Denim Dyeing with Indigo Dyes
Last updated: July 17, 2020
Denim garments were first to become very popular during the 18th century due to its durability and the resistance to tearing. Now denim has become a part of modern culture and fashion from ordinary cloth. Denim has embraced in our daily life like no other.
Firstly, let’s know what is denim?
Denim is a rugged, sturdy fabric in which the warp yarn is dyed, and the weft yarn remains undyed. It is generally 3/1 warp-faced twill fabric. Denim is different from other cotton fabric due to having a diagonal ribbing on its face side.
‘Serge de Nimes’ is a French word from which ‘Denim’ is derived. It refers to the city of Nimes.
Here is the process flow chart of denim manufacturing given above.
Now, let’s talk about denim dyeing. Generally, warp yarns are dyed with indigo dyes.
Indigo dyes were previously extracted from “Indigofera tinctoria,” i.e., leaves of indigo plants. Later, a German chemist named ‘Adolf von Baeyer’ developed first synthetic indigo dyes during the 1880s.
Approximately almost 80% of total denim is dyed with indigo and sulfur dyestuff. These dyes are pretty similar to vat dyes in terms of dyeing mechanism.
Reducing Agent: It makes the insoluble dye soluble by reduction. The most commonly used reducing agent is sodium dithionite (Na2S2O4).
Alkali: It helps to maintain a higher pH of 10.5-11.5 to obtain the ionized form of the dye. Mainly, Caustic soda (NaOH) is used.
Dispersing Agent: It prevents the agglomeration and aggregation of undissolved particles.
Wetting Agent: It helps to wet the fabric by reducing the surface tension of water.
Sequestering Agent: It makes hard water soft by deactivating the metal ions present in water.
As leuco Indigo is a weak acid, the ionization of the dye increases with the increase in pH. The optimum pH for higher substantivity for cotton remains around 11. pH around 10.5-11.5 gives a satisfactory result, and the depth and hue are not affected. Further increase in pH may affect the depth and hue.
The penetration of the indigo dye into the fiber is affected by the amount of Na2S2O4. Excess Na2S2O4 present in the dye-bath maintains the stability of leuco Indigo.
Up to a specific limit, the dye uptake increases with the increase of immersion time and then falls. 30 second immersion time seemed to be adequate. Prolonged immersion time reduces color depth as the re-reduction of oxidized Indigo occurs, which is retained by the cotton.
Maximum dye uptake occurs in room-temperature. The affinity of solubilized Indigo for cotton is reduced due to the increase in temperature. Typically, the re-reduction of oxidized Indigo does not occur at a lower temperature.
A part of the reduced dye is released due to lesser airing time. Optimum airing time for oxidation is ≥60 sec.
Four denim dyeing methods with Indigo are available.
At least 95% of world denim production is done by rope and slasher dyeing.
It was first started in USA at 1915. It is a supirior dyeing technology in which the warp wars are pre-treated and dyed in rope form.
Schemetic diagrams of rope dyeing are given below –
This continuous process was first introduced in 1970. The warp yarns are pre-treated and dyed in the form of a yarn sheet. This is multi-dip, multi-nip, and multi-airing indigo dyeing.
Beam Warping ⇒ Pre-Treatment ⇒ Washing ⇒ Warp Sheet Dyeing ⇒ Airing for Oxidation ⇒ After-Wash ⇒ Pre-Drying ⇒ Warp Yarn Sizing in Sheet Form ⇒ Weaving ⇒ Finishing
Schematic diagrams of slasher dyeing are given below –
Looptex Co., Italy in 1903 first developed it. It is almost similar to slasher dyeing, except it has only one dye bath.
Schematic diagrams of loop dyeing are given below –
For bottoming and topping, a combination of indigo dye with another dye in different dye-bath can be used. The most common process in sulfur bottoming/ topping.
Bottoming and topping are done when high intensity in color with acceptable fastness is required. As a diverse shade of blue color is very demandable, it is done.
Initially, dyeing with sulfur dye is done, followed by indigo. In this process, darker shade can be produced with less amount of indigo. There is a possibility of large removal of sulfur dyes in indigo dye-bath due to Sodium-dithionite.
The picture shows the inner layer with sulfur dye and outer layer with indigo dye.
Initially, dyeing with indigo is done, followed by sulfur dyeing. Much darker color than sulfur bottom can be obtained. The colors include black, blue-black, yellow-brown, and green. Slub appearance can be produced.
The picture shows the inner layer with indigo and outer layer with sulfur dye.
It is surface dyeing of yarn with indigo. As indigo dyes lack affinity towards fiber, they tend to adhere to the surface. If too much surface dyeing is done, it leads to poor color fastness.
1. Is denim a jean?
Denim is made of 100% cotton and twill structure, whereas jeans are one type of garment which is made of denim fabric. Denim can be defined as either pants, jackets, skirts, or shorts. On the other hand, jeans are specially defined as pants.
2. Is indigo dye sustainable?
Natural indigo is sustainable. After dye extraction, the residue can be composted and used as fertilizer. But synthetic indigo is not a natural dye.
3. Is indigo more blue or purple?
Nowadays, indigo color in referred to as color between blue and violet. Halfway between blue and purple is violet. So, indigo can be said as three-quarter blue and quarter purple.
4. Is synthetic indigo dye toxic?
Synthetic indigo dye is extracted from the Indigofera plant. Several toxic chemicals are required for synthesizing indigo dye, which also includes formaldehyde.
5. What does the Indigofera plant look like?
They are a shrub which are one to two meters high. Depending on the climate of the place, they can be of different types such as annual, biennial, or perennial.