Singeing, desizing, scouring and bleaching are
routine preparation processes, whereas
mercerization is an optional step for cotton
fabrics.
It is mostly done for cotton fabrics only.
The fabric is usually singed before
mercerization.
It can either precede or follow bleaching.
It can be applied to either yarns or fabrics.
Most good quality cotton sewing threads are
mercerized to improve their strength.
It is inexpensive and permanent, and due to
this, widely used for cotton.
Also used for linen, some HWM rayon and
lyocell.
Mercerization is a process in which the cotton
material is treated with 25 to 30% NaOH
solution under tension at room temperature.
Tension during mercerization is essential to
improve the luster.
It produces a permanent change in the
structure of the fibre.
It causes the flat, twisted, ribbon-like cotton
fibre to swell into a round shape and to
contract in length.
The fibre becomes much more lustrous than
the original fibre.
Tensile strength is increased by as much as
20%.
The natural twist or convolutions are very
much reduced and this phenomenon is called
deconvolution due to mercerizing.
The diameter of the fibre is increased.
The absorbency (moisture regain is increased
from 8 to 11%) and reactivity of fibres are
increased.
The machines available for mercerizing are:
(a) Chain mercerizing
(b) Chainless mercerizing
In chain mercerizing machine, the cloth is
first allowed to shrink and subsequently it is
stretched.
In chainless mercerizing machine,
continuous tension is applied without
allowing the fabric to shrink.
The machine consists of two 3-bowl padding
mangles containing NaOH solution, first one
at 40°C and second at room temperature.
The time of reaction with alkali may vary from
1 to 1.5 minutes, depending on the speed of
the machine.
From the second padding mangle, the fabric
reach the stenter unit.
The function of the stenter is to pull the cloth
to its original width and then washed by
counter current principle.
The length of the stenter is 50 to 75 feet.
The stenter frames hold the selvages tightly.
The first 20 feet is used to stretch the cloth to
its original width and then washing takes
place.
Washing is done by counter current principle,
i.e., by means of decreasing the strength of
alkali to fresh water.
Fresh water is introduced at the delivery end
of the machine and the first wash liquor that
the cloth meets in the stenter is of significant
alkali concentration.
The washed alkali is used for kier boiling and
other purposes.
After washing, the cloth reaches the
recuperator.
The function of recuperator is to remove any
NaOH present in the fabric by the use of
steam.
It is a cast iron chamber and contains two sets
of guide rollers, the top row being driven.
Low pressure steam is admitted into the
chamber, which impinges on the fabric, gets
condensed and removes the alkali efficiently.
The fabric leaves the recuperator through the
nip of a pair of squeeze rollers.
Then the fabric is given a hot wash.
Souring treatment is given by using dilute HCl
or sulphuric acid, to neutralize the excess
alkali.
The disadvantages are:
(a) Only fabrics having certain width can be
mercerized.
(b) Stenter length is very long, so avail more
floor space.
In chainless mercerizer, the impregnation
section is very much the same as that of chain
mercerizer.
After impregnation, the fabric passes over a
stabilizing step where, the expander rollers
(bowed rollers made up of cast iron) give the
necessary tension in the weft direction
maintaining it at the maximum width.
The tension in warp is being controlled by
oscillating rollers which has an influence over
weft tension also.
Though satisfactory mercerization is obtained
more economically than chain machine, the
width of the cloth in this machine cannot be
controlled.
It cannot give high luster as the chain
machine because the tension applied is lower.
The advantage of this machine is the
possibility of mercerizing more than one
fabric at a time, by passing them side by side
or one over the other simultaneously into the
machine.
As fabrics of two different widths can be
mercerized together, the production is
increased.
No risk of fabric tearing near the clips as this
machine involves no clips like chain machine.
The mercerization is done with low tension
and weaker solution of alkali.
It can be used for producing stretch fabrics.
During slack mercerization, the yarns shrink
and develop a higher degree of elasticity.
The finished fabric can be stretched and when
the tension is removed, the fabric will return
to its original length.
