Vat+dyes

punjab uni lhr

2. Saima Latif
Roll No 36
M.phil Organic
2
nd
Semester
Session 2015-2017

3. VAT DYES

4. Definition
•The word vat means, ‘Vessel’. The dyes take
their name from vatting. The vat dyes are
naturally coloring matter and kept in wooden
vat and make solubilise vat dyes by reduction
– so it is called vat dyes.

5. General
characteristics

6. •Vat dyes are one of the important class in the
synthetic dyes produced. These dyes are
characterised by their insolubility in water.
•They are applied to the fabric in a reduced, soluble
form, which has affinity for the substrate
and after the reduced dye been absorbed, the
fabric is taken out from the dye bath and left
in the air or immersed in solution, of a mild
oxidising agent to reproduce the dyeing shades
•Vat dyes are, in general, fast to washing to light,
etc.

7. •Vat dyes are insoluble in water, solubilised by
treatment with caustic soda and reducing agent, usually
hyposulphite, the resulting leuco compounds have affinity
for textile fiber, on exposure to air leuco compound
impregnated fiber reoxidises to the insoluble parent dye.

•Vat dyes mainly belong to indigoid and anthra quinoid
classes and are characterized by high fastness, specially
anthraquinoids, most valuable for dyeing and printing
cotton, wool and silk.
•pH is kept below a point at which damage to protein fiber
may occur.

8. CH
3
O
O
2H
O
Insoluble vat dye
CH
3OH
OH
OH
+
H
+
Vat acid
(non-substantive)
CH
3
O
-
O
-
Leuco vat dye
substantive
Absorption by fibre
oxidation

9. Chemical classification Of
Vat Dyes

10. •Until early times of the present century, the
only vat dyes known were those related to
indigo.
•But due to the constant research work in
the field, a new class, anthraquinone dyes
was found and is of much prominence these
days.

11. N
H
N
H
O O
Indigotin
S S
O O
Thioindigo
O
O
anthraquinone vat dyes
Indigo vat dyes

12. Indigo
•Indigo was first a naturally cultivated dye
which was principally grown in every part of
the world.
•Indigo has become quite important in
dyeing full shades of navy blue on wool,
for uniform cloth of good fasteners.
•While dyeing, it is reduced by hydrosulphite,
to make the alkali soluble leuco derivative or
to make indigo white.

13. Anthraquinone vat dyes
•Anthraquinone vat dyes are mostly solid in
the insoluble oxidised form, sometimes as
dry solids but more often as quneous
pastes.
They are applied in the reduced
form as 'Vats', the reduction 'being affected
by the use' of sodium hydrosulphite under
strongly alkaline conditions.

14. Steps Of Vat Dyeing Techniques
•A wide range of different techniques are
used in colouring processes with vat
dyes.
•All processes involve three steps:
· Vatting
· Oxidation
· After treatment.

15. vatting
•The step in which the reduction of the
dyestuff into its leuco-form takes place is
called vatting.
•Vat dyes are generally more difficult to reduce
than sulphur dyes. Various reducing agents
are used. Sodium dithionite (hydrosulphite) is
still the most widely employed although it has
some limits.

16. Limits of sodium dithionite
• Sodium dithionite is consumed by reduction of the
dye and also by reaction with atmospheric oxygen,
therefore an excess of reducing agent has to be
used and various techniques have been proposed
to reduce these losses.
•In addition sodium dithionite cannot be used for
high temperature or pad-steam dyeing processes
because over-reduction can occur with sensitive
dyes. In these application conditions and also for
printing, sulphoxylic acid derivatives are normally
preferred.

17. oxidation
•After absorption by the fibre, the dye in its
soluble leuco form is converted to the original
pigment by oxidation.

•This process is carried out in the course of wet
treatment (washing) by addition of oxidants
such as hydrogen peroxide, perborate to the
liquor.

18. After-treatment
•The final step consists in after-treating the
material in weakly alkaline liquor with a
detergent at boiling temperature.

•This soap treatment is not only aimed at
removing pigment particles, but also allows
the crystallisation of amorphous dye particles,
which gives the material the final shade and
the fastness properties typical of vat dyes.

19. Classification according to affinity for
fibre
•Vat dyeing conditions can vary widely in terms
of temperature and the amount of salt and
alkali required, depending on the nature of
the dye applied.
•Based on temperature, amount of caustic
soda, hydrosulphite and salt, used in dyeing,
vat dyes can be classified into four main
groups:

20. •IK dyes (I = Indanthren, K = cold) have low affinity,
they are dyed at 20 - 30 °C and require little alkali and
salt to increase dye absorption
• IW dyes (W = warm) have higher affinity, they are
dyed at 40 - 45 °C with more alkali and little or no salt
• IN dyes (N = normal) are highly substantive and
applied at 60 °C and require much alkali, but no
addition of salt.
• IK special class (Special dyeing dyes) The dyeing
time is usually 45- 60 mins

21. Solubilized Vat Dyes
•It's inconvenient to have to reduce your vat dyes in
order to dissolve them. However, it is impractical
to sell the reduced form of the dye, because it will
oxidize in the air, back to the insoluble form.
•The solution to this problem is for the
manufacturer to convert the soluble leuco acid
form of the dye to the leuco ester, such as by
reacting the leuco acid with sulfuric acid.

22. •The leuco form of this solubilized dye can be
regenerated by removing the ester group
chemically (with sodium nitrite in dilute
sulfuric acid) or by the action of light.
Drawback:
•The drawbacks of solubilized leuco esters are
their greater expense and their poorer
uptake into the fiber, which results in paler
shades.