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