Dyes are used to impart color to materials. There are two main types - dyes and pigments. Dyes are soluble organic compounds like synthetic dyes which can contain harmful chemicals. Pigments are finely ground insoluble particles. Natural dyes are plant-based or from insects but fade faster. Direct dyes are water-soluble and used for direct coloring. Azo dyes contain nitrogen groups and are made through a chemical reaction involving diazotization. Indigo is extracted from plants and involves reduction and reoxidation reactions. Disperse dyes are insoluble but used for polyester and require high temperatures.

2. Dye
Substance used to
import colour to
various materials
Organic compounds
(carbon)
Pigment
Finely ground solids
dispersed in a liquid or
blended with other
materials
Inorganic or organic
compounds
Give brighter colours
 Synthetic dyes can be harmful:
mercury, lead, chromium,
copper, sodium, chloride, toulene,
benzene
 Differnet things used to dye
fabric:
boiled shellfish, plants, mud… Natural dyes:
Plant-based, insects
Fade faster

3. DIRECT COLORING
 Direct dye = substantive dye
 water soluble compounds
 benzidine derivatives
 cheap, easily applied, bright colours
 positively and negatively charged groups
 Methyl orange
 Congo red:
 belongs to a group of azo dyes derived from benzidine
 synthetic dye
 mordant – a substance used to fix the colour to cotton fibers
 histology, acid-based indicator

4. COLORING AS A RESULT OF A
CHEMICAL REACTION
 Azo dye is a synthetic organic dye, containing
nitrogen as the azo group (-N=N-)
 Several categories, defined by fibers or methods
of application
aromatic amino
compounds
diazotization
Attachment of
an intermediate
to a phenol or
aniline

5. Indigo dye
 Organic compound
 Extracted from the leaves of Indigofera and Isatis
 Indican
 Simpler compounds:
 reduction to the soluble yellow leucoindigo
 reoxidized to indigo

6. DISPERSE DYES
 Water-insoluble dyes
 Dye polyester and acetate fibers
 The smallest molecules
 Based on an azobenzene or anthraquinone molecule
 High temperatures
+ nitro, amine, hydroxyl… groups
attached