Page 418 - Basic Principles of Textile Coloration

FASTNESS PROPERTIES OF AZOIC DYEINGS ON COTTON 407

In azoic dyeing, the dyer must choose appropriate components to produce the
required shade. He must select the method for application of the coupling
component and calculate the initial concentration required in the bath. This
usually involves considering which Naphtols have the appropriate substantivity for
the application method selected and which will give a shade close to the desired
colour with appropriate diazonium ion developers. A method of preparing the
diazonium ion solution must be selected and its concentration calculated. Finally,
the amounts of alkali for the coupling component solution and the amounts of
acid and buffers for the development bath must be calculated. Without prior
experience, this is a formidable task given today’s greater emphasis on being able
to dye on shade at the first attempt. Fortunately, the chemical suppliers provide
much practical information to make these steps as easy as possible. Even so, the
dyer must optimise the diazotisation and coupling steps and pay attention to the
conditions of soaping to obtain a dyeing with the correct shade and optimum
fastness.

19.4 FASTNESS PROPERTIES OF AZOIC DYEINGS ON COTTON

Correctly prepared dyeings with azoic combinations on cotton have fastness
properties often comparable, or only slightly inferior, to those produced using
quinone vat dyes. They complement the vat dyes because of the wide range of
orange, red and bordeaux shades that they provide.

The fastness to washing of azoic combination dyeings on cotton is usually very
good to excellent but only after careful elimination of particles of azo pigment
loosely adhering to exposed fibre surfaces. Intermediate drying or rinsing of fabric
containing the Naphtol, and the soaping of the final dyeing, are key processes
ensuring optimum fastness. The same argument applies to rubbing fastness. Deep
dyeings that have not been well soaped easily transfer colour onto adjacent white
fabric, even under conditions of gentle rubbing.

There are two other problems associated with the fastness properties of azoic
combinations on cotton. In pale shades, the dyeings often have much reduced
light fastness, particularly under humid conditions. Some sensitive azoic
combinations also give dyeings of only fair resistance to chlorine and peroxide
bleaching. Table 19.1 shows the typical variations in fastness grades for azoic
dyeings.