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 manufacturing process.

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 fiber.


Dyes are 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:

Anthraquinone 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.

Azo 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.

Triphenylmethane 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.

Properties 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.

Fast Acid Dyes
These are 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.

Milling acid dyes
These 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.

Metal 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.

Problems 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.


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