Classification of dyes

DYES AND ITS CLASSIFICATION

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Introduction

• Dyes are colored organic compounds that are used to impart color to various substrates, including paper, leather, fur, hair, drugs, cosmetics, waxes, greases, plastics and textile materials.

• A Dye is a colored compound, normally used in solution, which is capable of being fixed to a fabric.

• To be of use, dyes must possess the following four properties:

• 1. Colour • 2. Solubility in water • 3. Ability to be absorbed and retained by fibre (substantivity)

or to be chemically combined with it (reactivity). • 4. Ability to withstand washing, dry cleaning and exposure

to light.

• The dye has a colour due to the presence of chromophore and its fixed property to the acid or basic groups such as OH,SO3H, NH2, NR2, etc.

• The polar auxochrome makes the dye water-soluble and binds the dye to the fabric by interaction with the oppositely charged groups of fabric structure.

Classification

• There are several ways for classification of dyes. • Each class of dye has a very unique chemistry, structure

and particular way of bonding. While some dyes can react chemically with the substrates forming strong bonds in the process, others can be held by physical forces. Some of the prominent ways of classification are:• Classification based on the source of materials• classification of the Dyes- Based on the nature of their

respective chromophores.• Classification by methods of application.

Classification based on the source of materials

• A very common classification of the dyestuff is based on the source from which it is made. Accordingly the classification could be:

• Natural Dyes• Synthetic Dyes

Natural Dye• Natural dyes are dyes or

colorants derived from plants, invertebrates, or minerals.

• The majority of natural dyes are vegetable dyes from plant sources. E.g. roots, berries, bark, leaves, and wood.

• Other organic sources include fungi and lichens.

Synthetic Dyes• Almost all the colors that you see today are Synthetic dyes.

Synthetic dyes are used everywhere in everything from clothes to paper, from food to wood. This is because they are cheaper to produce, brighter, more color-fast, and easy to apply to fabric.

• E.g. Acid Dyes, Azo Dyes, Basic Dyes, Mordant Dyes, etc

Azo dye testing

Classification based on the Chromophore present• Dyes may be classified according to the type of chromophores

present in their structures. 1. Nitro and Nitroso Dyes2. Azo Dyes3. Triarylmethane Dyes4. Anthraquinone Dyes5. Indigo dyes

Nitro and Nitroso Dyes• These dyes contain nitro or nitroso groups as the

chromophores and –OH as auxochrome. • A few examples are:

Naphthol yellow SMordant green 4

Azo Dyes• Azo dye is a large class of synthetic organic dyes that contain

nitrogen as the azo group −N=N− as primary chromophore their molecular structures. More than half the commercial dyes belong to this class. These dyes are highly coloured and are prepared by diazotizing an aromatic amine and coupling with suitable aromatic compound.

Para Red

Methyl Orange

Congo Red

Bismarck Brown

Azo dyes account for approximately 60-70% of all dyes used in food and textile manufacture. In theory, azo dyes can supply a complete rainbow of colours, but yellow/red dyes are more common as blue/brown dyes.

Triarylmethane Dyes• Triarylmethane dyes are synthetic organic

compounds containing triphenylmethane backbones. These compounds are intensely colored and are produced industrially.

• In triarymethane dyes a central carbon is bonded to three aromatic rings, one is in the quinoid form. Auxochromes areNH2, NR2 and OH.

Malachite Green

• Malachite Green is used as a direct dye for wool and silk.• Phenolphtalein is used as acid base indicator

Phenolphthalein

Anthraquinone Dyes• Anthraquinone dye, any of a group of organic dyes having

molecular structures based upon that of anthraquinone.

• Alizarin is the main ingredient for the manufacture of the madder lake pigments known to painters as Rose madder and Alizarin crimson.

• Alizarin is also used commercially as a red textile dye.

Alizarin

Indigo Dyes• Indigo dye is an organic compound with a distinctive blue color

Historically, indigo was a natural dye extracted from plants. But today nearly all indigo dye is produced synthetically.

• It contains carbonyl chromophore.

• The primary use for indigo is as a dye for cotton yarn, which is mainly for the production of denim cloth for blue jeans

• Small amounts are used for dyeing wool and silk.

Indigo

Classification by methods of application

• Method used for application depends on the nature of both….Dye and Fabric.

• They are classified on the basis of technique employed for their application.

Direct Dyes• Carried out in a neutral or slightly alkaline dye solution.

• It contains acidic or basic auxochromes.

• Polar in nature.

• Used on cotton, paper, leather, wool, silk and nylon.

Martius yellow

Acidic Auxochrome

Fiber-NH2 + HO-Dye Fiber-NH3+……….-O-Dye

Vat dyes

• Insoluble in water.

• Soluble in sodium hydrosulfide (Na-S-H). Great affinity for cotton and fibers

• Indigo is a good example of Vat dyes.

• -OH binds the dye fast to cellulose fiber that contains ethereal oxygen and –OH group.

Mordant dyes• No natural affinity………used with the help of salts.• Mordants – Al or Cr oxides salts.

• Fiber is first treated with mordant & then with dye solution. (insoluble coordination complex between fiber and dye)

• Most suitable for wool and nylon.

Azoic dyes

Azoic dyes• Water insoluble.

• two components react to produce the dye-usually Phenol or Napththol or Aniline.

• This method of dyeing cotton is declining in importance due to the toxic nature of the chemicals used.

aniline Phenyl azo 2-napthol

Orange-red dye

Disperse dye• Insoluble in water……..but colloidal form can be formed.

• These colloidal fine particles are absorbed into the crystal structure of fabric.

• Used to dye nylon,orlon,polyesters and cellulose acetate.