Page 416 - Basic Principles of Textile Coloration

APPLICATION OF AZOIC DYES 405

fabric with alkaline brine removes excess coupling solution adhering to the fibres.
If this is not done, azoic pigment forms in excessive amounts on the fibre surfaces
giving a product with reduced fastness to rubbing and washing. The cotton
material containing the naphtholate ion must be kept cool and not allowed to dry
out.

Coupling is carried out at room temperature using an excess of the diazonium
ion component. It is common to add some salt to the development bath to prevent
bleeding of the coupling component into the solution and formation of suspended
azoic pigment. Addition of a non-ionic surfactant assists penetration of the
diazonium ion into the cotton and helps to disperse any pigment that forms in the
solution so that it does not deposit on the fibre surfaces. The rate of coupling
varies widely depending on the electrophilic nature of the particular diazonium ion
and depends on the pH value of the bath. This must be close to the optimum
value. If the solution is too acidic, coupling may be very slow. If it is too alkaline,
the diazonium ion may react with water or decompose. The addition of buffering
salts, such as sodium acetate, bicarbonate or phosphate, helps to neutralise the
strong acid in the diazonium ion bath. The more electrophilic high-energy
diazonium ions have electron-attracting substituents and can couple at pH values
as low as 4–5. Under these conditions, the concentration of free naphtholate ion is
very small. As the electrophilic nature of the diazonium ion decreases, the pH of
the coupling bath must increase to allow a higher concentration of free
naphtholate ion. Low energy diazonium ions only couple in weakly alkaline
solution around pH 8 or higher.

After coupling, the goods are rinsed with a weakly acidic solution and then with
water. Soaping under alkaline conditions with good mechanical action is essential
to remove azoic pigment that has formed or deposited on the cotton fibre surfaces.
This ensures the optimum fastness to washing and rubbing. It also helps to develop
the true shade of the dyeing. During soaping there is often a noticeable change in
shade of the dyeing associated with changes in the physical form of the pigment
particles within the fibres. This may involve both crystallisation and aggregation of
the particles. Soaping helps to achieve the maximum light and chlorine fastness.

Correction of faulty dyeings by stripping is not easy. Treatment with an alkaline
solution of hydros in the presence of a cationic surfactant destroys the colour of
the azoic pigment by reduction of the azo group. It is essential to remove the
primary amines produced to avoid coloration from their oxidation by air. Boiling
alkaline solutions will remove most Naphtols.