Dyeing and Dyes

Dyeing

The most important view of dyeing is for colouring textile material. Other than textiles there are other materials that are coloured by dyeing viz. paper products, leather products, cosmetics, foods etc. The dyeing usually is done to make the product attractive. Dyeing of textile materials is a process of applying a dye so that the materials not only change their color but also lastly retain the dye. The process that results in the materials acquiring one color is called plain dyeing or simply dyeing; the application of color to the material at separate places or of several colors forming a design is called pattern printing. The subject of dyeing includes two objects: the dyestuffs and the fiber and relation between them at certain conditions.

Dyes

Generally, a dye may be defined as a color substance which when applied to the fabrics a permanent color and the color is not usually removed by washing with water, soap or no exposure to sunlight. In other words, compounds containing charomophore and auxochrome groups are called dyes. Chromophore group is responsible for dye colour due to their unsaturation or multiple bonds e.g. --NO2, – N=O, --N = N--, quinonoid structure, >C = O etc.
Auxochrome group is responsible for dye fiber reaction e.g –OH, COOH, SO3H, –NH2, Cl- etc.
All colored substances are not necessarily dyes. For example, though both picric acid and trinitrotoluene are yellow in color but only picric acid can fix to a cloth where trinitrotoluene does no fix to a cloth and so it is not a dye.


Direct dyes

The characteristic features of direct dyes are:
(1) The shapes are long, narrow and flat
(2) The constitutive groups are -OH, -NH2 , -N=N– which may form hydrogen bond with --OH groups of the cellulosic chain and
(3) Two or more ion forming groups usually SO3Na which help the dissolution of dyes in water.

Direct dyes are used to dye animal fibers as well as cotton or the vegetable fibers without mordant directly. Some of them are also employed for dyeing union goods (cotton and wool or cotton and silk). These dyes are also called salt dyes, because of the fact that dyeing is usually carried out in presence of common salt or Glauber’s salt to the dye bath. The dye is applied to the fabric by immersing the fabric in hot boiling solution and then removing and drying the fiber. Addition of common salt increases the solubility of the dye and hence cause better exhaustion of it forms the dyeing solution.