The production process of denim given in the following link gives the general idea of the process, the series of blogs like this give the brief explanation of the process and machines. Here in this blog we will go forward to next and important step, which is Dyeing Process:https://bindaprocess.blogspot.com/2022/02/denim-production-process.html
Introduction
With the warp
yarns dyed using blue indigo dyes and
the filling yarns being left undyed for the special denim appearence.
There are a number of alternatives for
denim dyeing that are routinely used to
obtain specific fabric appearance or
performance with the advent of denim
garment washing techniques, the
construction and dyeing.
Techniques used
are of utmost importance, the selection of
yarn the consistencies of the indigo
dyeing process and its modifications
have become crucially important in
determining the quality and performance of
indigo denim products.
The properties of the
Indigo dyes account for the wide variety
of color designs that are available on
denim products. Indigo is unique as a
major textile dye because the indigo dyeing
process is naturally ring dyes the cotton
yarns, unlike almost all other
commercially successful dye stuffs.
The Indigo dye
concentrates in the outer layers of the
yarn,during the dyeing
process this produces an intense ring
of color around a white core in the yarn. Thus the name ring dyeing occurs.It's
considered a dyeing defect also indigo dye
an intense blue is insoluble in
water and will not dye cotton fiber in
order to dye cotton yarn the Indigo must
first be converted into a water-soluble
leuco form and then applied. This chemical
process is known as a reduction.
Reducing solutions containing sodium
hydrosulfide and sodium hydroxide
chemically change the indigo dye into
its soluble form this process also
converts the dye from its intense blue
color to a very pale greenish yellow
color. The outer layers of the yarn
readily absorbed this leuco form of indigo. once dyed the yarn the Indigo is
returned to the insoluble form by oxidation.
During oxidation the ropes of yarn passed
through the air in a process called
skying. The oxygen in the air converts the
dye back to its original
insoluble form and it's blue color returns in
traditional rope.
Steps in Dyeing
The following
steps are involved in processing Scouring, Indigo
dye application, Rinsing and drying. For substantial dyeing, process are defined here:
https://bindaprocess.blogspot.com/2022/02/textile-sustainable-denim-dyeing.html
The
ropes are first fed into one or more
scouring baths which consists of
wetting agents and detergents. The
purpose of these baths is to remove
naturally-occurring impurities found on the
cotton fiber such as dirt , minerals , ash and
pectin. Additionally the
waxy layer on the fiber which repels
water is also remove.
It is very important to
remove these materials to guarantee
uniform wetting and dyeing.The ropes are
subsequently fed into two or more water
rinsing baths the key to this process is
the dyeing segment. where the Indigo dye is
added in layers to the outer surfaces of
the yarn in rope.
The yarn is passed into a vat of soluble
indigo dye that exhausts on the
surface of the yarn and is then oxidized
this results in a small amount of dye
being deposited on the surface yielding
only a light blue dyed yarn. In order to
obtain a deeper blue indigo shade the
dye must be built in layers by
multiple passes of the Rope of yarn into the
subsequent vats of soluble dye and then into
the air for oxidation.
Each of these
cycles is called a dip. Normally this
process is repeated from three to nine
dips in order to build up a rich deep blue
color. If even darker shades are
desired a sulphur black or sulfur Navy dye
can be applied to the yarn prior to the
Indigo dyeing this is known as a sulfur
bottom.
If the sulphur dye is applied
after the indigo it's known as a Sulfur Topping.
Here 3 in 1 box or bottoming box plays roles that are defined here for effective dyeing:
https://bindaprocess.blogspot.com/2022/02/denim-rope-dyeing-automation-2022.html
when a sulfur bottom is
required the scoured ropes of yarn are then fed
into a bath of a reduced sulfur
black dye similar to indigo sulphur
dyes are water insoluble, they must be
reduced to a water-soluble form prior to
application.
Unlike indigo , the sulphur dye
can penetrate into the core of the
cotton fiber thus darkening the overall color
shade. Once the reduced sulfur dye is applied
to the ropes of yarn the dye is
allowed to oxidized to its normal
water insoluble form.
The ropes are then
fed into one or more water rinse paths
these baths remove any unfixed sulphur
dye that might contaminate the
indigo dyeing process.
Dyeing range for
application of the Indigo dye
typically 12 to 36 individual ropes of yarn are
simultaneously fed side-by-side into
the range. The ropes are kept separate
from each other throughout the
various parts of the derange.
The ropes are then fed into the reduced
indigo dye baths once reduced the
indigo dye takes on a pale greenish
yellow shade the yarns are then skyed as
much as 30 to 40 feet into the air to allow
the oxygen in the air to convert the
indigo back to its normal water insoluble
blue shade.
As the yarns oxidize they
change from a light greenish yellow shade
through various darkening shades
of blue green until they become the
deep blue indigo shade.
Denim shades
require as few as three or as many as nine to tweleve dips and skying processes. The
ropes are then rinsed in several water
baths to remove any unfixed or
surface dye deposits another.
Option would be
to add a sulfur top after the indigo
dye has been applied. The sulphur dyes
still migrates toward the core of the
fiber but the sulfur top uses a different
type of color performance. Especially in garment washing
procedures the sulfur top process is
followed by one or more water rinses to remove
any unfixed dye , after rinsing the yarn. Ropes pass through squeezed rolls to
mechanically extract water from the yarns, are then dried and coiled into large
tubs.
The typical drying apparatus
is multiple stacks of drying cans these
metal cylinders are filled with
pressurized steam. Care must be taken not to
over dry the yarn or the dye will
excessively migrate to the surface of the
yarn increasing the tendency of the
color to crock.
Additionally if
the surface of the drying can is too
hot the yarn can be overstressed
producing an undesirable shiny or ironed
appearance.
Coiling takes place after
drying the ropes, exit the drying cans and
are lifted high into the air and then coiled
into large trucks or tubs. Each
individual rope is placed into a separate truck, after drying, the color of the yarn is
checked either visually or instrumentally.
with many indigo dye ranges the color
of the yarn is continuously
monitored by a color spectrophotometer
which is electronically
linked to the controls of the indigo dye
baths. This type of control system
can automatically adjust the dynamics of
the process to obtain the most
consistent color from the beginning to the
end of the many thousands of
yards of yarn contained within a single
dye lot.
Once the warp yarns are rope tied it's
then necessary to change the yarn
alignment from rope form to sheet form prior
to entering the next process. which is slashing
or sizing beaming or Rebeaming. Which involves pulling the dyed ropes of yarn out
of storage tubs and moving them
upward to a guiding device sometimes called
a satelliete.This upward travel allows the
ropes to untangle before reaching
the Beamer head. Once the ropes come down
from the guiding device they go through
tensioning rollers to help further
separate the yarns prior to going through a
comb at the warper. For subsequent process in sizing and weaving.
The complete dyeing process describe in the blog is also available in video at this link:
https://youtu.be/qp2Z_pYsRGM
