and Young’s modulus of cotton
mercerized with 300 gpl NaOH solution at 20 °C for 60 sec are plotted. The
tension is varied to allow 10% shrinkage to 4% stretch. The elongation and
modulus are indicated on secondary vertical axis. It is clear that the
enhancement in both tenacity and modulus is higher at higher tension. It is
easy to visualize this since it results in better alignment of molecular
chains in fibre axis direction (higher orientation). However, in case of
elongation, it is reverse. With increasing tension, the elongation at break
decreases monotonically. It is to be expected as increasing orientation would
lead to lower elongation. At negative tension though (shrinkage occurring)
the breaking elongation is higher than that of unmercerized cotton.
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Hence
it can be concluded that if improving the tenacity of cotton yarn is the main
objective, mercerization should be carried out under tension.
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Effect
of Alkali concentration
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The
effect of alkali concentration on tenacity, breaking elongation and Young’s
modulus of cotton mercerized with NaOH solution of variable concentration at
20 °C for 60 sec at constant length are plotted in the figure given below.
The alkali concentration is varied from 100 to 300 gpl. Again, the elongation
and modulus are indicated on secondary vertical axis.
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Machines
for Mercerization:
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Fabric
Mercerization with Tension
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1.
Stenter type or chain mercerizing machine
2.
Chainless mercerizing machine
3.
Chainless and pad less mercerizing machine
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