Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl. They are a commercially important family of azo compounds, i.e. compounds containing the linkage C-N=N-C. Azo dyes are widely used to treat textiles, leather articles, and some foods.
Azo dyes are organic compounds bearing the functional group R−N=N−R′, in which R and R′ are usually aryl. They are a commercially important family of azo compounds, i.e. compounds containing the linkage C-N=N-C. Azo dyes are widely used to treat textiles, leather articles, and some foods.
These dyes are non ionic in nature and are applied on the fibres which cannot dyed by any other dyes like Reactive dyes, acidic dyes, direct dyes. These dyes make dispersion in the water which form particles. So we have to make soluble it.
These dyes are non ionic in nature and are applied on the fibres which cannot dyed by any other dyes like Reactive dyes, acidic dyes, direct dyes. These dyes make dispersion in the water which form particles. So we have to make soluble it.
Dye is a coloured substance that chemically bonds to the substrate to which it is being applied. This distinguishes dyes from pigments which do not chemically bind to the material they colour. Dye is generally applied in an aqueous solution, and may require aftertreatment to improve the fastness of the dye on the fibre.
Dyes are usually soluble in water whereas pigments are insoluble. Dyes selection is very important when dyeing different types of textile fibre. Types of chemical dyes are Direct, Reactive, Vat, Sulphur dyes etc. All dyes required different chemical auxiliaries and different after treatment process. Each dyes possess different light, washing & rubbing fastness and having different characteristics also.
Bleaching, a process of whitening fabric by removal of natural colour, such as the tan of linen, is usually carried out by means of chemicals selected according to the chemical composition of the fibre.
Styles of printing; Printing thickeners including synthetic thickeners; Printing auxiliaries; Printing of cotton with reactive dyes, wool, silk, nylon with acid and metal complex dyes, Printing of polyester with disperse dyes; Pigment printing; Resist and discharge printing of cotton, silk and polyester; Transfer printing of polyester; Inkjet printing.
IT INCLUDE THE REACTION IN DETAILS OF THE DIAZONIUM SALT AND STUDY OF NITRILR GROUP. UNDER SAOME CRITICAL CONDITION YOU HAVE TO BE CARRIED OUT THE THIS DIAZO REACTION.
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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. • 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
6. • 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
naptholate
7. 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
8. • Time of impregnation
• 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
9. Diazotisatio
n
• 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
10. • 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
11. 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
N=N-Base
ONaa
12. 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
13. 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