The reactive dyes fall essentially into two classes:
1. Those that react by nucleophilic substitution mechanism based on the presence of halogen
substituents in a heteroatomic systems e.g. the chlorotriazinyl dyes.
D – X-+ Cell-OH-
D – O – Cell + HX-
Among the principle reactive systems of the type are the halogen substituted triazine, pyrimidine , pyrozine, quinoxaline, thiazole and pyridazone groups.
2. Those that reacting with cellulose by nucleophilic addition to a carbon – carbon double bond, usually activated by an adjacent electron – attracting
Dye – SO2 – CH = CH2 + Cell – OH -
Dye – S02 – CH2 – CH2 – O Cell
1.On the basis of Reactive group of reactive dyes there are two types
a) Halogenated heterocycles (Example: Diclorotriazine, Tetrachloropyrimidine, Monochlorotriazine etc)
b) Activated vinyl compound (Example: Vinyl sulphone, Vinyl acrylamide, Vinyl sulphonamide etc)
2.On the basis of Reactivity of reactive dyes there are three types
a) High reactivity (Example: Dichlorotriazine)
b) Moderate reactivity (Example: Vinyl sulphone)
c) Low reactivity (Example: Trichloropyrimidine, Dichloroquinoxaline etc)
Modern classification of reactive dyes: Reactive dyes have recently been classified as
1. Alkali-controllable reactive dyes, which have relatively high reactivity and only moderate substantivity. The reactive dyes are applied at relatively low temperatures and level dyeing requires careful control of the addition of the alkali to initiate the fixation stage. Examples include DCT, DFCP and VS reactive dyes.
2. Salt-controllable dyes. These are dyes of relatively low reactivity towards cotton under alkaline conditions and therefore the'dyeing temperature will be as high as 80°C. They have appreciable substantivity and level dyeing requires careful addition of salt to promote exhaustion. Examples in this class include TCP, MCT as well as MFT reactive dyes.
3. Temperature-controllable dyes, which undergo fixation at high temperatures even under neutral conditions. The NT dyes are in this class.A
If the reactivity of the dye is increased considerably, the rate of reaction with the fiber increases. Therefore, the dyeing can be carried out in a short time. However, in this case the rate of hydrolysis of the dye also increases, leading to deactivation of a part of the dye. This results in wastage of the dye. If, on the other hand, the reactivity of the dye is decreased, the extent of hydrolysis can be reduced considerably. However, this results in the slower rate of reaction with the fiber also.
Role of electrolyte in the dyebath:
When a fiber is immersed in water, a negative electrostatic charge develops on its surface. This charge repels any dye anions present in the solution, so that, the fiber cannot be dyed satisfactorily. If, however, the dyebath also contains an electrolyte such as sodium chloride or sodium sulphate, a diffuse layer of positive sodium ions forms at the fibre surface, neutralizing its charge. The dye ions are then able to approach sufficiently closely to the fiber for the inherent attractive forces between the dye and the fiber to operate.
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