This document deliberates on the dyeing of cellulosic fiber with vat dyes.

1. 1
VAT DYE
Gezu K.
2019

2. Introduction
 Vat dyes are one of the oldest types and water insoluble dyes.
 Vat dyes ‐ a series of dyes of different chemical constitution … contain two or
more keto (C=O) groups
 They are called dyes because:
i. Chemical reduction into a water-soluble leuco form
ii. Substantivity in leuco form for cotton

3.  Insoluble in water
 Have best overall fastness properties. .. often used for fabrics that will be
subjected to severe washing and bleaching conditions (toweling, industrial and
military uniforms)
 Not bright colors
 No substantivity for cellulose
 Have a limitation with regards to use of strongly alkaline solutions
 Mostly complex dye structures with no ionic groups
CHARACTERISTICS OF VAT DYES

4. Cont. …
 Vat dyes are used predominantly for dyeing cellulosic fibers.
 Although leuco dyes have substantivity for wool and nylon, technical reasons
(damage in wool and poor fastness on nylon) vat dye application is restricted to
cellulose only.
 The vat pigment and the leuco compound often have quite different colors
 The primitive color is developed by subsequent oxidation

5.  Characterized by containing at two or more keto group [C=O]
 Mostly complex dye structures with no ionic groups
Chemical Characteristics

6. MECHANISM OF DYEING WITH VAT DYES
Converting a water-insoluble keto-substituted colorant by
reduction to a water-soluble leuco compound.
This penetrates into the fiber, where it is reoxidised back to the
original insoluble form.
Fixation based on secondary forces and insolublization

8. The process of reduction of vat dye in aqueous alkaline solution is known as
VATTING.
Sodium dithionite [sodium hydrosulphite or hydros] is used as reducing agent and
caustic soda as alkali .
Hydros is a very strong reducing agent, more effective at high pH and at higher
concentration.
Control over concentration & temperature for optimum reduction
Rate of reduction depends on pigment dispersion
DISPERSION STABILITY FOR PROPER REDUCTION
Na2S2O4 & NaOH

9. CLASSIFICATION OF VAT DYES
Based on chemical structure
Method of application

10. Indigo
Anthraquinonoid vat dyes: Based on
anthraquinone which give leuco compounds of
relatively high affinity
Based on chemical structure
a) Dyes derived from indigo – both natural and synthetic
Anthraquinone
Indigoid vat dyes: Derivatives of indigo which give leuco
compounds of relatively low affinity
b) Dyes derived from anthraquinone – most of
vat dyes
Indigo dyes give more brilliant colors than
anthraquinone But their light and wash fastness are
not as good

11. Classification Based on Application/dyeing properties
 IN [INDANTHRENE NORMAL]
 IW [INDANTHRENE WARM]
 IK [INDANTHRENE COLD {KALT}]

12.  Use of concentrated NaOH
Have high molecular weight
 High vatting temperatures (60 °C)
 High dyeing temperatures (60 °C)
 High substantivity
 No salt addition to the dyebath
THE IN (INDANTHRENE NORMAL)

13. IW [INDANTHRENE WARM] DYES
 Moderate concentration of NaOH
 Lower vatting temperature than IN dyes (50 °C)
 Lower dyeing temperatures than IN dyes (50 °C)
 Moderate substantivity
 Some amount of salt addition to aid exhaustion

14. IK [INDANTHRENE COLD {KALT}] DYES
 Low concentration of NaOH
 Lower vatting temperature (40 °C)
 Dyeing at room temperature (20°C).
 Poor substantivity
 Considerable amount of salt addition for good exhaustion

15.  The vatting temperature is often higher than the subsequent dyeing temperature.
To avoid decomposition of hydros and decreases the risk of over-reduction

16. 1) Reduction of the pigment to the soluble leuco compound
2) Absorption of the leuco compound by the cotton during dyeing
3) Oxidation of the absorbed leuco compound in the cotton, reforming the
insoluble pigment inside the fibers.
4) Soaping to remove pigment loosely adhering to the fiber surfaces and to develop
the true shade and fastness properties
Key Steps in Vat Dyeing Of Cotton
Air Oxidation Or Chemical Oxidants
Recrystallization/Aggregation
Preparation of the vat containing the leuco forms of the dyes
Water-soluble leuco compound

17. APPLICATION OF VAT DYES ON CELLULOSE
i. Reduction of pigment to soluble leuco compound, a process called vatting;
Converting a water-insoluble keto-substituted colorant by reduction to a water-
soluble enolate leuco compound.
ii. Absorption of leuco compound by cotton; water-soluble enolate leuco
penetrates into the fiber, where it is reoxidised back to the original insoluble
form.
iii. Fixation based on secondary forces and insolublization. Oxidation of absorbed
leuco compound in cotton, reforming insoluble pigment inside the fibers.

