⇨ Vat dyes have an outstanding fastness to external agencies, like light, washing, bleaching, rubbing, perspiration, etc.
⇨ Therefore, vat dyes are used for shirting,
window curtains, sarees, casement cloth, etc.
⇨ They can be classified into three distinct
chemical groups: indigoid vat dyes, anthraquinonoid vat dyes, and sulfurized
vat dyes.
⇨ Vat dyes are forming leuco compounds with a
reducing agent due to reduction, which are soluble in water, and the parent colour
can be generated by oxidation.
⇨ Vat dyes (indigo) can be printed by the glucose and
hydrosulphite process by direct style of printing.
⇨ Glucose
Process:
●
The fabric is padded with a 25% solution of glucose, dried, and printed with indigo (vat dye) with
printing paste prepared under:
|
Ingredients |
Quantity (Parts) |
|
Indigo (vat dye) |
50 |
|
Caustic soda (70˚Tw) |
240 |
|
Water |
260 |
|
Gum Arabic paste |
450 |
|
Total |
1000 |
●
After printing and
drying, the fabric is steamed at 100°C in air-free moist steam for a few
minutes.
●
Steamed fabric is passed
through the solution for 15 minutes for oxidizing the dye as prepared as
follows:
|
Ingredients |
Quantity (Parts) |
|
Sodium bicarbonate |
20 |
|
Ammonium sulphate |
10 |
|
Total (adjusted with water) |
1000 |
●
Finally, fabric is
washed, soaped, and dried.
⇨ Hydrosulphite
Process:
●
The fabric is printed
with indigo (vat dye) with printing paste prepared as under:
|
Ingredients |
Quantity (Parts) |
|
Indigo (vat dye) |
150 |
|
Alkaline thickening |
350 |
|
Formosul |
80 |
|
Alkaline thickening |
420 |
|
Total |
1000 |
●
After printing and
drying, the fabric is steamed at 102°C for 2–3 minutes in a Rapid ager.
●
Steamed fabric is
oxidized in a chrome bath.
●
Finally, fabric is
washed, soaped, and dried.
●
Alkaline thickening is
prepared by mixing 30 parts maize paste and 100 parts British gum paste with
870 parts caustic soda (77˚Tw)
⇨ Potash
Rongalite Method: (for anthraquinonoid
vat dyes)
●
This is the most popular
method for printing the vat dyes.
●
The fabric is printed
with vat dye by preparing printing paste as under:
|
Ingredients |
Quantity (Parts) |
|
Vat dye |
200 |
|
Glycerine |
50 |
|
Solution Salt B |
30 |
|
Starch-tragacanth
paste |
500 |
|
Potassium carbonate |
120 |
|
Rongalite C |
100 |
|
Total |
1000 |
●
After printing, the
fabric is dried as quickly as possible and steamed for 5–10 minutes in
air-free moist steam at 100–102°C.
●
Steamed fabric is rinsed
and then oxidized at 50°C for 2–5 minutes in a solution containing:
|
Ingredients |
Quantity (Parts) |
|
Sodium bicarbonate |
2 |
|
Acetic acid |
5 |
|
Total (adjusted with water) |
1000 |
●
Finally, the fabric is
washed, soaped at a boil, rinsed, and dried.
References:
