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  Dyes, Chemistry tutorial

Introduction:

The Dyes industry plays a significant role in the total growth of chemical industry. The preparation and usage of dyes is one of the traditional forms of human activities. The very first synthetic organic dye, mauveine, was discovered via William Henry Perkin in the year 1856. Synthetic dyes now accessible in various shades or colours are amongst the oldest chemicals produced by man. Dyes are now available in broad variety of shades to meet up the requirement of various shades in all materials. Textile, garment and carpet industries are the main consumer of dyes. The growth of the textile industry consists of a direct impact on the growth of dyes industry.

Dyes:

Dyes are colored, ionizing and aromatic organic compounds which consist of affinity to the substrate to which it is being applied. Dyes are usually applied in an aqueous solution which might as well require a mordant for enhancing the fastness of the material on which it is applied.  Different hazardous waste stream is produced throughout manufacture of dyes and intermediates. Most of the dyes such as azo dyes, benziidine dihydrochloride and benzidine based dyes have been banned in several parts of the world. The environmental pressure has necessitated changes in the process Chemistry and technology and use of safer and eco-friendly raw materials.

Classification of Dyes:

1) Based on source of materials:

Dyes are categorized based on the source from which it is made up into natural dyes and synthetic dyes. 

2) Based on Dyeing process:

Dyes are categorized based on dyeing process into acid dyes, basic dyes, direct or substantive dyes, fibre-reactive dyes, vat dyes, disperse dyes, azo dyes, sulphur dyes, mordant dyes and so on.

3) Based on Chromophore:

Dyes are categorized on the basis of nature of their chromophore to acridine dyes, Anthraquionone dyes, azo dyes, diazonium dyes, nitroso dyes, arylmethane dyes, phthalocyanine dyes, indophenols dyes, azin dyes, oxazin dyes, oxazone dyes, thiazin dyes, thiazole dyes, fluorine dyes and rhodamine dyes and so on.

4) Based on Application:

Based on application, the dyes are categorized into food, laser dyes, cosmetics and drug dyes, leather dyes, solvent dyes, contrast dyes, carbine dyes and so on.

5) Based on nuclear structure:

Dyes on the basis of their nuclear structure can be categorized into cationic dyes and anionic dyes.

6) Based on colour index:

Colour index is compendium of dyes where each and every colorant is assigned a generic name and a constitution number. The colour index identifies twenty six kinds of dyes by chemical classification. 

Chromophore and Auxochrome:

Dyes are fundamentally ionizing and aromatic compounds having chromophores that make the dyes proficient in their capability to absorb radiation. Auxochrome is a group of atoms joined to a chromophore. This is responsible for giving solubility and cohesiveness to the dyes.

Dyeing process-based dyes:

1) Acid dyes:

Acid dyes are the water-soluble anionic dyes and insoluble in acid bath. They are employed for dyeing of wool, nylon, silk, acrylic fibre, paper and leather. 

2) Basic dyes:

Basic dyes are the water-soluble cationic dyes. They are generally amino and substituted amino compounds. Basic dyes are employed for dyeing acrylic fibre, wool, cotton, paper and so on.

3) Direct dyes:

Direct dyes are employed in a neutral or slightly alkaline dye bath without addition of mordant. They are employed for dyeing cotton, wool, silk, paper and nylon. They are usually azo dyes. 

4) Disperse dyes:

Disperse dyes are employed for dyeing synthetic fibres such as cellulose acetate, polyesters, nylon and acrylic fibres. They are applied as finely divided materials in the presence of dispersing agent.

5) Vat dyes:

Vat dyes encompass highly complex structures and are insoluble in the water. They are employed after reduction in alkaline liquor that generates water-soluble alkali salt.

6) Reactive dyes:

The reactive dyes react to form covalent bond that directly react by the fibre and give excellent wash resistance.

7) Mordant dyes:

A mordant is the chemical which joins with the dye and fibre generally to enhance the fastness of the dye. As the principal modern mordants are dichromate and chromium complexes, mordant dye generally signifies chrome dye. Most of the mordant dyes result various colours by different mordants. They can be employed with wool, silk, wool blends, cotton and some modified-cellulose fibres.

8) Azoic dyes:

Azoic dyes are dyes which are not applied directly however are produced in the fibre itself. In azoic dyeing, colours are made on the fibre via coupling diazotized materials whereas in contact with fibre. Final colour is controlled through the choice of diazoic and coupling components.

9) Sulphur dyes:

Sulphur dyes are most generally employed dyes manufactured for cotton in terms of volume. They are prepared by reacting sulphide and polysulphides by chlorinated aromatics.

Production process of Dyes:

Dyes are synthesized in the reactor, filtered, dried and blended by other additives to generate the final product. The synthesis step comprises reactions like sulphonation, halogenation, amination, diazotization, and coupling followed via separation methods which might comprise distillation, precipitation and crystallization. In common, organic compounds like naphthalene are reacted by an acid or an alkali all along by an intermediate (like a nitrating or a sulphonating compound) and a solvent to make a dye mixture. The dye is then separated from the mixture and purified. On completion of the manufacture of actual color, finishing operations comprising drying, grinding and standardization are performed. These are significant for maintaining the consistent product quality.

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