Introduction Of Dyes

Presented By

Mr. N. P.badgujar

Introduction

Dye: A coloured organic comp. or mixture that may be used

for imparting colour to a substrate such as cloth, paper,

plastic or leather.

Requisites of a True Dye:

Suitable colour

Attractive colour.

Able to attach itself.

Must be water soluble

Substrates being dyes must be affinity to dye.

After fixation of dye, it must be fast to washing, cleaning etc.

Shade & fastness of a given dye may vary depending on the

substrate.

GENERAL INTRODUCTION TO THE CHEMISTRY OF DYES

Dyes possess colour because

1) Absorb light in the visible spectrum (400–700 nm)

2) Have at least one chromophore (colour-bearing group),

3) Have a conjugated system, i.e. a structure with alternating double

and single bonds.

4) Exhibit resonance of electrons, which is a stabilizing force in

organic compounds.

• Most dyes also contain groups known as auxochromes (colour

helpers), examples of which are carboxylic acid, sulfonic acid,

amino, and hydroxyl groups.

• While these are not responsible for colour, their presence can

shift the colour of a colourant and they are most often used to

influence dye solubility.

Dyes versus Pigments

Solubility : organic colourants fall into two classes,

viz. dyes and pigments

The key distinction is that dyes are soluble in water

and/or an organic solvent.

Pigments are insoluble in both types of liquid media.

Dyes are used to colour substrates to which they

have affinity.

• Pigments can be used to colour any polymeric

substrate but by a mechanism quite different from

that of dyes.

Classification Systems for Dyes

1. Chemical Classification:

• The most appropriate system for the classification

of dyes is by chemical structure,

• Advantages:

it readily identifies dyes as belonging to a group

that has characteristic properties.

EX. Azo dyes (strong, good all-round properties,

cost-effective) and anthraquinone, dyes (weak,

expensive).

• Azo yellow, an anthraquinone red, and a

phthalocyanine blue.

Usage Classification:

• Classification by usage or application is the principal

system adopted by the Colour Index. Because the

most important textile fibers are cotton and

polyester.

• The most important dye types are those used for

dyeing these two fibers, including polyester. cotton

blends.

• Other textile fibers include nylon, polyacrylonitrile,

and cellulose acetate.

Classification of Dyes by Use or Application Method

1. Reactive Dyes:

It possible to achieve extremely high washfastness

properties by relatively simple dyeing methods .

Chemical structures are much simpler, their

absorption spectra show narrower absorption bands,

and the dyeing are brighter.

• EX. Azo (including metallized azo), triphendioxazine,

phthalocyanine, formazan, and anthraquinone .

• High-purity reactive dyes are used in the ink-jet

printing of textiles, especially cotton.

2. Disperse Dyes:

Water-insoluble non-ionic dyes for application to

hydrophobic fibers from aqueous dispersion.

Used predominantly on polyester and to a lesser

extent on nylon, cellulose, cellulose acetate, and

acrylic fibers.

Used in Thermal transfer printing and dye diffusion

thermal transfer processes for electronic

photography.

Direct Dyes.

Water-soluble anionic dyes, when dyed from aqueous solution in

the presence of electrolytes, are substantive to, i.e., have high

affinity for, cellulosic fibers.

Uses: dyeing of cotton and regenerated cellulose, paper,

leather, and, to a lesser extent, nylon.

Polyazo compounds, along with some stilbenes,

phthalocyanines, and oxazines.

After treatments, frequently applied to the dyed material to

improve wash fastness properties.

Include chelation with salts of metals (usually copper or

chromium), and treatment with formaldehyde or a cationic dye-

complexing resin.

4. Vat Dyes :

• Water-insoluble dyes are applied mainly to cellulosic

fibers as soluble leuco salts after reduction in an

alkaline bath, usually with sodium hydrogen sulfite.

• The leuco forms are reoxidized to the insoluble keto

forms and after treated, usually by soaping, to

redevelop the crystal structure.

• The principal chemical classes of vat dyes are

anthraquinone and indigoid.

5.Sulfur Dyes:

• These dyes are applied to cotton from an alkaline

reducing bath with sodium sulfide as the reducing

agent. Numerically this is a relatively small group

of dyes.

• The low cost and good wash fastness properties of

the dyeing make this class important from an

economic standpoint.

6. Cationic (Basic) Dyes:

• Water-soluble cationic dyes are applied to paper,

polyacrylonitrile modified nylons, and modified polyesters.

• Using for silk, wool, and tannin-mordanted cotton when

brightness of shade was more important than fastness to

light and washing.

• Basic dyes are water-soluble and yield colored cations in

solution.

• Principal chemical classes are diazahemicyanine,

triarylmethane, cyanine, hemicyanine, thiazine, oxazine,

and acridine.

• Some basic dyes show biological activity and are used in

medicine as antiseptics.

7. Acid Dyes:

• Water-soluble anionic dyes are applied to nylon,

wool, silk, and modified acrylics.

• Used to some extent for paper, leather, ink-jet

printing, food, and cosmetics.

8. Solvent Dyes:

• Water-insoluble but solvent-soluble dyes are devoid

of polar solubilizing groups such as sulfonic acid,

carboxylic acid, or quaternary ammonium.

• Used for coloring plastics, gasoline, oils, and waxes.

• The dyes are predominantly azo and anthraquinone,

but phthalocyanine and triarylmethane dyes are also

used.