May 06, 2008

Dyeing Equilibrium - A Practical Dyer's Guide to Reactive Dyeing of Cotton - Part-4

In the previous post we have discussed the chemical aspects only and the processes of reaction with cellulose and the water have been treated as if they were occurring quite separately. But in fact this is not the case and we have just seen the importance of exhaustion in obtaining a good efficiency.

We will now examine what happened in a two stage dyeing process in which the dyestuff is exhausted from the neutral dye bath in the first stage and then at a later stage the solution is made alkaline so that the reaction begins.

In the first stage of neutral dyeing, no decomposition of dye takes place and the process is exactly the same as the dyeing of a direct dye. The only difference is the lower degree of exhaustion of reactive dyes. At the end of the 1st stage we have two equilibrium.

When alkali is added to the system, chemical reaction begins. In the dye bath hydrolysis with water occurs, while in the fibre, the dissolved dye will also hydrolyse, but the adsorbed dye will mainly react with the fibre although the possibility of aqueous hydrolysis cannon be excluded. The hydrolysed dye (DOH) will have similar properties to the parent dye ( or direct dye) and will not get adsorbed on the fibre surface. Finally, when all the reactive dye present has been destroyed one way or another, new equilibrium will be set up between the hydrolysed dye in the dye bath, in solution inside the fiber and adsorbed on cellulose while the combined dye is present as a separate component, not taking part in this.