The document discusses azo dyes, which are a class of dyes containing the azo group (-N=N-) and are produced through a reaction between a coupling compound and a diazo compound. Azo dyes are primarily used in dyeing cellulosic materials due to their excellent light fastness and brightness but require a two-bath dyeing process involving an impregnation bath and a developing bath. Additionally, advancements in azo dye technology, such as piezoelectrochemical effects for dye degradation, are also explored.

1. Bahir Dar University
Ethiopian Institute of Textile and Fashion
Technology(EiTEX)
Azo dyesAzo dyes
Prepared By: Bademaw Abate

2. Introduction
The dyes containing insoluble azo group-N=N-)
are known as azoic dyes.
These dyes are not found in readymade form.
Azoic dyes are produced by a reaction between
two components. The components are:
1. Coupling Compound ( Naphthol)
2. Di-azo –compound or diazo base or diazo
salt.
The colored substance formed from this
colorless compound is insoluble in water and
washing fastness of this shade is excellent.

3. Cont….
• Azo dyes contain the characteristic azo group –N=N–
which is present to a greater or lesser degree innearly all
classes of dyes.
• Over half of all commercial dyestuffs belong in this
general category.
• By varying the chemical composition it is possible to
produce acid, basic, direct, reactive, disperse and mordant
dyes containing the azo chromophore.

4. Monoazo dyes with one azo group, e.g. the acid
dye, Orange II:
Disazo dyes with 2 azo groups must not be confused with
Diazotizable direct dyes. Most of the substantive dyes belong to
this class, e.g. DiaminBrown M:

5. Properties/significance of Azoic Dyes:
● These dye stuffs always contain Azo groups in its
chemical structure.
● Light fastness property is admirable.
● Brightness of shade is also admirable.
● It is directly insoluble in water.
● These dye stuffs are always used in dyeing cellulosic
material.
● Here dyeing operation is completed by two bath
arrangement. One is called impregnation bath and
another is called developing bath

6. Cont…
● Alkali resistance is poor to good, Index 2 – 4.
● Suitable for lighter shade dyeing (Light resistance:
Poor to very good, Index 2 – 7).
● This dye stuff is called developed dye due to● This dye stuff is called developed dye due to
formation of dye in fiber during dyeing process.
●Addition of salt increases the substantivity.
● Napthol dispersed in alcohol, T.R. oil.

7. Mechanism
The synthesis of an azo dye requires two organic
compounds- a diazonium salt and a coupling component.

9. Dye molecules bind to the fiber molecules in cloth in one of the
following ways:via electrostatic attraction, by van der Waals forces,
through hydrogen bonding or even by covalent bonds. In general, the
greater the number of polar groups that are present in the fiber
molecule, the easier the fabric is to dye.

10. Why is azoic color called ‘Ice’ and
‘Magic’ color?
Ice Color:
• The coupling component is finally soluble in diazotization
reaction.
• Diazotization is a chemical reaction where the base (Aromatic
Amine) is to be converted to a solubilized form by theAmine) is to be converted to a solubilized form by the
chemical reaction with NaNO2 at ice temperature (0 – 5˚C).
• That’s why this dye is called Ice color

11. Cont…
Magic Color:
• Two steps are required by dyeing with azoic color.
• In the first step textile goods are dyed by Napthol
color.color.
• In the 2nd bath, Base or Salt is used for dyeing. After
2 – 3 minutes dyeing in 2nd bath, the azoic color is
seen in the textile goods magically. For this reason
this dye is called Magic color.

12. Why Azoic dyes are called ‘Napthol’
and ‘Pigment’ color?
• Napthol color:
• Azoic coupling components are insoluble in water. To
make them soluble in water the textile materials are
impregnated in a solution of Napthol and NaOH. As
the first coupling component is Napthol color.the first coupling component is Napthol color.
• Base + Napthol → Azoic dyes
• Soluble Soluble Insoluble
• Azoic dyes contain Azo group and final color is
insoluble in water, so it is called Azoic Pigment.

13. General Procedure for dyeing cellulosic
material with Azoic Dyes:
Two bath of individual recipe is used for dyeing
with azoic dyes.
1. Impregnation Bath.
2. Developing Bath.2. Developing Bath.

