Vat Dyeing Process: Properties & Mechanism

What is Vat Dyeing?

Vat dye: ‘Vat’ means Vessel. Vat dyes are naturally obtained coloring matter from ancient times, kept in a wooden vat, and made solubilized in vat by the process of fermentation, which is called vat dye.  

Properties of Vat Dyes: 

• Vat dye is water insoluble and can’t be applied directly on textiles except vatting.
• Mainly used for cellulose fiber, in protein fiber dyeing PH should be controlled.
• Rubbing fastness is not good. 
• Various shade is found. 
• The dyeing process is difficult.  
• Costly.  
• The washing fastness of vat dye is very good with a rating of 4-5. 

Chemistry/Mechanism of Dyeing With Vat Dyes: 

1. Aqueous dispersion: The insoluble vat dyes, in this stage, is dispersed in water.
2. Dye absorption: To achieve adequate exhaustion, an electrolyte is added at 600C temp. to the dye liquor. 
3. Re-oxidation of Vat Dye: In this stage, the leuco form of vat dye is oxidized by atmospheric oxygen & converted to its original color and insoluble form. 
4. Soaping of Vat Dye: During the previous stage some insoluble vat dye may be deposited on the surface of the material. This has to be removed to prevent poor rub-fastness as well as shade changing. By soaping off, this surface dye can be removed.  

Classification of Vat Dyes: 

▪ On the basis of chemical structure, mainly two types:

1. Indigoid: 
2. Anthraquinone: 

▪ On the basis of the Application Method: 

1. IN (Normal dyeing group) 
2. IW (Warm dyeing group) 
3. IK (Cold dyeing group) 
4. IN Special (Special dyeing group)

Vatting and Dyeing Conditions in Different Methods: 

Group	Vetting   temp. (0C)	Sodium hydrosulfite (gm/L)	Sodium   hydroxide(gm/l)	Sodium hydro  sulphite (gm/L)	Common   salt (gm/L)
IN	50-600	50-600	1.5-10	1.5-10	No
IW	40-500	40-500	0.4-0.45	0.4-6.25	3-25
IK	500	20-300	0.4-3	0.4-4.5	6.25-50
IN special	Dyes require individual treatment

Common Methods of Application of Vat-Dye:  

1. Conventional or reduced vat color method. 
2. Pigmentation or Pigment Padding System. 
3. Pad Jig 
4. Pad steam. 
5. Vat acid process. 

Application of Vat Color Method on Cotton Goods: 

▪ Recipe: 

Dyestuff: 3% 

Caustic Soda: 13% 

T.R oil: 3% 

Sodium Hydro Sulphite: 8% 

Temp.: 600 C (Vatting) 

700 C (Drying chamber) 

M: L= 1:30 

Time: 45-60 min. 

▪ Insoluble vat dye is converted into leuco compound. Dye, T.R. oil, and required hot water are mixed as recipes & stirred gently to form a paste.  

▪ Then addition of sodium hydrosulfite & caustic, soluble leuco compound is formed. 

▪ In the dye bath, the rest of the water is mixed & closed by cover not coming into contact with air. 

▪ Fabric is heated for 45-60min at 600C & rinsed. 

▪ Then washed with acetic acid to that remove alkali.  

▪ Then the fabric is kept in the air by washing with pure water. Dye becomes stable by treatment with the oxygen of air and Sulpher in dye.  

Cotton Dyeing With Vat-Dyes by Pad-Steam (Continuous Method): 

▪ Recipe: 

Dyestuff: 2-3% 

Dispersing agent: 1-2% 

Gum: 1-2% 

Caustic soda: 1-2% 

Sodium hydrosulfite: 2-3% 

M: L= 1:20 

Temp.: 55-650C (Drying chamber) 

600C (Padding) 

102-1050C (Steam fixation) 

▪ In this method, the fabric is at first pigment padded.  

▪ Then it is dried and again padded with sodium hydroxide and hydrosulfite.

▪ Then for the fixation, the fabric is passed through the steam chamber for about 30-60.

▪ The fabric is applied soap wash, ringing, and dry wash one after another.

Cotton Dyeing With Vat-Dyes by Pad-Jig Method (Semi-Continuous Method): 

▪ Recipe:  

✔ For padding: 

Dyestuff: 2-3% owf 

1. R oil: 0.1-0.2% 

Gum; 1-2% 

Water: As required 

✔ For fixation: 

Sodium Hydro Sulphite: 2-3% 

Caustic Soda: 1-2% 

▪ Fabric is padded with vat dye at a speed of 80 yds per minute & temp. 600C, then on a Jigger in caustic soda or hydrous liquor at 50-800C 

▪ The deposited fabric pigment is reduced by soap wash. 

▪ Then the fabric is oxidized to make the soluble vat dye insoluble again.

▪ Then the fabric is washed with soap and rinsed with water. 

Cotton Dyeing with Vat-Dyes by Vat Acid Process: 

▪ In this method, stirring continuously & treatment with acetic acid results leuco-compound precipitated as a dispersed product by neutralizing NaOH. 

▪ Larger molecules of dyestuff are converted into small molecules, known as side treatment. 

▪ Material is again brought to substantive soluble form by treatment with NaOH. 

▪ Then Oxidation is done for fixation in fabric & dried after soap wash & cold wash. 

Causes of Poor Rubbing Fastness: 

• Large quantity dye molecules in small water.  
• More vatting time. 
• Not proper reduction during dyeing i.e. Insoluble at last. 
• Not proper soap washing after dyeing. 
• Uses of Hard water. 
• Performs in Iron vessels. 

Over Reduction of Vat dyes: Over reduction from anthrone or ex-anthrone that does not reoxidize easily, but may be reduced further to Anthrone which can isomerize to Anthranol.  This may lead to a loss of color value and off-shade dyeing. 

Trade Name of Vat Dye: 

Vat dye Country 

• Artex/Meta Vat India 
• Cibanone Switzerland 
• Caledon U.K. 
• Paradone U.K. 
• Indan threm Germany 
• Tina Switzerland

Solubilized Vat-dyes: If vat dye is converted into the soluble form, called solubilized Vat dye.  Solubilize the vat dye found in powder form & prepare the colorless solution.  

Dyeing Process With Solubilised Vat Dyes: 

▪ Solubilize vat dye used in two steps: a. Dyeing, b. Generation/Development.

▪ Dyeing Recipe: 

Dyestuff: 1% 

Common salt: 10% 

M: L= 1:10 

Temp: 600C, Time: 30-45min. 

▪ Development recipe: 

NaNO2: 1% 

H2SO4: 1-2% 

M: L= 1:15 

Temp.: 40-500 C 

▪ Dye is dissolved in small water. Cotton is dyed with the recipe at 50-600C for 30-40 min. 

▪ After that, dyed cotton is taken to 2nd bath after cold wash & squeezed, treated at 40-500C  for 15 min & color is developed. 

▪ Then after squeezing. Cotton is washed with 2% soap & 2% soda & then dried. 

Differentiate Between Soluble and Insoluble Vat Dyes: 

Insoluble vat dye	Soluble vat dye
1. Insoluble in water	1. Soluble in water
2. Obtained directly.	3. It primarily has no affinity to textile.
3. It primarily has no affinity to textiles.	3. It has an affinity to textile mills.
4. Oxidation is easy.	4. Oxidation is difficult.

Stripping of Vat Dyes: Excess color or dye is removed from the fabric with the help of sodium hydrosulfite.