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:
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