The name “acid dye” derives from the use of an acidic dye bath.
Most pre-metallised and mordant dyes are acid dyes. In case of mordant dyes,
the dyeings are aftertreated with a suitable metal ion mordant, usually
chromium. In fact, mordant dyes are often referred to as chrome dyes. The metal
in pre-metallised dyes is incorporated into the dye molecule during the
Acid dyes are usually sodium salts of sulphonic acids, or less frequently of
carboxylic acids, and are therefore anionic in aqueous solution. They will dye
fibres with cationic sites. These are usually substituted ammonium ion groups
in fibres such as wool, silk and nylon. These fibres absorb acids. The acid
protonates the fibres amino groups, so they become cationic. Dyeing involves
exchange of the anion associated with an ammonium ion in the fibre with a dye
anion in the bath. The strength (fastness) of this bond is related to the
desire/ chemistry of the dye to remain dissolved in water over fixation to the
normally very large aromatic molecules consisting of many linked rings. Acid
dyes usually have a sulphonyl or amino group on the molecule making them
soluble in water. Water is the medium in which dyeing takes place. Most acid
dyes are related in basic structure to the following:
type: Many acid dyes are synthesised from chemical intermediates which form
anthraquinone-like structures as their final state. Many blue dyes have this
structure as their basic shape. The structure predominates in the levelling
class of acid dye.
dyes:The structure of azo dyes is based on azobenzene, Ph-N=N-Ph (see right
showing cis/ trans isomers) Although Azo dyes are a separate class of dyesuff
mainly used in the dyeing of cotton (cellulose) fibers many acid dyes have a
similar structure, most are red in color.
related:Acid dyes having structures related to triphenylmethane predominate in
the milling class of dye. There are many yellow and green dyes commercially
applied to fibers that are related to triphenylmethane.
Acid dyes are thought to attach to fibers by ionic bonds, hydrogen
bonds, and Van der Waals forces. They are normally sold as the sodium salt,
therefore they are in the form of anions in solution. Animal protein fibers and
synthetic nylon fibers contain many cationic sites, therefore there is an
attraction of the anionic dye molecule to a cationic site on the fiber. The
strength (fastness) of this bond is related to the tendency of the dye to
remain dissolved in water vis-a-vis its tendency to be fixed to the fiber.
The chemistry of acid dyes is quite complex. Dyes are normally very large
aromatic molecules consisting of many linked rings. Acid dyes usually have a
sulfonyl or amino group on the molecule making them soluble in water. Water is
the medium in which dyeing takes place.
of Acid Dyes:
Levelling Acid Dyes
Dyeing wool with leveling acid dye requires sulphuric or formic acid in the
dyebath, along with glauber’s salt. Considerable amounts of a strong acid are
needed to achieve good exhaustion, typically 2-4% owf of sulphuric acid.
Because of the case of migration of levelling acid dyes during dyeing, the
fastness to washing of their dyeing is only from poor to moderate. Their light
fastness, however, ranges from fair to good. If the dye molecules do aggregate
in solution at the maximum dyeing temperature, the aggregate are quite small,
or there are enough individual molecules present in the solution for good
penetration into the pores of the wool.
Wool contains about 820 mmol kg-1 of amino groups, some of which converts into
ammonium ions in the presence of sulphuric acid, with a bound bisulphate anion.
During dyeing, a dye anion displaces the bisulphate ion associated with an
ammonium ion site. The wool is far from being saturated with dye anions.
The added glauber’s salt act as a retarding and leveling agent. It promotes
leveling and reduces the dyebath exhaustion. As dyeing proceeds, more acid is
then gradually added to decrease the bath pH.
Levelling dyes give decreasing exhaustion on increasing the dyebath pH to
values above 4, and with increasing temperature. These effects are consistent
with a simple ion exchange process that is exothermic.
usually monosulphonated acid dyes of somewhat higher molecular weight than
typical leveling dyes. They dye wool by essentially the same dyeing method
using acetic acid (1-3% owf) and glauber’s salt (5-10% owf). These dyes are
used wehere level dyeing is necessary but when the washing and perspiration
fastness of leveling acid dyes are inadequate.
anionic dyes have higher molecular weights and greater substantivity for wool
than leveling or fast acid dye. They are medium to high wet fastness. Some
milling dyes have poor light fastness in pale shades. Generally not combinable.
Used as self shades only.
complex acid dyes
More recent chemistry
combined transition metals with dye precursors to produce metal complex acid
dyes with the highest light fastness and wet fastness. These dyes are also very
economical. They, however, produce duller shades.
The invention provides a process for dyeing a textile substrate comprising wool
fibres, which process comprises bringing the substrate into contact with an
aqueous dyebath containing an acid dyestuff or a mixture of acid dyestuffs
having(a) a build-up power on wool of from 90 to 98% at pH 4.5; together with(b)
a migrating ability on wool of from 25 to 40%, at pH 4.5;(a) and (b) being
determined under specific conditions, in the presence of a levelling agent
which is the alkoxylation product of an aminesubstituted by a fatty saturated
or unsaturated residue, the aqueous dyebath containing a mixture of acid
dyestuffs when the substrate is a wool/synthetic polyamide fibre blend. These
dyestuffs give level, fast and reproducible dyeings of a highquality.
of acid dyes:
Temperature and pH control
i) Unequal access of the fibres to the dye solution, resulting from densely
packed fibres or yarns and from poor agitation of the dyebath.
ii) Variation of the temperature throughout the dyebath and the goods.
iii) Uneven pH in the bath and the material.
Dyeing damaged wool fibres:
i) Dyeings with colored patches of different depths caused largely by uneven
treatment with chemical during processes such as scouring, bleaching or
chlorination, or incomplete and non-uniform removal of residual chemicals,
ii) Skitteriness: The uneven dyeing of individual wool fibres whose tips have
degraded more from the greater exposure to the elements during the growth of
the wool fleece.