Classification of Dyes

Commercial dyes are usually a mixture of large complex and have often unreported molecular structure and diverse properties and also vary widely in chemical composition [12]. There are several ways for classification of commercial dyes. It should be noted that each class of dye has a very unique chemistry, structure, and particular way of bonding. Agreeing with Agustina [36], dyestuff can be classified according to their origin (natural/synthetic), chemical and/or physical properties or characteristics related to the application process. Another categorization is based on the application. Dyes are also classified based on their particle charge upon dissolution in aqueous medium [37] such as cationic (all basic dyes), anionic (direct, acid, and reactive dyes), and non-ionic (dispersed dyes). A systematic classification of dyes according to chemical structure is the colour index [36]. However, due to the complexities of the colour nomenclature from the chemical structural system, the classification based on application is often favourable [32]. The classification based on chemical structure for the common class of the dyes is presented in Table 1, whereas Table 2 represents the classification based on dye application.

Table 1. Functional structure based dyes classification

Class

Functional Groups

Example & Molecular Formula

Azo dyes

Reactive Black 5,

C26H21N5NaA9S6

Anthraquinone

dyes

Reactive Blue 4, C23Hi4Cl2N6O8S2

Indigoid dyes

Vat Blue 5,

C16H6Br4N2O2

Nitroso dyes

Acid green 1, C30H15FeN3Na3O15S3

Nitro dyes

Acid Yellow 24, C10H5N2NaO5

Triarylmethane

dyes

Basic red 9, C19H17N3.ClH

Table 2. Application based dyes classification updated and modified from [38]

Class

Sources

Application Process

Functional Chemicals

Acid

Nylon, wool, silk, paper, inks and leather

Usually from neutral to acidic dye baths

Azo

(including premetallized), anthraquinone, triphenylmethane, azine, xanthenes, nitro, nitroso

Basic

Paper,

polyacrylonitrile, modified nylon, inks, polyester

Applied from acidic dye baths

Cyanine, Diaz cyanine, hemidiazacyanine, diphenymethane, azo, xanthenes

Direct

Cotton, rayon, paper, leather and nylon

Applied from neutral or slightly alkaline baths containing additional electrolytes

Azo, Phthalocyanine, stilbene and oxazine

Disperse

Polyester, polyamide, acetate, acrylic and plastics

Fine aqueous dispersion often applied by high temperature/pressure or low temperature carrier methods

Azo, anthraquinone, styryl, nitro and benzodifuranone

Fluorescent

brighteners

Soaps, detergents, all fibres,paints,oil and plastic

From solution, dispersion or suspension in mass

Naphtha, stilbene, pyrazoles, coumarin

Food, cosmetics and drugs

Food, drug and cosmetics

Azo, anthraquinone, carotenoid and triarylmethane

Reactive

Cotton, wool, silk and nylon

Reactive site on dye reacts with functional group on fiber to bind dye covalently under influence of heat and pH(alkaline)

Azo, anthraquinone, phthalocyanine, oxazine and basic

Sulphur

Cotton and rayon

Aromatic substrate vatted with sodium sulphide and reoxidized to insoluble sulphur-containing products on fiber

Indeterminate structures

Vat

Cotton, rayon, wool

Water insoluble dyes solubilized by reducing with sodium hydrogen sulphite, then exhausted on fiber and reoxidized

Anthraquinone(including polycyclic quinones) and indigoids

 
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