2. Introduction
• Azo Dyes are not ready made dyes but are produced within the substrate
using two different components i.e. “napthols” and “bases” (also known
as “coupling components” and “diazo components”)
• The coloured compound formed by the chemical combination of two
colourless component is insoluble in water and therefore exhibits
extremely good fastness to wet treatments
• About 24 amines are banned as they are carcinogenic
3. Properties of Azo Dyes
• Azo dyes are insoluble in water
• They are not ready made dyes. The colour is formed in the fibre by two
components usually referred to as “napthols” and “bases”
• The dyed goods have very good to excellent light fastness
• The dyed goods have good washing fastness
• The dyed goods suffer from poor rubbing fastness and this can be
overcome by proper soaping treatment
4. Mechanism of Dyeing
• The steps involved in dyeing cotton is stated below
– Impregnation with solution of napthol. This step is called
“naptholation”
– Development of the chromophore (azo group). This is the
“diazotisation” step
– Development of coloured compound. This is the “coupling” step
– Removal of the surface colour from the dyed fibre. This step is called
“soaping”
5. Naptholation
• Napthols are insoluble in water
• They can be dissolved in water by converting them into sodium compounds,
which are soluble in water
• For this caustic soda is used
Napthol – OH + NaOH Napthol – ONa
(Insoluble in water) (Sodium salt of napthol – soluble in hot water )
• Treat the material with the napthol solution in room temperature for 20 – 30
minutes
• In some cases the addition of common salt or Glauber salt increases the
absorption
• Squeeze the naptholated goods evenly, so as to leave it uniformly naptholated
6. Factors affecting Naptholation
• Substatntivity
– Low substantivity napthols
– Medium substantivity napthols
– Higher substantivity napthols
– Still higher substantivity napthols
• Concentration of the impregnation bath
– The depth of the shade in azoic dyeing is directly proportional to the
amount of napthol fixed on the fibre which in turn depends on the
concentration of the bath and M:L ratio
• Time of impregnation
7. • Temperature
– Affinity of napthol is more at lower temperature and decreases with
increase in temperature
– The best temperature of impregnation is 20 – 250C
• Addition of common salt / glauber salt
8. Diazotisation
• Bases are insoluble in water
• They are converted into water soluble salts by using hydrochloric acid
Base – NH2 + HCl Base – NH3
+Cl-
• Base is converted into diazonium salt, when the hydrochloride of the base
is treated with nitrous acid at low temperature for 20 – 30 minutes
• This is known as “diazotisation”
HCl + NaNO2 HNO2
Base – NH3
+Cl- + HNO2 Base – N=NCl + 2H2O
9. • The diazotised base in the state will be highly acidic due to excess HCl in it
• For good coupling a pH of 5 -6 is required
• Sodium acetate is added to convert the excess HCl into weak acid (acetic
acid)
CH3COONa + HCl CH3COOH + NaCl
• Coupling will also be slowed down if the liquor becomes alkaline which
can occur easily on account of NaOH brought into the liquor by the
naptholated substrate
• This excess alkali can be neutralised by adding an alkali – binding agent
e.g. aluminium sulphate
10. Coupling or Development
• The naptholated material is entered and developed in the base solution
for 15 – 20 minutes at room temperature
• This process is called “coupling” or “developing”
• The pH should be maintained 5 to 6 during this step
Napthol – ONa + Base – N=N-Cl Napthol + HCl
ONa
N=N-Base
11. Soaping
• After dyeing, the material is rinsed in cold water to which 1 g/l HCl is
added and then it is soaped at the boil for 30 minutes to remove unfixed
dye
• The soaping process is important to increase the rubbing fastness of the
dye
12. Fastness properties of Azo dyes
• Azoic dyes have good to very good to excellent light fastness which is
attributed to the very stable electron arrangement of the dyes
• Their washing fastness is also very good due to the fact that azoic dyes are
insoluble in water
• Azoic dye suffer from poor rubbing fastness due to the formation of
insoluble azoic dye on the surface of the fibre
