| Acid dye is a member of a class of dye that is applied | | | | Classes of acid dyes |
| from an acidic solution. In the home or art studio, the | | | | Equalising/levelling acid dyes: Highest level dyeing |
| acid used in the dyebath is often vinegar (acetic acid) | | | | properties. Quite combinable in trichromatic shades. |
| or citric acid. | | | | Relatively small molecule therefore high migration |
| In textiles, acid dyes are effective on protein fibers, i.e. | | | | before fixation. Low wet fastness therefore normally |
| animal hair fibers like wool, alpaca and mohair. They | | | | not suited for apparel fabric. |
| are also effective on silk. They are effective in dyeing | | | | Milling acid dyes: Medium to high wet fastness. Some |
| the synthetic fiber nylon but of minimal interest in | | | | milling dyes have poor light fastness in pale shades. |
| dyeing any other synthetic fibers. | | | | Generally not combinable. Used as self shades only. |
| Acid dyes are generally divided into three classes | | | | Metal complex acid dyes: More recent chemistry |
| which depend on fastness requirements, level dyeing | | | | combined transition metals with dye precursors to |
| properties and economy. The classes overlap and | | | | produce metal complex acid dyes with the highest light |
| generally depend on type of fiber to be coloured and | | | | fastness and wet fastness. These dyes are also very |
| also the process used. | | | | economical. They produce, however, duller shades. |
| Acid dyes are thought to fix to fibers by hydrogen | | | | Health and safety |
| bonding. They are normally sold as the Sodium salt | | | | Any dyes including acid dyes have the ability to induce |
| therefore they are in solution anionic. Animal protein | | | | senstisation in humans due to their complex molecular |
| fibers and synthetic Nylon fibers contain many cationic | | | | structure and the way in which they are metabolised in |
| sites therefore there is an attraction of anionic dye | | | | the body. This is extremely rare nowadays as we |
| molecule to a cationic site on the fiber. The strength | | | | have a much greater understanding through |
| (fastness) of this bond is related to the desire/ | | | | experience and knowledge of dyestuffs themselves. |
| chemistry of the dye to remain dissolved in water | | | | Some acid dyes are used to colour food. We wear |
| over fixation to the fiber. | | | | fabrics every day exposing our skin to dyes. |
| Structures | | | | The greatest risk of disease or injury due to dyes is |
| The chemistry of acid dyes is quite complex. Dyes are | | | | by ingestion or exposure to dye dust. These scenarios |
| normally very large aromatic molecules consisting of | | | | are normally confined to textile workers. Whereby the |
| many linked rings. Acid dyes usually have a sulphonyl | | | | dye itself is normally non toxic, the molecules are |
| or amino group on the molecule making them soluble in | | | | metabolised (usually in the liver) where they may be |
| water. Water is the medium in which dyeing takes | | | | broken back down to the original intermediates used in |
| place. Most acid dyes are related in basic structure to | | | | manufacture. Thus many intermediate chemicals used |
| the following: | | | | in dye manufacture have been identified as toxic and |
| Many acid dyes are synthesised from chemical | | | | their use retricted. There is a growing trend among |
| intermediates which form anthraquinone-like structures | | | | governments to ban the importation of dyes |
| as their final state. Many blue dyes have this structure | | | | synthesised from restricted intermediates. For |
| as their basic shape. The structure predominates in the | | | | example: the dye CI Acid red 128 is banned in Europe |
| levelling class of acid dye. | | | | as it was found to metabolise in the body back to |
| Azo dyes: | | | | ortho-toluidine, one of its chemical intermediates. Many |
| The structure of azo dyes is based on azobenzene, | | | | intermediates used in dye manufacture such as |
| Ph-N=N-Ph (see right showing cis/ trans isomers) | | | | o-toluidine, benzidine etc. were found to be |
| Although Azo dyes are a separate class of dyesuff | | | | carcinogenic. All the major chemical companies have |
| mainly used in the dyeing of cotton (cellulose) fibers | | | | now ceased to market these dyes. Some, however, |
| many acid dyes have a similar structure, most are red | | | | are still produced but they are found to be totally safe |
| in color. | | | | when on the fiber in its final state. The use of these |
| Acid dyes having structures related to | | | | dyes is declining rapidly as cheap and safer |
| triphenylmethane predominate in the milling class of | | | | alternatives are now easily available. |
| dye. There are many yellow and green dyes | | | | The incident concerning the dye Sudan 1 is an example |
| commercially applied to fibers that are related to | | | | of a suspected toxic dye finding its way into the food |
| triphenylmethane. | | | | chain. Such incidents are extremely rare. |