Vat dyes are insoluble in water but can be converted to water-soluble leuco derivatives through reduction in an alkaline solution. The leuco derivatives penetrate cellulose fibers and are then oxidized back to an insoluble form within the fibers. Vat dyeing involves reduction, diffusion of the leuco form into fibers, rinsing, oxidation back to insoluble form within fibers, and soaping. Vat dyes are known for their excellent light and wash fastness due to their large molecule size and insolubility once oxidized within fibers.
2. Vat dyes
• Vat dyes are a class of dyes that are classified as such because
of the method by which they are applied. Vat dyeing is a
process that refers to dyeing that takes place in a bucket or
vat.
• Originated in Europe, vat dyes were so named because of the
vats used in the reduction of indigo plants through
fermentation.
• Before chemical reducing agents were readily available, vat
dyes were converted to their soluble leuco form by
fermentation of organic matter in wood tubs called vats. This
method of reduction and application is the source of the
name for this class of dyes.
Dr.
Muhammad
Mohsin
3. General properties of vat dyes
• Insoluble in water
• Are converted into water-soluble leuco-
derivatives by reduction in alkaline
solutions
• Leuco-derivatives possess substantivity for
cellulose on which the colour is
developed by subsequent oxidation
Dr.
Muhammad
Mohsin
4. Fastness
• Vat dyes are, as a class, the most light-fast of all dyes.
However, not all individual vat dyes are equally resistant to
light.
• Vat dyes are especially fast to light and washing
• Once the vat dyes have been regenerated inside the fiber,
they are very insoluble. This accounts for their excellent wash
fastness.
• The wash fastness rating of vat dyes is about 4 –5. The
excellent wash fastness of textile material, colored with vat
dyes is attributed to the large vat dyes molecule as well as its
aqueous insolubility.
Dr.
Muhammad
Mohsin
5. Common phases in vat dyeing by
exhaust method
Reduction
• Conversion of insoluble vat pigment into soluble sodium leuco-vat
anions
Diffusion
• of sodium leuco-vat anions into cellulosic fibres
Rinsing
• Removal of excess alkali and reducing agents by washing off
Oxidation
• of the soluble dye into insoluble pigmentary form within the cellulosic
fibres, and
Soaping
• during which the isolated molecules of vat pigments are re-orientated
and associate into a different, more crystalline form
Dr.
Muhammad
Mohsin
6. Preparing dye dispersions
• Disperse the dye into 5-8 times its weight
of soft water at 20-25°C, while stirring at a
high speed.
• Alternatively, mix dye to a smooth paste
with the gradual addition of about 5 times
its weight of soft water at 20-25°C and
then dilute as required.
Dr.
Muhammad
Mohsin
7. Vat Dyeing
• A bath containing the vat dye +sod. hyrosulphite+
sod. Hydroxide‐exhaustion of the dye on to the
fabric, the dyed material is removed, squeezed and
exposed to air when the leuco vat dye is converted to
insoluble dye form.
• Sometimes other oxidising agents like sodium borate,
hydrogen peroxide or potassium dichromate may be
used to hasten the oxidation.
• At the end of this treatment, the dyed material is
washed, soaped at the boil with a solution of
detergent and soda ash for 10‐15 mins, washed and
dried.
Dr.
Muhammad
Mohsin
8. Vat dyes classification
(Temperature)
According to the temperature required for dyeing vat
dyes are classified as:‐
• Cold dyeing dyes( II class) 20‐30 degree
• Warm dyeing dyes(IW class) 30‐40 degree
• Normal dyeing dyes(IN class) 40‐50 degree
• Special dyeing dyes ( IK special class) low temp
The dyeing time is usually 45‐60 min
Dr.
Muhammad
Mohsin
10. Classification of vat dyes (Based
on chemistry)
•Indigo derivatives
•Anthraquinone derivatives
Dr.
Muhammad
Mohsin
11. Reducing agent requirements
• A level of reducing power sufficient to reduce all commercial
vat dyes to their water soluble form, quickly and economically
• Conversion of the vat dyes into products from which the
original pigment can be restored (no over-reduction)
• Sodium hydrosulphite, commonly known as hydros
• Although a part of the hydros is used up in the reduction of
vat dyes, a large part of it may be destroyed by its reaction
with oxygen in the air (oxidation), particularly at higher
temperatures.
Dr.
Muhammad
Mohsin
12. Factors of rate of dye reduction
Dye-related
• Amount of dye
• Particle size of the dye
Auxiliary-related
• Concentration of the reducing agent
• Concentration of alkali/pH during reduction
Process related
• Temperature
• Time
Dr.
Muhammad
Mohsin
13. Effect of temperature
• At low temperature, the rate of exhaustion is low which might
promote levelness but the rate of diffusion is also low.
• At high temperature, the rate of exhaustion is high which
might decrease levelness but the rate of diffusion is high.
• Maximum exhaustion, penetration and levelness can be
obtained by starting the dyeing at low temperatures in
the leuco stage and slowly raising the temperature.
• Some dyes may not be stable to very high temperatures, so
the stability of dyes to temperature must be taken into
account
Dr.
Muhammad
Mohsin
14. Rinsing before oxidation
Purpose
• To remove any loose dye, excess of reducing agent
and alkali to lower the pH and establish conditions
favorable for oxidation.
Parameters
• The higher the temperature and/or pH of
the rinsing bath, the lower is the colour strength.
• Very high pH and temperature during rinsing may
also result in the dulling of the shade.
• The ideal is to do rinsing thoroughly at low
temperature at a rinsing bath pH value of 7
Dr.
Muhammad
Mohsin
15. Oxidation
Purpose
• To convert the water-soluble leuco from of the vat dye, back into
the insoluble pigment form
Parameters
• The type and concentration of oxidising agent,
• The type of pH regulator and pH during oxidation,
• Temperature during oxidation
• Oxidizing agent
The oxidizing agent must provide a level of oxidation potential
sufficient to oxidize the reduced vat dye into insoluble pigment, with
no over-oxidation i.e., beyond the oxidation state of the original
pigmentary form of the dye
• The optimum pH for oxidation is 7.5-8.5
• The higher the temperature, the faster is the oxidation, the
optimum temperature being 50-60°C
Dr.
Muhammad
Mohsin
16. Soaping
Purpose
• To remove any dye that is not diffused into the fibre
• To stabilize the final shade.
• This results in improved fastness properties and resistance to
any shade change to a resin or other finish or to consumer
use.
Parameters
• Time,
• Temperature,
• Type and concentration of soaping auxiliaries
Dr.
Muhammad
Mohsin
