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Classification of Dyes Based on Application

Dec. 13, 2021

Dyes are chemical substances used to give colour to fabrics, foodstuffs and other objects to beautify and distinguish them. They can be permanently fixed to fabrics/objects and are resistant to the action of water, soap, light, acids and alkalis.

 

Acid dyes - These are azo dyes, characterised by sulphonates or carboxylates. These are usually applied to wool, silk and nylon and have no affinity for cotton. Examples - Orange I, Orange II, Methyl Orange, Martius Yellow and Naphthol Yellow.

 

Basic dyes - These dyes contain unsubstituted amino groups ( -NH 2 ) or substituted forms ( -NR 2 ) as chromophores (colour-bearing groups) or auxiliary pigments (colour-enhancing groups). They form water-soluble cations in acid solutions, which then react with anionic sites on the fabric and thus attach to them. These are used to dye modified nylon, polyester, paper, leather, wool, cotton, etc. Examples - Aniline Yellow and Malachite Green.

 

1-Nitroanthraquinone CAS 82-34-8     

1-Nitroanthraquinone CAS 82-34-8     

 

Direct dyes - These also belong to the azo dye group and are used to dye fabrics directly by placing them in an aqueous solution of the dye. These are suitable for those fabrics that can form hydrogen bonds with the dye. They are used for dyeing wool, silk, nylon, rayon and cotton. Examples - Martius Yellow and Congo Red.

 

Disperse dyes - These are water-insoluble dyes and are dispersed in reagents such as phenol and cresol before being applied to synthetic fibres. They are used to dye nylon, polyester and polyacrylonitrile. Examples- Celliton Fast Pink B and Celliton Fast Blue B.

 

Fibre reactive dyes - These dyes contain a reactive group that binds directly to the hydroxyl or amino group of the fibre. Due to the irreversible chemical reaction, the colouring is fast and long-lasting. They are used for dyeing nylon, wool and silk.

 

Insoluble azo dyes (Ingrain Dyes) - These dyes are synthesised directly on the surface of the fibre. The fabric to be dyed is soaked in an alkaline solution of phenol or naphthol and then treated with a diazotized amine solution to produce an azo dye on the surface of the fabric. The colour imparted by this dye is not very fast. These are used to dye nylon, cotton, silk, polyester, etc. For example - Nitroaniline Red.

 

 1-Aminoanthraquinone CAS 82-45-1

1-Aminoanthraquinone CAS 82-45-1

Reduction dyes - These are insoluble dyes that are first reduced to a colourless, colourless compound in a wooden barrel by an alkaline reducing agent such as sodium hyposulphite and then applied to the fabric. After a period of time, the fabric is exposed to air or oxidising agents and insoluble coloured dyes are formed on the fabric. These are used to dye cotton fibres. Examples - Indigo.

 

Mordant dyes - These dyes do not dye the fabric directly, but require a mordant as a binder between the dye and the fabric. For acid dyes, the mordant is a metal hydroxide and for basic dyes it is tannic acid. The mordant adheres to the fabric and then interacts with the dye to form an insoluble coloured complex called a colour deposit. Depending on the mordant used, the dyestuff will produce different colours. Example - Alizarin (a mordant dye) is red with Al and Sn salts, brownish red with Cr mordants and blackish purple with Fe mordants.

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