Page 189 - Basic Principles of Textile Coloration

178 AN INTRODUCTION TO DYES AND DYEING

length, texture, and cross-section, as well as the cloth construction. Since a
client’s coloured sample is rarely of the same material as that to be dyed, dyehouse
laboratories devote considerable time to dyeing trials aimed at developing recipes
to reproduce the desired colour on the given goods.

Coloration of a textile material is achieved in a number of different ways:
(1) direct dyeing, in which the dye in the aqueous solution in contact with the

material is gradually absorbed into the fibres because of its inherent
substantivity;
(2) dyeing with a soluble precursor of the dye, which forms an insoluble pigment
deep within the fibres on treatment after dyeing;
(3) direct dyeing followed by chemical reaction of the dye with appropriate
groups in the fibre;
(4) adhesion of the dye or pigment to the surface of the fibres using an
appropriate binder.

All of these methods but the last require that the fibres, at some stage, absorb the
dye, or an appropriate precursor, from an aqueous solution. This process is
essentially reversible. Note, however, that precipitation of a pigment and reaction
with the fibre are irreversible chemical processes.

Dyeing is either a batch exhaustion process, or a continuous impregnation and
fixation process. In the exhaust technique, all the textile is in repeated contact
with all the dye liquor during dyeing and the fibres gradually absorb the dyes.
Careful control of the dyeing temperature, pH and auxiliary chemical
concentrations is often necessary to obtain level, well-penetrated dyeings. This is
essential if the initially absorbed dye is unable to migrate from heavily dyed to
poorly dyed areas during the process. In continuous impregnation methods, the
fabric passes through a small bath containing the dye solution and then two
rubber-covered rollers squeeze out the excess solution. This process is called
padding. There is no migration of the dye from the point of impregnation except
into the fibres, which is assisted by the pressure rollers. Each small segment of
fabric encounters the dye liquor only once so padding must be uniform across the
fabric width and along its entire length. After padding, the dyes must diffuse into
the fibres. This step is called fixation. It may be as simple as rolling up the fabric
and batching it for several hours, or as complex as a rapid thermal treatment in a
steamer or hot air oven.

Other operations often precede or follow the actual process of dyeing.
Pretreatments include scouring and bleaching, sometimes as separate continuous