Acid dye is a member of a class of dye that is applied from an acidic solution. In the home or art studio, the acid used in the dyebath is often vinegar (acetic acid) or citric acid.

In textiles, acid dyes are effective on protein fibers, i.e. animal hair fibers like wool, alpaca and mohair. They are also effective on silk. They are effective in dyeing the synthetic fiber nylon but of minimal interest in dyeing any other synthetic fibers.

Acid dyes are generally divided into three classes which depend on fastness requirements, level dyeing properties and economy. The classes overlap and generally depend on type of fiber to be coloured and also the process used.

Acid dyes are thought to fix to fibers by hydrogen bonding. They are normally sold as the Sodium salt therefore they are in solution anionic. Animal protein fibers and synthetic Nylon fibers contain many cationic sites therefore there is an attraction of anionic dye molecule to a cationic site on the fiber. 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.

Structures

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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 sulphonyl or amino group on the molecule making them solublue 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.

Classes of acid dyes

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Equalising/levelling acid dyes: Highest level dyeing properties. Quite combinable in trichromatic shades. Relatively small molecule therefore high migration before fixation. Low wet fastness therefore normally not suited for apparell fabric.

Milling acid dyes: 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 produce, however, duller shades.

Health and safety

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Any dyes including acid dyes have the ability to induce senstisation in humans due to their complex molecular structure and the way in which they are metabolised in the body. This is extremely rare nowadays as we have a much greater understanding through experience and knowledge of dyestuffs themselves. Some acid dyes are used to colour food. We wear fabrics every day exposing our skin to dyes.

The greatest risk of disease or injury due to dyes is by ingestion or exposure to dye dust. These scenarios are normally confined to textile workers. Whereby the dye itself is normally non toxic, the molecules are metabolised (usually in the liver) where they may be broken back down to the original intermediates used in manufacture. Thus many intermediate chemicals used in dye manufacture have been identified as toxic and their use retricted. There is a growing trend among governments to ban the importation of dyes synthesised from restricted intermediates. For example: the dye CI Acid red 128 is banned in Europe as it was found to metabolise in the body back to ortho-toluidine, one of its chemical intermediates. Many intermediates used in dye manufacture such as o-toluidine, benzidine etc. were found to be carcinogenic. All the major chemical companies have now ceased to market these dyes. Some, however, are still produced but they are found to be totally safe when on the fiber in its final state. The use of these dyes is declining rapidly as cheap and safer alternatives are now easily available.

The incident concerning the dye Sudan 1 is an example of a suspected toxic dye finding its way into the food chain. Such incidents are extremely rare.

Dyes