1. Problem Solving with Reactive Dyes:
Reactive dyes are one of the most commonly used application class of dyes for cotton materials, Two important aspects of reactive dyeing, namely dye variables and system variables, are discussed in this section, along with important characteristics of reactive dyeing such as exhaustion, migration and leveling, fixation and colour yield, and washing off and fastness. A significant portion of this section also deals with the problem of the reproducibility and difficulties in obtaining right-first-time dyeing.
Dye Variable in Reactive Dyeing
The major dye variables that affect reactive dyeing are dye chemistry, substantivity, reactivity, diffusion coefficient and solubility. Each of these will be briefly discussed below.
Reactive dyes have a wide variety in terms of their chemical structure. The two most important components of a reactive dye are the chromophore and the reactive group. The characteristics governed by the chromophore are colour gamut, light fastness, chlorine/bleach fastness, solubility, affinity, and diffusion. The chromophores of most of the reactive dyes are azo, anthraquinone, or phthalocyanine. Azo dyes are dischargeable. Dis-azo dyes have the disadvantage of being much more sensitive to reduction and many of them are difficult to wash-off.
Anthraquinone dyes exhibit relatively low substantively and are easier to wash-off. Most of them possess excellent fastness to light and to crease-resistant finishes, but they are not dischargeable. Phthalocyanine dyes diffuse slowly and are difficult to wash off.
Metal complex dyes containing copper possess rather dull hues, but show a high degree of fastness to light and to crease-resistant finishes. Their substantivity is fairly high; 1:2 complexes diffuse relatively slowly, so a longer time is needed to wash-out unfixed dye completely.
The dye characteristics governed by the reactive group are reactivity, dye fibre bond stability, efficiency of reaction with the fiber, and affinity. Dyeing conditions, especially the alkali requirement and temperature as well as the use of salt also depend on the type of reactive group. Dyes based on s-triazine do not have good wet fastness properties in acidic media and, due to their high substantivity, have poor wash-off properties.