18.  The most important reducing agent in
vat dyeing is sodium dithionite
(Na2S2O4), referred to as
hydrosulphite or hydros.
Reduction of Vat dyes
 Vat dyes are available as fine powders or grains,
and as liquid dispersions or pastes.
 The insoluble pigment is extensively milled with
dispersants such as sodium ligno-sulphonates to
produce very small particles
In the presence of NaOH- alkali

19. Control over concentrations & temperature for optimum reduction
Rate of reduction depends on pigment dispersion
Dispersion stability for proper reduction

20.  Some Quinone vat dyes can be over-reduced  reduction of more than one pair
of keto groups (C=O)
 Over-reduced vat dyes give have poor substantivity for cotton; more difficult to
oxidize; produces duller shades; gives lower color yield.

21. Others ... sodium sulphoxylate formaldehyde and sodium sulphoxylate
acetaldehyde, particularly in printing and in continuous dyeing processes.
CONT’D
Others auxiliaries .... neutral salts (to increase substantivity), wetting, sequestering and
dispersing agents, leveling agent.
Help to overcome dyeing problems caused by inadequate pretreatment.

22. ALKALI PREVENT VAT ACID FORMATION
Caustic soda ‐ is used for:
1. The formation of the leuco: pH = 12‐13 needed.
2. The transformation of cellulose into alkali‐cellulose ,
3. Neutralization of acid products of decomposition of hydros
CONT’D

23. ABSORPTION AND LEVELLING
 Alkaline leuco vat dye exhausts very fast onto cellulose until equilibrium is
attained.
 The higher the substantivity the higher is the exhaustion
 High concentration of Na+ from sodium hydrosulfite and sodium hydroxide
facilitate exhaustion

24. Because of this, many dyes have a rapid strike; dye bath may be 80–90%
exhausted within 10 minutes.
The more rapidly dye exhausts, the greater is the risk of obtaining an unleveled
dyeing.
High electrolyte content Use of retardants

25. Adsorption Isotherm For Vat Dyes On Cotton

26. SUBSTANTIVITY OF LEUCO VAT DYES FOR COTTON
 H-bonds b/n hydroxyl groups and phenolate ion groups, or amino or amide
substituents are important
 Molecules of leuco vat dyes are large and also coplanarity of structure is
essential.

27. OXIDATION AND SUBSEQUENT PROCESS/SOAPING
 Once the dye has been absorbed by the fiber in the form of a leuco derivative, its
oxidation takes place.
The leuco derivative must be oxidized
Any excess hydrosulphite must be eliminated
The excess of alkalinity in the fiber must be neutralized
 Leuco dye is then reconverted into its original form by oxidation.
 Carried out with oxidizing agents like hydrogen peroxide, sodium perborate,
Sodium dichromate or percarbonate.
 The higher the temperature and the lower the pH, the higher the rate of oxidation
reactions

28. Dyes that are easy to reduce are more difficult to oxidize, and vice versa.
Chemical oxidants preferred because they give more rapid and uniform oxidation
throughout the material.

29. SOAPING
 Removes pigment deposited on fiber surface that would wash or rub off in use.
 Involves thoroughly washing goods in a detergent solution at or near boiling
point.
 It improves the light, washing and rubbing fastness of the dyeing.
 Soaping probably involves re-crystallization of oxidized and dispersed vat dye
particles
 Crystallization results in slight change in hue and improvements in fastness
properties.
Neutralizing with acetic acid solution and final rinsing.

30. Two methods:
1. Dyeing with reduced vat ‐ the dye in the form of soluble leuco is put into
contact with the cellulosic fiber, is absorbed by it and then oxidized in the
fiber itself.
2. Dyeing by pigmentation ‐ the insoluble dye in the form of a fine dispersion is
deposited on the surface of the fiber; reduced in an alkaline solution which
causes rapid absorption, finally, the process is continued as in the reduced vat
system.
Dyeing Processes

31. DYEING METHODS
Both batch and continuous methods are used
Batch – (hot method, warm method and cold method) ‐ leveling
problem due to high dyeing rate
Continuous method – mostly used method (most common is pad‐steam
process)

32. Exhaust Dyeing: Typical Vat Dyeing Cycle
Leuco Process
In the leuco process the material to be
dyed is entered into a prepared dye
liquor that contains the fully vatted
dye, alkali (caustic soda) and
reducing agent (hydrosulphite),
together with various amounts of
 Salt,
 Dispersing agent,
 Sequestering agent and
 Levelling agent, as required.

