Textile

1. Acid Dye
Properties of Acid Dyes
1. Acid dyes are called so because their
application happens in acidic pH. It is also
consistent with the fact that they are salts
organic acids.
2. Acid dye produces a wide range of brilliant
shades.
3. They do not have any affinity for cellulosic fiber
4. They have good affinity for protein and
polyamide fibers like wool, silk and nylon.
5. Chemically acid dyes resemble direct dyes.
They are generally represented as DSO3Na. After
dissolution they produce a coloured anion
(DSO3-) and cation (Na+) where D represents
chromogen part of the dye molecule.

2. Characteristics of Acid Dyes
Characteristics Levelling acid
dye
Milling Acid dye Super Milling
Acid Dye
Chemical Added
to dye bath
Sulphuric acid Acetic acid Ammonium
acetate
pH of the dye
bath
2-4 4-6 6-7
Levelling
property
Good Moderate Very poor
Fastness to wet
treatment
Poor Good Very good
Molecular
weight
Low High High
Solubility in
water
High Low Low
State of division
in water
Molecularly split Colloidal Colloidal
Affinity to silk
and wool
Low (wool>silk) High (wool>silk) Very High
(wool>silk)

3. Factors Affecting Acid Dye on Protein Fiber
1. Effect of Electrolyte -
a. Salt as retarding agent – Salts function as retarding agent for acid dyes that
need to be used at lower pH values i.e. molecularly split acid dyes or levelling
type of acid dyes.
b. Salt as exhausting agent – In case of milling and supermilling acid dyes which
are applied from a neutral bath, the addition of salt promotes exhaustion. The
presence of electrolyte reduces negative charge on surface of fiber and enables
movement of dye into the fiber.
2. Effect of temperature – Increase in temperature increases rate of exhaustion. For
milling and super milling dyes exhaustion starts at 60 C and reaches maximum at 70 C.
3. Role of acid –
• Protein fibers contain amino acid and are represented by
H2N-W-COOH (wool) H2N-S-COOH (silk).
• In aqueous solution they become
+H3N-W-COO-
• In presence of acid anionic potential of the protein fiber is neutralised
HCl – H+ + Cl-
+H3N-W-COO- + H+ + Cl- -Cl+H3N-W-COOH

4. •This facilitates incorporation of anionic dye molecule in the modified fiber as below
-Cl+H3N -W-COOH + D-SO3- D-SO3H3N-W-COOH
The process is fast in case of levelling type of dyes and further electrolyte is added
to control the rate of exhaustion . In case of milling and super milling type of acid
dyes salts are added to facilitate ionisation of dyes in the solution which otherwise
remain in colloidal form in the aqueous solution.
Application Of Acid Dye On Wool