Acid dye are easily applied on wool, silk and nylon fibres.
During dyeing, H2SO4 or CH3-COOH is present
due to perform dyed. They are water soluble and produce color anion and sodium
cation.
RSO3Na ---------- RSO3- +Na+
Wool fibre polymer may be represent as:
H2N-W-COOH ------------- Wool
So there are present amino group of polymer part and other
part contain carboxyl group.
Under certain condition, The hydrogen ion attached to the
carboxyl group is transferred to the amino group at the other end of the
macromolecule so that the two ends of the keratin molecular chain acquire
opposite electrical charges:
H2N-W-COOH----------------H3N+-W-COO-
Thus when wool fibre is entered in a acid solution,
negatively charged carboxylate group take up hydrogen ions released from
hydrochloric acid group and the chloride anions, also released from HCl are
absorbed by positively charged amino ends of the keratin macromolecule.
HCl ====H+ + Cl-
NH3+-W-COO- H+
Cl- ========= ClNH3-W-COOH
When wool is immersed in acid solution, the concentration of
acid is decreased by the absorption of acids with the formation of an
electrically positive site whenever an amino group is present in the keratin
molecule i.e. acids act as exhausting agents while dyeing acid dyes on wool. This
absorption continues until equilibrium absorption and sodium chloride is formed
as a result of forward reaction.Thus if the salt reaction is increased in the
dye bath, the rate of backward reaction is increased. In fact act as a
retarding agent in the dyeing of acid dyes on wool. In order to get uniform
dyeing results the dyeing is started in the presence of salt with a small
amount of an acid in the cold, followed by raising the temperature gradually.
In order to exhaust the dye bath more of the acid is added to the dye bath and
the dyeing aontinued..
Dyeing process sequence:
Dyeing
Neutralization
Soap wash & boiling