Introduction to dyes
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Transcript of Introduction to dyes
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.