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