Page 190 - Basic Principles of Textile Coloration

DYEBATH AND FABRIC PREPARATION 179

operations, or as batch processes in the dyeing machine. These remove natural
and synthetic impurities from the goods and should result in uniform water
absorbency. After dyeing, the material is rinsed to remove adhering solution but it
may need additional treatment while still in the dyeing machine. These
aftertreatments may include:
(1) washing in detergent, at or near the boil, to remove any unfixed dye or loosely

adhering pigment from the fibre surfaces – this process is called soaping;
(2) treatment with chemicals to improve the fastness properties of the dyeing –

this often causes a change in shade and complicates colour matching;
(3) application of simple finishing chemicals such as softeners.

10.3 DYEBATH AND FABRIC PREPARATION

A number of preparatory steps are essential before dyeing. Once the goods are in
the dyeing machine, they must be thoroughly wetted out to remove trapped air.
This is to ensure that the dye solution will have equal access to all fibre surfaces.
Wetting-out may involve running the material in the dyeing machine in warm or
hot water, in the presence of a wetting agent if wetting is likely to be difficult.
Fabric that has been scoured or bleached in the dyeing machine must be rinsed to
remove residual chemicals. It will already be completely wet but must be run in
water and the pH value checked, and adjusted if necessary, before the dyes are
added.

Dyeing is often one of the final processes in production. Many types of dyeing
faults, such as colour blotches and spots, colour shading from side-to-side, end-to-
end or face-to-face of the material, water marks, and crack or crease marks, are
often a consequence of poor preparation, or dye selection, or dyeing technique.
For uniform dyeing, the fabric’s absorbency must be uniform. While preparation
may not totally remove all the natural and synthetic chemicals present in the grey
material, the residues should be minimal and uniformly distributed.

Even the preparation of the solution or dispersion of the dyes is not necessarily
as simple as it might seem. Water-soluble dyes must be completely in solution,
otherwise undissolved particles may cause deeply coloured spots where they
contact the cloth surface. Dye powders are normally pasted with warm water. A
wetting agent, or some alcohol, often helps in wetting of the powder. Cold, warm
or even boiling water is then poured onto the paste and the mixture stirred well.
Granular forms can often be simply dispersed in water and dissolved by stirring.
Boiling will usually dissolve all the dyes if the concentration is high or if they are of