Introduction

A dye can generally be explained as a coloured substance that has an affinity to the substrate to which it is being applied. At the extremely basic level utilize of colour in identifying individual components of tissue sections can be completed primarily through dyes. Even though there are other means, dyes are though, the largest groups that can simply be manipulated to our liking. Dyes are applied to various substrates for instance to textiles, leather, plastic, paper in liquid form.

Definition of Dyes

By definition dyes can be said to be coloured, ionizing and aromatic organic compounds that demonstrate an affinity towards the substrate to which it is being applied. It is usually applied in a solution that is aqueous. Dyes might as well need a mordant to get better the fastness of the dye on the material on which it is applied. A mordant is an element which aids the chemical reaction that takes place between the dye and the fiber so that the dye is absorbed.

Different Classification of Dyes

There are several methods for classification of dyes. It should be noted that each class of dye has an extremely exceptional chemistry, structure and meticulous way of bonding.  While several dyes can react chemically with the substrates forming strong bonds in the procedure, others can be held through physical forces. Several of the prominent ways of categorization is specified hereunder.

a. Organic/Inorganic

b. Natural/Synthetic

c. By area and method of application

d. Chemical classification- Based on the nature of their respective chromophores.

e. By  nature  of  the  Electronic  Excitation  (i.e. energy transfer colourants, absorption colourants and fluorescent colourants).

f. According to the dyeing methods Anionic (for Protein fibre) Direct (Cellulose) Disperse (Polyamide fibres)

Chemical Classification of Dyes

Table: Chemical Classification of Dyes

Industrial Classification of Dyes

Majority of the dyestuff is primarily consumed via the textile industry, so at this level an organization can be done according to their performances in the dyeing procedures. Worldwide around 60% of the dyestuffs are depend on azo dyes that are utilized in the textile ending procedure. Major divisions of dyes in textile finishing are as listed below;

(a) Acid dyes: They are water-soluble anionic dyes that are applied to fibers these as silk, wool, nylon and modified acrylic fibers using neutral to acid dye baths. Attachment to the fiber is characteristic, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber. Acid dyes aren't substantive to cellulosic fibers. Most synthetic food colors fall in this group.

(b) Basic dyes: Such are water-soluble cationic dyes that are mostly applied to acrylic fibers, but find several utilizes for wool and silk. Generally acetic acid is added to the dye bath to assist the uptake of the dye onto the fiber. Basic dyes are as well utilized in the colouration of paper.

(c) Direct or substantive dyeing: This is normally carried out in a neutral or slightly alkaline dyebath, at or near boiling point, through the addition of either sodium chloride (NaCl) or sodium sulfate (Na₂SO₄). Direct dyes are employed on cotton, paper, leather, wool, silk and nylon. They are as well utilized as pH indicators and as biological stains.

(d) Mordant dyes:  This class of dye requires a mordant that improves the fastness of the dye against water, light and perspiration. The choice of mordant is extremely significant as different mordants can transform the final color significantly. Most natural dyes are mordant dyes and there is hence a huge literature base describing dyeing techniques. The most significant mordant dyes are the synthetic mordant dyes, or chrome dyes, utilized for wool; such comprise several 30% of dyes utilized for wool, and are particularly helpful for black and navy shades. The mordant, potassium dichromate, is applied as an after-treatment. It is significant to note that many mordants, particularly those in the heavy metal category, can be hazardous to health and extreme care must be taken in using them.

(e) Vat dyes: They are basically insoluble in water and incapable of dyeing fibres directly. Though, reduction in alkaline liquor creates the water soluble alkali metal salt of the dye, that, in this leuco forms, has an affinity for the textile fibre. Subsequent oxidation reforms the original insoluble dye. The colour of denim is due to indigo, the original vat dye.

Reactive dyes: They utilize a chromophores attached to a substituent that is capable of directly reacting with the fibre substrate. The covalent bonds that attach reactive dye to natural fibers make them among the most permanent of dyes. 'Cold' reactive dyes, these as Procion MX, Cibacron F, and Drimarene K, are very easy to utilize since they can be applied at room temperature. Reactive dyes are via far the best choice for dyeing cotton and other cellulose fibers at home or in the art studio.

Classification Based on the Source of Materials

(a) Natural Dyes: These are dyes obtained from natural sources - plant, animal or mineral. Roots, nuts and flowers that grow in the backyard are all sources of colouring pigments known as Natural Dyes. Many natural dyes had been earlier  created and used at home long before the chemist created dyes in laboratories, there were dyers who extracted colours from flowers, leaves, roots, the outer  and  inner  bark  of  trees  as  well  as  their  heartwood.  An ordinary instance is Indigo.

Indigo 

(b) Synthetic Dyes: The 1^st human-made (synthetic) organic dye, mauveine, was determined through William Henry Perkin in the year 1856. Thousands of synthetic dyes have since been arranged. Synthetic dyes rapidly swapped the traditional natural dyes. They cost less, they offered a huge range of new colours, and they imparted better properties upon the dyed substances.

Classification Based on Application

A number of other classes have as well been established, depend on application that comprises the subsequent:

(a) Leather Dyes - employed for leather.

(b) Oxidation Dyes - Used mainly for hair

(c) Optical Brighteners - Used primarily for textile fibres and paper.

Solvent Dyes - For application in wood staining and production of coloured lacquers, solvent inks, waxes and colouring oils and so on.

(e) Fluorescent Dyes - An extremely innovative dye. Utilized for application in sports good and so on.

(f) Fuel Dyes - As the name suggests it is utilized in fuels

(g) Smoke Dyes - Utilized in military activities.

(h) Sublimation Dyes - For application in textile printing.

(i) Inkjet Dyes - Writing industry including the inkjet printers

(j) Leuco Dyes - Has a broad variety of applications including electronic industries and papers.

Classification based on International Trade Commission

However the most popular classification is the one that is advocated through the US International Trade Commission. This system classifies dyes into 12 kinds:

Type 1: Acridine dyes, derivatives of acridine

Type 2: Anthraquinone dyes, derivatives of anthraquinone

Type 3: Arylmethane dyes

-  Category 1:  Diarylmethane dyes, based on diphenyl methane

-  Category 2:  Triarylmethane   dyes, derivatives of triphenyl methane

Type 4: Azo dyes, based on -N=N- azo structure

Type 5: Cyanine dyes, derivatives of phthalocyanine

Type 6: Diazonium dyes, based on diazonium salts

Type 7: Nitro dyes, depend on a -NO₂ nitro functional group

Type 8: Nitroso dyes, depend on a -N=O nitroso functional group

Type 9: Phthalocyanine dyes, derivatives of phthalocyanine

Type 10: Quinone-imine dyes, derivatives of quinone

-  Category 1: Azin dyes

   Class (a): Eurhodin dyes

   Class (b): Safranin dyes, derivatives of safranin

-  Category 2: Indamins

-  Category 3: Indophenol dyes, derivatives of indophenols

-  Category 4: Oxazin dyes, derivatives of oxazin

-  Category 5: Oxazone dyes, derivatives of oxazone

-  Category 6: Thiazin dyes, derivatives of thiazin

Type 11: Thiazole dyes, derivatives of thiazole

Type 12: Xanthene dyes, derived from xanthene

Category 1: Fluorene dyes, derivatives of fluorene Pyronin dyes 

-  Category 2: Fluorone dyes, based on fluorone Rhodamine dyes, derivatives of rhodamine

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