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, ^C16^H6^Br4^N2^O2
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