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Application of vat dyes - dyeing

Application of vat dyes 

Application of vat dye on cotton involves the following four steps.

(1) Vatting
(2) Dyeing
(3) Oxidation 
Second step Explanation - 

(2) Dyeing

Absorption of reduced and dissolved dyestuff by fibre from an alkaline reduction bath while being exposed to an exhausting or retarding agent. 

The sodium salt of the leuco vat colour is adsorbed on the fibre's surface within the second step, where it diffuses into the fibre's interior to dye it. 

Since the insoluble dye in a very vat has no affinity for the fibre, adsorption and diffusion are only possible after the dye is fully dissolved. 

Keeping the hydrated oxide and hydros concentrations and dyeing temperature within the prescribed range is consequently crucial.

To stay the dye soluble and in substantive form during dyeing, a small overabundance of hydrated oxide and hydros is required.

Application of vat dyes - dyeing


1. Na2S2O4 (Hydros) + O2 (Atmospheric Oxygen) > Acidic product  

2. NaOH (Caustic soda) + Acidic product > Neutralised bath  

  • The atmospheric oxygen breaks down sodium hydrosulfite, creating an acidic byproduct that partially neutralises the alkali (caustic soda -NaOH). 
  • As a result, the hydroxide concentration within the dye bath drops. 
  • The dyestuff is kept in soluble condition by adding fresh amounts of hydroxide and hydros.
  • When the leuco dye that the fibre has absorbed is prematurely oxidised because of depletion, which might happen even toward the conclusion of the dyeing process, patchy dyeing with dark and bright patches within the dyed material results. 
  • Therefore, from the start to the finish of the dyeing, it's crucial to take care of sufficiently alkaline and adequate reducing conditions within the dye bath. 

Mechanism of dyeing i.e these chemicals function via two different mechanisms - 

(i) These substances fight with the dye molecules for the accessible sites within the fibre, slowing down the speed of dyeing.
(ii) alternatively, they combine with the dye molecules to form loose complexes that aren't absorbed by the fibre. 

These complexes don't seem to be softened during the later stages of dyeing when the temperature is higher. The dye molecules are also released for adsorption by the fibre surface. 

Q. Why do we use excess hydros with caustic i.e 1 gpl caustic soda(NaOH) with 2 gpl hydros (Na2S2O4)?
Ans: Hydros reacted very fast with atmospheric oxygen and decomposed into Acidic products due to which hydros quantity decreased very fast so we added excess i.e 2 gpl hydros with 1 gpl caustic soda.

Test for good dyeing 

Using phenolphthalein paper and vat yellow paper, respectively, the presence of hydrated oxide and hydros during dyeing is checked A blue solution appears when there's a satisfactory amount of hydros present.

A new amount of hydros has to be applied if the blue colour isn't visible. With phenolphthalein paper, which turns from white to a deep pink within the presence of alkali, the alkalinity, or the presence of hydrated oxide, is decided.

Vat yellow paper

Phenolphthalein paper

If blue colour appears then satisfactory hydros are present

If intense pink colour appears then satisfactory Alkali is present

If the blue colour does not appear then a fresh quantity of hydros needs to be added

If the pink colour does not appear then a fresh quantity of alkali needs to be added

Questions -

  1. Which are the steps for applying vat dyes to the fabric?
  2. Describe the dyeing process of vat dyes.
  3. Which are dyeing reactions involved in the vat dyeing process?
  4. What is a test for applied vat dyes?


Hussain, T. (2007, November 19). Important considerations in dyeing with vat dyes.; Fibre2Fashion.

Kim, M. R., Kim, H., & Lee, J. J. (2013). Dyeing and fastness properties of vat dyes on meta-aramid woven fabric. Fibers and Polymers14(12), 2038–2044.

Patwary, E. M. Z. (2012, May 4). What is vat dye. Textile Fashion Study; Engr. Mohammad Zillane Patwary.

Saha, S. (2018, October 21). Vat Dyes. Blogger.

Sharmin, F. (2017, November 18). Dyeing of cotton with vat dye. Textile Study Center.

The Editors of Encyclopedia Britannica. (2015). vat dye. In Encyclopedia Britannica.

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