synthesis of vat dyes are vatting, dyeing, oxidation, after treatment. it gives proper results on cotton materials.

lets know about vat dyes,

What is vat dye

First off all i want to say that, vat dye have activity to insoluble in water. but it becomes soluble by the vatting process. this mean, insoluble vat dye is converted to soluble vat dye with help of vatting process.

A process in which insoluble vat dye are converted to soluble vat dye, this process is called vatting.

You know that, in case of vat dyeing process, first required vatting befor dyeing. If you follow this process, it gives even dyeing and it help improved shade of materials.

Some chemical are to be uses for vatting process like, sodium hydroxide, sodium hydro sulphite and more. vat dye are widely uses for colouration of cellulose textile materials. but it expensive compare with other dyes.

Vat dyes have good fastness properties compare with all the dyes.


You know that vat dyes are naturally obtained. vat dyes are gives excellent performance on cellulose textile materials. but the disadvantage of this dyes are, it is expensive process.

Synthesis of vat dyes

Steps involved in vat dyes;

  1. Vatting
  2. Dyeing
  3. Oxidation
  4. Aftertreatment

1. Vatting

In case of vat dyeing process, first required vatting. after vatting process, fabric is ready for dyeing. so, first vatting is done then dyeing is done.

solubilised vat dye is required, for vat dyeing process. as a result they gives even dyeing. some chemical is required for process, such as sodium hydrosulphite, sodium hydroxide and more.

Insoluble dye is reduced and Solubilised by using sodium hydrosulphite (Na2S2O4) and Sodium hydroxide.

for example,

Conversion into sodium salt of leuco vat dye

Here, I m going to explain how to convert sodium salt to leuco vat dye with the help of structure.

synthesis of vat dyes

2. Dyeing

In vat dyeing process, ready to dye bath with the help of high amount (add excess) of caustic soda and hydros (Na2S2O4) to keep the dye in soluble and substantive form during dyeing. Na2S2O4 gets decomposed by atom Oxygen and produce acidic product. in which in trun neutralised a part of the alkali.

Hence the concentration of sodium hydroxide in the dye bath decreases. Now, add fresh quantities of naoh and hydrose are add. Due to this it maintain the dyestuff in soluble form.

Cell – OH has higher affinity for most of sodium salt of leuco vat dyes necessary controle over temperature and salt.

Now, dye solution is add in 2 installment.

To promote, level dyeing add non – ionic dispersing agent. No other chemical are to be add during reduction and Solubilisation except Na2S2O4 and NaOH.

The dyeing take place as,

  1. Temperature – room temperature to at boil
  2. PH – Alkaline
  3. Time – minimum 45 to 60 mint.

3. Oxidation

After dyeing, some time oxidation is also required for better performance.

In this step, dyed textile materials is oxidized to restore original dye structure, to trapped dye molecules inside the fibre and to develop actual shade.

Oxidation may be carried out in open air improter oxidation leads to faulty dyeing and also covered uneven dyeing.

The oxidation treatment has to be carried out under mild conditions. for example, – low temperature and minimum time period. but must not over oxidation.

For oxidation treatment, some important oxidation agent are required such as H2O2, sodium perborate, sodium dichromate in presence of acetic acid and sodium hypochlorite.

4. After treatment

The leco vat dye absorbed by the fibre is converted into it’s original insoluble form inside the fibre during oxidation. At the same time dye particles in the residual liquor are also oxidised and get loosely deposited on the fibre surface. these dye particles are not for finely fixed to the fibre.

Therefore, it responsible for poor rubbing fastness properties of dyed materials. It is necessary to treat the dyed goods in a detergent bath to develop the true shade with good fastness properties. and soaping is carried out for removing loosely dye particle from the surface of materials.

For this soaping process, boiling solution of soap and synthetic detergent is used.

Quick test for vatting;

In order to check whether vatting has been completed or not, a drop of vatting solution is placed on a filter paper and absorbed. If liquor is incompletely vatted, the unvatted dye will be visible at center of the place that was spotted. When a dye is perfectly solubilised the dye solution spreads uniformly.

Test for good dyeing process

The presence of caustic soda and hydrose during dyeing is checked by means of phenolphthalein paper and vat yellow paper.

Presence of hydrose is indicated by appearance of a blue colour within seconds of spotting the vat yellow paper with the dye bath solution. If the blue colour does not appear, a fresh quantity of colour hydrose needs to be add.

The presence of caustic soda is checked with phenolphthalein paper, which change from white to pink in the presence of alkali.

Diffrence between vat dyes and Solubilised vat dyes

what is difference between vat dyes and Solubilised vat dyes?


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