Vat Dyeing Process | Textile Dyeing Process With Vat Dye

Last Updated on 05/09/2021

Vat Dyes:
The name ‘vat’ comes from the wooden vessel which was first used for the reduction and application of vat dyes. Vat dyes are made from naturally not synthetically. Vat dyeing is widely used for coloration of cellulose materials such as cotton. Vat dye can’t be directly applied and required vatting because it is insoluble in water. However, they can be made soluble by reduction in alkaline solution which allows them to affix to the textile fibers. Subsequent oxidation or exposure to air restore the dye to its insoluble form. Indigo is the original vat dye. These dyes are the fastest dyes for cotton, linen and viscose. They are used with mordants to dye other fabrics such as wool, nylon, polyesters and acrylics. But vat dyes fastness properties is good among the all dyes. The dye takes their generic name from vatting. The vat dyes are naturally obtained coloring materials from the ancient time and kept into wooden vat and make soluble in vat by the process of fermentation- so it is called vat dyes.

vat dyeing

What is Vatting?
Vatting is the process of converting the insoluble vat dyes into its soluble form. Because our ultimate goal is to dye the substrate evenly in an aqueous medium. And the insoluble vat dye displays no substantivity towards cellulose, but the soluble form does. With the help of a suitable reducing agent and alkali, we can convert the insoluble vat dye into its soluble leuco form. This usually results in a change of color.

Pretreatment for Vat Dyeing:
Before vat dyeing, pretreatment is very important; it should ensure that the substrate has a high and uniform dye uptake and absorbency, is virtually free of husks, has a sufficiently high standard of whiteness, but must not cause any significant damage to the cellulose.

Pretreatment is carried out by boiling off in the presence of alkali and dispersing and sequestering agents. Further improvement in the absorbency can be achieved by a treatment with a high concentration of alkali (causticising or mercerizing). Prior to dyeing, size is removed from woven fabrics by the usual methods.

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Since the vat dyeing of cellulosic fibers is carried out in relatively strong alkaline baths, it is also possible – in certain circumstances – to dye grey material. For example, with suitable auxiliaries grey yarn can be scoured and dyed simultaneously by the high temperature process above 100oC.

Typical Recipe for Vat Dyeing:
A typical recipe for a dye liquor would be:

  • Caustic soda (27% by weight), 5 ml l–1;
  • Hydrosulphite, 1.5 g l–1;
  • Stock vat, 62.5 ml l–1.

The stock vat is composed of:

  • Indigo Pure (BASF CI Vat Blue 1), 80 g l–1;
  • Dispersing agent, 4 g l–1;
  • Wetting agent, 1 g l–1;
  • Caustic soda (27% by weight), 130 ml l–1;
  • Hydrosulphite, 60 g l–1.

Application of Vat Dye:
The application of a vat dye consists of the following steps:

  • Application of the vat dye in insoluble, disperse form (pigmentation)
  • Reduction of the dyestuff by appropriate reduction agents
  • Dissolution of the reduced (leuco) form of the dye
  • Exhaustion of the dye from the dyebath/adsorption of the reduced dye in continuous dyeing processes
  • Levelling of the dye to achieve uniform dyestuff distribution
  • Oxidation of the reduced dye into the insoluble pigment form
  • Soaping to achieve the final particle distribution and tone
  • Rinsing to remove chemicals and neutralise the material

The basic steps in the application of vat dyes are summarised in below Figure.

Basic steps in the application of vat dyes
Fig: Basic steps in the application of vat dyes

Vat Dyeing Process:
The vat dyes are generally not soluble in water. However, solubilized vat dyes are also available but are usually more expensive as compared to generally available insoluble vat dyes. Based on their chemistry, vat dyes can be classified into two main groups: indigo derivatives and anthraquinone derivatives. In general, fastness properties of anthraquinone-based vat dyes are usually better as compared to those of indigo-based dyes. Indigo blue vat dyes are commonly used for producing indigo denim, with different wash-down and worn-out looks. Based on application properties, vat dyes are classified into four main types: IN vat dyes; IW vat dyes; IK vat dyes; IN special dyes. Major differences in the above four groups of vat dyes include: their leveling properties, dyeing temperature, and amount of alkali, salt and leveling agent required during dyeing.

