Page 311 - Basic Principles of Textile Coloration

300 DYEING CELLULOSIC FIBRES WITH DIRECT DYES

of dye must then be dissolved in a bath of reduced volume. It is important to
ensure that the dye is, in fact, in solution at the beginning of dyeing when the
temperature is relatively low and after addition of salt.

14.5 THE AFTERTREATMENT OF DYEINGS WITH DIRECT DYES
Aftertreatment of dyeings of direct dyes on cellulosic materials aims to improve
the washing fastness by increasing the dye’s molecular weight. This makes it less
soluble and of slower diffusion. Some of these processes decrease the light fastness
of the dyeing. Aftertreatments are difficult and costly to carry out, and often give
changes in hue that greatly impede shade correction and colour matching. Many
of the direct dye aftertreatments now have limited use because reactive dyes give
dyeings of much better washing fastness.

14.5.1 Diazotisation and development
Diazotisation of direct dyes with primary aromatic amino groups, followed by
coupling of the diazonium ion with an appropriate developer, can be a very
effective aftertreatment. Primuline, CI Direct Yellow 59 (7, in Figure 14.5), is a
classic example of such a dye. Diazotisation involves treating the yellow dyeing
with an acidic solution of sodium nitrite at room temperature or lower. Both amine
and phenol developers can be used. The diazotised dye in the material is sensitive
to light and heat so immediate coupling in a second bath containing the developer
is necessary. A final wash removes any dye deposited on the fabric surface to
ensure good fastness to washing and rubbing. Although the washing fastness
improves by about one grade, there is often a considerable change in hue. For
example, a dyeing with Primuline, diazotised and developed with 2-naphthol (8),
turns from yellow to red. This technique is useful for cheap navies and blacks with
a washing fastness of 3–4 but is little used today.

14.5.2 Coupling with diazonium salts
Direct dyes with free positions ortho and para to hydroxyl or amino groups react
with an appropriate diazonium ion to introduce one or two additional azo groups.
p-Nitroaniline, diazotised in a freshly precipitated suspension of its hydrochloride,
has been widely used for this purpose.