33. High Degree Of Fastness
Pre-oxidation
Ancillary Chemicals
[Chelating Agents, Wetting Agents, Sequesterants , Dispersants]
Over-reduction
Over-oxidation
Effective Pretreatment
General features Of Vat Dye
Washing fastness
Light fastness

34. VAT DYEING FAULTS
 Unlevellness  [inadequate oxidation, salt, temperature]
 Poor penetration  (rapid heating result surface deposition)
 Dull shade  due to over-reduction
 Staining  [inadequate vatting, rinsing & circulation]
 Poor rub fastness  [inadequate vatting, poor soaping]
 Incorrect shade  [Soaping too short or low energy]
 Fiber damage

35. DENIM AND INDIGO VAT
 DENIM is a warp faced twill fabric made from
cotton yarns
 The warp predominates on the surface
 Dyed yarns in the warp and undyed yarns in the
weft
 Small white flecks distributed in a darker basic
color
 JEANS made from blue denim is most popular
 Inexpensive, durable, versatile

37.  Indigo is applied in a series of ‘dips’, with intermediate squeezing and atmospheric
oxidation.
 Reduction of indigo to soluble leuco-indigo
 Multiple dip-squeeze-oxidize of the yarn
 Rinsing, soaping & drying
 Blue-dyed warps wash down to an attractive blue
 No staining on the white yarns
 STONE WASHING FOR FADED LOOK

40. The goods are threaded through each box and skyed
The first box is used to wet out the material.
In subsequent boxes, the goods are immersed in the leuco Indigo solution for
10–30 s, squeezed and skyed for 2 min to oxidise the leuco dye to Indigo.
This process of several dips and oxidations is then repeated in a second series of
boxes, and so on.
Deep shades built up by repeated dipping in the dye bath after each oxidation.
Several rinsing and washing boxes complete the process.

41. SULPHUR DYES
SULPHUR dyes constitute the largest class of dyes in terms of quantity
Like vat dyes applied to cellulosics by redox mechanism
The insoluble pigment is converted into the substantive leuco compound by
reduction and the leuco form is subsequently oxidized inside the fiber.
Chromophore based on sulphur containing compounds
The dyes contain THIAZINES AND THIOZOLES
Limited color gamut [Known for black]
SULPHUR BLACK

42. THIAZOLES
THIAZINES
Sulphur containing aromatic heterocyclic linked by sulphide bonds

43.  On treatment of an aqueous dispersion of the insoluble pigment with sodium
sulphide, the sulphide links are reduced forming individual heteroaromatic units
with thiol groups.
DYEING PROCESS WITH SULPHUR DYES
These are soluble in the alkaline solution in the form of thiolate ions that have
substantivity for cellulose.
After dyeing, the thiolate ions in the fibers can be re-oxidized back to the
polymeric pigment.
Low to moderate substantivity for cellulose
Salt addition to promote exhaustion
RINSED, OXIDIZED SOAPED

44. Reduced
Sulphur dyes are important for black, navy, brown, olive
and green colors in medium to heavy depths, being
relatively inexpensive.

45.  SODIUM SULPHIDE [Na2S] IS USED AS THE REDUCING AGENT

46. Sulphur dyes have the dullest range of colors of all dye classes but are relatively
inexpensive.
When black color and dull shades are needed, with good fastness at reasonable cost,
sulphur dyes are irreplaceable.
Cotton dyed with some sulphur blacks becomes tendered on storing under warm
humid conditions. Formation of sulphuric acid due oxidation of dye.
Jig and pad steam for woven fabrics
Large amounts of sodium sulphide used in the application of sulphur dyes, pose a
significant environmental problem.
OVER-REDUCTION

47. CATIONIC [BASIC] DYEING
 Dyes containing basic functional groups that are protonated
 Organic cations
 Dye fibers with anionic sites by a process of ion exchange
 Mostly applied for acrylic, wool and silk
 Low substantivity for cotton

48.  Dye adsorption by acrylic fibers involves interaction between anionic
sulphonate/sulphate polymer end groups and cationic dye molecules
TRIPHENYL METHANE ANTHRAQUINONE