Dyeing is the
process of coloring textile materials by immersing them in an aqueous solution
of dye, called dye liquor. Normally the dye liquor consists of dye, water and
an auxiliary.
The general
theory of dyeing explains the interaction between dye, fibre, water and dye
auxiliary. More specifically, it explains:
a. forces of repulsion which are developed
between the dye molecule and water; and
b. forces of attraction which are
developed between the dye molecules and fibres.
Dye molecules
are organic molecules which can be classified as:
1. anionic - in
which the colour is caused by the anionic part of the dye molecule;
2. cationic -
in which the colour is caused by the cationic part of the dye molecule;
3. disperse -
in which the colour is caused by the whole molecule.
The first two
dye molecule types are applied from an aqueous solution. The third is applied
from an aqueous dispersion.
Textile fibres
are organic compounds and develop a slight negative surface charge or potential
when immersed in an aqueous solution. Since the dye molecule and textile fibre
both become slightly negatively charged in aqueous solution, there is a
tendency for the dye and the fibre to repel each other.
In addition to
dissolving the dye, water acts as the medium through which the dye molecules
are transferred into the fibre.
The polar
groups in the dye molecules attract water molecules, and cause the dye to
dissolve in water. This attraction between water and dye is on the whole
undesirable, as the dye molecules resist leaving the water and entering the
fibre. In some instances, however, it is desirable to reduce the rate at which
dye leaves the water and enters the fibre so as to ensure a uniform coloration
of the fibre.
The addition of
an electrolyte to the dye liquor of anionic dye increases the uptake of the dye
by the fibre. The electrolytes used in dyeing dissociate completely in the
aqueous dye liquor. This increases ‘the forces of repulsion between the dye
molecules and water so the dye is attracted by the fibre.
The addition of
electrolytes to the dye makes the dye liquor more ionic and thus increases the
forces of repulsion between the electrolyte and the dye molecules. This
attracts the dye to the fibre and increases the chances of the dye molecules
entering the fibre.
Dyeing at
temperatures from 100° to 130°C, under pressure to about 170 kPa (about 1¾
kg/cm2) is termed high temperature dyeing. Hydrophobic fibres, such
as polyester fibres, are dyed in this way.
These chemical
compounds include carriers or swelling agents, levelling agents, anti-foaming
agents, dispersing agents, detergents and wetting agents.
The most widely
accepted explanation is that carriers help to swell the fibre and make it
easier for the dye molecules to enter the polymer system. As a general rule
carrier is only used to dye polyester fibres with disperse dyes.
Leveling
agents
The addition of
levelling agents to the dye liquor helps produce a more uniform colour in
textile fibres. Levelling agents which tend to slow down the dye uptake of the
fibres are also termed retarding agents or retarders. Levelling agents are
surface active agents, and are chemically related to soaps, synthetic
detergents and wetting agents. They may be anionic, cationic or non-ionic organic
compounds.
Dyeing always
leaves some dye molecules on the surface of the fibres of the dyed textile
materials. It is essential that these dye molecules be removed when dyeing is completed.
If these dyes are not removed they may result in two problems:
a. poor
rub-fastness which may result in the dye rubbing off onto adjacent materials;
b. poor
wash-fastness which may result in other fabrics being coloured by this excess
dye during laundering.
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