Azoic Dye & dyeing process of fabric with Azo dye
What is Azoic Dye ?
Azoic dye are not ready-made dye but are produced by reaction of two compnents Di-azo component or Base/Salt and Coupling component ( Naphthol ).
Dye formation in fibre occurs on the basis of coupling reaction.
RN2Cl +R/-ONa –> R-N=N-R/-OH
The coloured substance formed Azoic dye from these essentially colourless compound is insoluble in water and the washing fastness is excellent.
Chemical structure of azoic dye:
Why they are so called
The dyes containing insoluble azo group(-N=N-) are known as azoic dyes.These dyes are not found in ready made form.
Azoic dyes are produced by a reaction between two components
The components are:
1)Coupling Compoun ( Napthol )
2) Di -azo-compound or diazobase or diazosalt
How they are formed
The coloured substance formed from this colorless compound is insoluble in water and washing fastness of this shade is excellent.
They have excellent coloring properties,again mainly in the yellow to red range, as well as light fastness.
The light fastness depends not only on the properties of the organic azo compound,but also on the way they have been absorbed on the pigment carrier.
Since the application is carried out in an aqueouss medium, the two compounds combine in the solution phase, the pigment is partly loosely deposited on the fibre surface and partly it is suspended in the dye bath,the surface deposition leads to poor rubbing fastness.
Different names of azoic dyes
- Ice color: The coupling component,which is finally soluble in diazotisation reaction,the ice water (0o– 5oC) required to the process, so it is called ice color.
- Mazic color: It is called mazic color in views of its versatility. Diffierent colors are obtained by combining different bases with same naphthol.Again using the same base with different varieties of naphthol,a large no. of different shades can be produced.
- Napthol color: To make the fabric coloured, the fabric is first impregnated with sodium salt of B-napthol (Beta napthol) or arylamides to make substantive towards cellulose, as the first coupling component is naphthol, so it is called naphthol color.
- Pigment color: Azoic dyes contain azo group and the final color product is insoluble in water,so it is called azoic pigment
Base + Napthol=Azoic dyes
Soluble soluble Insoluble
Properties of Azoic dyes
- Cellulosic cotton but also jute,nylon,polyster,hemp acetate,rayon,linen are dyed by azoic dyes
- Insoluble azo group present in it
- It is directly insoluble in water.
- Color is formed from solubilised components but the ultimate colored product is always insoluble.
- Wash fastness-very good.
- Brightness of azoic dye is good
- This dye is called developed dye due to formation of dye in fibre during dyeing process
- Addition of salt increase substantivity
Chemistry of dyeing or Chemical reaction during dyeing with Azoic dye
Dyeing process of azoic dyes includes three main chemical reaction stages. They are:
1)Naphtholation: Naphthols are insoluble in water and converted into soluble form by treating with alkali. The fabric is in pregnated with B(beta)-naphthol and NaOH solution and converted into B(Beta)-Naphtholate.
2) Diazotisation: A base containing amino group reacts with sodium nitrate to a solution of the hydro chloride of the base in the presence of excess of HCL at 0o-5oC
3) Coupling: The impregnated material is treated in a bath containing diazonium solution to carry out coupling and azoic color is produced
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