14. Impregnation Bath (1st bath):
Recipe:
• Dye Stuff → 3% (According to the wt of the
material)
• T.R. oil → 2%• T.R. oil → 2%
• NaOH → 2%
• Hot water → 3 times
• Cold water → 7 times
• HCHO → 1.5%
• Temperature → 40˚C – 50˚C
• Time → For a few minutes (2 – 3)

15. Developing Bath (2nd bath):
Recipe:
Salt Bath
……………………..
Fast salt → 6%
Hot water (30˚C – 40˚C) → 2 times
Base Bath
…………………
Fast base → 3%
HCl → 2.5% - 3%
NaNO → 1.5%Hot water (30˚C – 40˚C) → 2 times
Cold water → 8 times
NaCl → 2% – 3%
Time → 2 – 3 minutes
NaNO2 → 1.5%
Cold water → 10 times
Temperature → 0˚C – 5˚C
Time → 15 (2 – 3) minutes
In major cases, we use base bath.
It gives very good color in low cost though it is insoluble in
water.

16. Function of different chemicals used in
Napthol Dyeing:
Caustic Soda:
Soluble the dye material.
Produce alkali medium.
T.R. oil:
Used as softening agent.Used as softening agent.
Used as dispersing agent.
HCHO:
Ensure the actual level dyeing properties.
For proper dyeing.
Salt:
To increase the substantivity of the azoic color in textile
materials.

17. Procedure:
• Dissolve NaOH in small quantity of hot water with
Napthol & T.R. oil in a bath and make a paste.
• Then add slowly in it, the rest hot water and stir (mix)
constantly.
• After dissolving color perfectly and above mentioned• After dissolving color perfectly and above mentioned
cold water and finally the solution of Napthol is thus
obtained, this is cooled to 50˚C if necessary, HCHO may
be added in the impregnated bath.
• Now the bath is ready for steeping textile goods for a
few minutes (In this bath, any naptholated goods,
excepting the napthol AS – G will look yellow).

18. Cont..
• In the developing bath salt is dissolved with hot water
and then cold water added in the bath.
• Finally add NaCl in the developing bath. Now
treating the above naptholated or impregnated goodstreating the above naptholated or impregnated goods
for 2 – 3 minutes in this bath.
• It will be noted that the color will develop the goods
magically.
• Then squeeze the goods and boiling it in a soap bath
followed by washing & drying.

19. • Now steep the textile goods for a while after
developing a bath contains 1.5% of HCl in times of
water.
• It is done only for neutralization of goods. Finally 3%• It is done only for neutralization of goods. Finally 3%
soap and 1.5% soda is taken in a bath containing 15
times and treat the goods for a few minutes and wash
the goods in fresh plain water.
• Then squeeze and dry.

20. APPLICATION OF AZOIC DYES TO COTTON
Reaction of Azoic Dye
Naphtholation:
Naphthols are insoluble in water and they are converted into
water soluble compound by treating with alkali.
Diazotization:
A base containing amino group (-NH2) reacts with the NaNO2
(Sodium Nitrite) to form a solution of diazonium chloride of
that base in presence of excess HCl at 00-50C temperature.that base in presence of excess HCl at 00-50C temperature.
Coupling/ Developing:
The impregnated material is treated in a bath containing
diazonium solution to carry out to coupling and thus color is
produced inside the fabric. The PH maintaining is important.
R N=N Cl+ R’ O Na = RN=NR’OH +NaCl

21. Cont…
• During dyeing of protein fiber (Silk, Wool)
with azoic dye, we should be very careful not
to damage the protein fiber due to high alkali
concentration.concentration.

22. New Advancement of Azoic
• A newly discovered piezoelectrochemical
(PZEC) effect for the direct conversion of
mechanical energy to chemical energy is
applied for the mechanically inducedapplied for the mechanically induced
degradation of a dye of C.I. Acid Orange
(AO7) from aqueous solution in the presence
of BaTiO3 microdendrites.

23. • The mechanism of the PZEC degradation of
the dye depends on the ultrasonic vibration
used, in which the formation of the strain-
induced electric charges on the dendrite
surface is due to the deformation and localsurface is due to the deformation and local
charge accumulation on the BaTiO3.
• With sufficient applied electric potential,
strained piezoelectric dendrites in AO7
aqueous solution triggered the decomposition
reaction

24. A New Mechanism for Azo Dye Decolorization in Aqueous
Solution through Vibrating Piezoelectric Microfibers

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