A dyes is a coloured substance, which can be applied in solution or dispersion to a substance such as textile fibres (cotton, wool, silk, polyester, nylon) paper, leather, hairs four, plastic materials, wax, a cosmetic base, giving it a coloured appearance.
- Conditions which a dye must satisfy
A substance can be used as dye for the textiles only if it satisfies the following conditions:
- It must have a suitable colour.
- It must be able to fix itself or capable of being fixed to the fabric from the solution.
- When fixed, it must be fast to light resistant to the action or water, soap detergents etc. during washing or the organic solvents during dry cleaning etc.
- Classification of dyes
Dyes are classified either according to their constitution or method of application. The classification can be done as:
- Classification based on constitution
This classification is based on the distinguishing structure units presents in the dye.
- Classification based on application
Depending upon the process of application the dyes are classified as:
- Acid Dyes
- Basic dye
- Direct Dyes
- Disperse Dyes
- Fibre-reactive dyes
- Insoluble azo dyes
- Vat dyes
- Mordant dyes
- Acid Dyes
The sodium salts of azo dyes containing sulphonic acid (-SO3H) and carboxylic acid (CO2H) group are called acid dyes.
These do not have affinity for cotton and hence can not be used to dye cotton. Typical examples of acid dyes are orange –l. orange –ll, methyl organe, methyl red And congo red.
- Basic Dyes
These dyes are the salt of the coloured bases containing amino groups (-NH2 or –NR2) as auxochromes. These include azo and triphenylmethane dyes. Some common examples of this class are aniline yellow, butter yellow, chysodine G and malachite green.
- Direct dyes
These are water-soluble dyes. As the name suggests, these are those dyes which can be directly applied to the fabric from an aqueous solution. These are most suitable for fabric which can form hydrogen bonds with the dyes.
Thus these are usually used for dyeing cotton, wool, silk, rayon and nylon. Example: congored and martius yellow.
- Disperse dyes
These are water insoluble dyes and are applied to the fabric in form of a dispersion of the finely divided dye in a soap solution in the presence of some stabilizing agent such as phenol, cresol or benzoic acid.
Example: (i) Celliton fast pink B and
(ii) Celliton fast blue B.
Illustration 6: Which dye is obtained from the bodies of certain snail?
Solution: Royal purple
- Fibre reactive dyes
These are those dyes which contain a reactive group. Which combines directly with the hydroxyl or the amino group of them, because of the formation of permanent chemical bonds between the fibre and the dye, the colour of the dyed fabric is fast and has a long life. Dyes which are derivatives of 2,4-dichloro-1,3,5-triazine are important example of fibre reactive dyes.
- Ingrain dyes or insoluble azodyes
These are obtained by coupling of phenols, naphthols arylamines, amio-phenols adsorbed on the surface of a fabric with a diazonium salt. As there is only surface absorption of the dye 0n the fabric, the colour is not fast. Example: para red nitroaniline red.
- Vat dyes
Vat dyes are insoluble in water and hence can not be used directly for dying. Therefore, they are first reduced to a soluble colourless in large vats with a reducing from (leucoform) agent. Such as an alkaline solution of sodium hydrosulphite. Under these alkaline condition, the leufoform develops affinity for cellulose fibres. Therefore, these dyes are mainly used to dye cotton fibres. Example: Indigosol O.
- Moradant dyes
These dyes are primarily used for dying of wool in the presence of metal ions. The metal ion binds to the fabric and the dye acting as ligand coordinates to the metal ions. The same dyes in the presence of different metal ions impart different colour to the fabrics. Alizarin imparts rose red, blue brownish red, violet and red colour to the presence of Al3+, Ba2+, Cr3+, Cr2+ and Sr2+ ions respectively.
Illustration 7: b (i) Indigo belongs to which class of dyes?
(ii) Whichdye is used in blue jeans? ‘
Solution(i) Vat dyes
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