Vat dyes are insoluble in water but can be converted to soluble leuco compounds using reducing agents like sodium hydrosulphite and caustic soda. These leuco compounds are applied to cotton and oxidized back to insoluble dyes. Vat dyes are classified by application temperature and chemical requirements, with IK requiring the lowest temperatures and chemicals and IN-Special requiring the highest. The vat dyeing process involves vatting to solubilize the dye, dyeing to apply it to cotton, oxidation to fix it, and after treatment including soaping to improve fastness.

1. VAT DYES

2. Properties
• Vat dyes are insoluble in water
• They are converted to their soluble “leuco” state by means of
sodium hydrosulphite (reducing agent) in the presence of caustic
soda
• “Leuco” compound have substantivity to cotton
• Vat dye have excellent washing and light fastness
• Vat dyes are expensive compared with the other classes of dye

3. Classification based on Method of Application
• IK
– Have optimum affinity at low temperature i.e. at 25 – 300C
– Require minimum addition of caustic soda and sodium hydrosulphite
• IW
– Are exhausted at moderate temperature i.e. at 40 – 500C
– Requires moderate caustic soda
– Requires moderate concentration of exhausting agent for better
exhaustion
• IN
– Have maximum affinity for cellulose at 50 - 600C
– Requires relatively high concentration of caustic soda in the dyeing bath
– The exhaustion of the dye bath is achieved even without the addition of
common salt
– Require retarding agents during dyeing

4. Classification based on Method of Application
• IN-Special
– Require relatively high concentration of caustic soda, sodium
hydrosulphite,
– Require high vatting and dyeing temperature
Group Vatting
Temperature
(0C)
Dyeing
Temperature
(0C)
Caustic soda
(g/l)
Hydros (g/l) Common salt
(g/l)
IK 50 25-30 1.0 – 3.0 1.0 -5.0 6.0 – 50
IW 50 40 – 50 1.0 – 5.0 1.0 – 6.5 3.0 – 25
IN 50 – 60 50 – 60 1.5 – 10 1.5 – 10 None
IN-Special Dyeing require special condition

5. Application/ Mechanism of Vat dyeing
• Vatting
– Conversion of insoluble commercial vat dye powder into its soluble
sodium salt
• Dyeing
– Application of reduced and dissolved vat dye to the fibre from an
alkaline reducing bath in presence of exhausting or retarding agent
• Oxidation
– The conversion of soluble salt of leuco vat dye absorbed by the fibre
into insoluble form by aerial or chemical oxidation
• After treatment
– Treating with soap solution to get good fastness properties

6. O
O
ONa
ONa
Reduction Solubilisation
Insoluble
Vat Dye
Soluble Vat
Dye
OH
OH
Oxidation
[H]
[O]

7. • Vatting
– The conversion of insoluble vat dyes into their soluble form involves
the following steps
• Reduction of insoluble vat dye into weakly acidic leuco form
• Neutralizing the leuco vat dye with sodium hydroxide to give water
soluble sodium salt of leuco-vat dye
– The reduction followed by solubilising is called vatting
– To keep the dye in soluble sodium salt form, a sufficient excess of both
caustic soda and hydrosulphite soda is required to be maintained in
the bath
– Chance of sodium hydrosulphite to get decomposed to form acid
• In expose to air
• With increase in temperature

8. • Dyeing
– This is the application of the dye to the textile material
– The get exhausted inside the textile material
– Have the following steps
• Adsorption
• Absorption or penetration

9. • Oxidation
– The dyed goods are subjected to an oxidation for conversion of soluble
sodium leuco-vat dye into insoluble vat dye
• Aerial oxidation
• Chemical oxidation (Hydrogen peroxide, sodium dichromate in
presence of acetic acid etc.)
– The oxidation treatment has to carried out under mild condition (i.e.
at low temperature and minimum time) to prevent oxidation of
cellulose
– Due to over-oxidation change in tone may occur

10. • After Treatment
– Dye particles in the residual liquor are also oxidized and get loosely
deposited on the fabric surface
– These dye particles are not firmly fixed to the fibre and are therefore
responsible for poor rubbing fastness properties
– During soaping the loosely held dye particles are removed and held in
suspension to prevent them depositing back on the material
– The soaping treatment is carried out at boil
– The soaping treatment is done with soap and soda ash followed by
washing treatment for 20 – 30 minutes

11. Dyeing Auxiliaries
• Reducing Agent
– The most important reducing agent for Vat dyeing is sodium
hydrosulphite (Na2S2O4)
• Alkali
– The most important is caustic soda (NaOH). Solubilising agent.
• Electrolyte
– Neutral salts e.g. common salt (NaCl) can increase the substantivity of
leuco dyes for the fibre