Sunday, 9 January 2011

Why Different types of Reactive Dyes Dye differently



Almost all the reactive dyes are built on a similar structure (Remazol Dye from Hoechst is the exception). This structure consists of (1) a chromophore (the color-bearing group), (2) a reactive group (usually a heterocyclic carbon-nitrogen ring system), and (3) a "leaving group" which is part of the carbon-nitrogen group which is generally a halogen compound (chlorine family).

Influence of Dye characteristics in reactive dyeing:

The major dye variables that affect reactive dyeing are dye chemistry, substantivity,Reactivity and solubility.

Dye chemistry:

Reactive dye has a wide variety in terms of their chemical structure. The two most important component of a reactive dye are the chromophore and the reactive group. The characteristics governed by the chromophore are color gamut, light fastness, chlorine / bleach fastness, solubility, affinity and diffusion.

The dye characteristics governed by reactive group are reactivity, dye-fiber bond stability, and efficiency of reaction with the fiber and affinity. Dyeing conditions, especially the alkali requirements and temperature as well as the use of salt also depends upon the type of reactive group.

Substantivity:

The affinity of dye for a given substrate ( textile material) is called substantivity

Substantivity more depends upon chromophore as compared to reactive system. A high substantivity may results:
• Lower dye solubility.
• High primary exhaustion.
• A high reaction rate.
• Lower diffusion coeffecient.

A low sensitivity of dyes to the variation in the processing conditions such as time, temperature, pH, material to liquor ratio may results:
• Less diffusion.
• Less migration and levelness.
• More difficult to the removal of unfixed dyes.

Substantivity is also the best measure of the ability of a dye to cover dead cotton or immature cotton fibers. Covering power is best when the substantivity is either high or very low. An increase in the dye substantivity may be affected by:
• Lower concentration of dyes.
• Higher concentration of electrolyte.
• Lower temperature.
• Higher pH upto 11.
• Lower liquor to material ratio (M:L)

Reactivity:

High dye reactivity entails a lower dyeing time and lower efficiency of fixation. To improve the efficiency of fixation by reducing dye reactivity requires a longer dyeing time and therefore less effective than an increase in substativity. Also there is wide range of temperature and pH over which the dye can be applied. Altering the pH or temperature, two dyes of intricsically different reactivity may be made to react at a similar rate can modify reactivity of dye.

Solubility:

Dyes of better solubility can diffuse easily and rapidly into the fibers, resulting in better migration and leveling. Increasing the temperature, adding urea and decreasing the use of electrolyte may affect on increase in dye solubility.

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