General phases in dyeing with vat dyes are as follows:

  1. Reduction: conversion of insoluble vat dye into soluble sodium leuco vat anions, with the help of a reducing agent (sodium dithionite) and alkali (sodium hydroxide)
  2. Diffusion: penetration of the reduced / solubilized sodium leuco-vat anions into fibers
  3. Rinsing: removal of excess alkali and reducing agent from the dyed material
  4. Oxidation: conversion of vat dye absorbed in the fibers back into insoluble form
  5. Soaping: during which the vat dye molecules absorbed by the textile material are re-orientated and associate into a more crystalline form.

Vat dyeing is complex from other dyeing. Its dyeing process are given below:
The required water for dyeing is taken in the dye bath and it is maintain at proper temperature (50º- 60ºC) the reducing and dyeing temperatures vary from dyestuffs to dyestuff. The vatted dye solution may than be added to the dye bath containing the required amount of caustic soda sodium hydro sulphate, kept at recommended temperature.

The well scoured wet yarn is entered in the dye bath and turned several times, so that the affinity of the color may be uniform. The yarn is then kept completely immersed under the dye liquor and the dyeing is continued for one hour. The yarn is turned from time to time. Care should be taken to keep the bath at required temperature and also to keep the yarn thoroughly immersed under the liquor.

The exhaustion agents or retarding agents are added to the dye bath depending upon the dyestuffs taken, during the entire dyeing period. Excess quantities of both sodium hydroxide (NaoH) and sodium hydro sulphate (Na2S2O4) should be present in the dye bath in order to keep the dye in the soluble form. At the end of the dyeing the partly or completely exhausted dye bath must be kept in a distinctly reduced condition; otherwise oxidation of the residual vatted dye takes place in the dye bath itself leading to the appearance of turbidity. This is ensured by adding sufficient sodium hydro sulphate. The dyed goods may then be removed from the dye bath and excess liquor which contains the unexhausted vat dye, sodium hydroxide, sodium hydro sulphate is removed as for as possible from the goods.

The dyed goods are rinsed with cold water and then subjected to an oxidation treatment by exposure to atmospheric oxygen. This is called “air oxidation” or “airing” but the oxidation may be accelerated by using stronger oxidizing agent such as sodium per borate or hydrogen peroxide or sodium dichromate in the presence of acetic acid. This process is usually referred to as chemical oxidation.

During the oxidation step the sodium salt of leuco vat dye absorbed by the fiber is oxidized and converted into insoluble dye in the fiber. At the same time the vatted dye contained in the residual liquor in the goods being dyed also gets converted into the insoluble form which is loosely deposited on the fiber surface. This loosely deposited dye on the surface of the fiber has to be removed for achieving optimum fastness properties especially rubbing and washing fastness properties. This is achieved by soaping process. The dyed material is treated in hot soap solution or a synthetic detergent solution for 15 – 30 minutes. After the soaping treatment the dyed goods should be rinsed thoroughly and finally the dyed material is dried.

Precautions in Vat Dyeing:
To get uniform level dyeing results, the concentration of sodium hydro sulphite and caustic soda must be in exact quantity to reduce the dye bath during whole the process. Excessive quantity of reducing agent and alkali decrease the exhaustion rate of the dyeing. Similarly insufficient quantity of chemicals produce adverse effect on dyeing so to control this balance Phenolphthalien paper and Hydro papers are used.

  1. Phenolphthalien Paper: It is white paper which turns in pink color while contact with caustic soda.
  2. Vat Yellow Paper: It is gold yellow paper which turn in blue-violet color if hydro is available in dye bath.

It is also advisable that dyeing with two or three dyes combination must be from same group of dyes. So behavior of dyes is an important chapter to get uniform results. Vat dyeing is the most difficult dyeing as compared to other dyeings.

Conclusion:
The vat dyes have excellent light fastness due to stable electron arrangement in the chromophore (colour-bearing group) of the dye molecule and presence of numerous benzene rings. Vat dyes have excellent wash fastness owing to the aqueous insolubility of the oxidized dye absorbed in the fiber and due to large vat dye molecules trapped within the polymer system. However, vat dyes are usually very expensive and need more expertise for their application because of greater number of steps involved in dyeing.

References:

  1. Textile Dyes by N. N. Mahapatra
  2. Industrial Dyes: Chemistry, Properties, Applications Edited by Klaus Hunger
  3. Textile Chemistry by Thomas Bechtold, Tung Pham
  4. Cellulosics Dyeing Edited by John Shore
  5. Textile Engineering-An Introduction Edited by Yasir Nawab
  6. Textile Dyeing by N. N. Mahapatra

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