Question

Write a note on (any 2) of the following :
(a) Direct dyes
(b) Azoic dyes
(c) Disperse dyes

Hint:

Direct dyes colour cotton directly with salt; disperse dyes colour polyester at high temperature.

Answer 1

Step 1: (a) Direct dyes: what they are:
Direct dyes are water-soluble dyes that are applied straight to cellulosic fibres such as cotton, linen, jute and viscose.
They need no mordant, hence the name 'direct'.

Step 2: Direct dyes: application:
The dye is dissolved in water, and salt (common salt or Glauber's salt) is added to push the dye into the fibre.
Dyeing is done at or near boiling point, then the cloth is rinsed.

Step 3: Direct dyes: properties:
They are cheap, easy to apply and available in many bright shades.
Wash fastness is poor, so an after-treatment with a cationic fixing agent or copper salts is often given.
Light fastness varies from poor to good.

Step 4: (c) Disperse dyes: what they are:
Disperse dyes are non-ionic dyes with very low solubility in water.
They are used for hydrophobic (water-repelling) fibres: polyester, acetate, nylon and acrylic.

Step 5: Disperse dyes: application:
The dye is ground to a fine powder and spread in water with a dispersing agent.
Polyester is dyed at about 130 degrees C under pressure (HTHP), or by the carrier method at the boil, or by thermosol at about 200 degrees C.
Acetic acid keeps the bath at pH 4.5 to 5.5.
After dyeing, a reduction clearing wash removes loose dye from the surface.

Step 6: Disperse dyes: properties:
They give bright shades and good light and wash fastness on synthetics.
They can sublime (turn to gas) at high heat, which can stain other cloth.

Final Answer:
Direct dyes colour cotton and other cellulosic fibres directly from a salt bath; disperse dyes colour polyester and other synthetic fibres from a fine dispersion at high temperature.