Thermodynamic Modeling of Bifunctional Dye Hydrolysis under High Shear Boundary Layer Flow
High fluid shear compresses boundary layers, accelerating hydroxyl mass transfer and driving hydrolytic dyestuff loss in bifunctional reactive dyeing.
Hydroxyl Ion Hydrolysis
Chemical Degradation
Nucleophilic attack by water molecules in the presence of alkali leads to the inactivation of reactive dye molecules before they can bond with the fibre. This secondary reaction converts the reactive group into a non-reactive form that cannot create a covalent bond with the cellulose hydroxyl groups. Such a transformation represents a permanent loss of dyestuff that eventually must be washed away from the textile.
Fixation Rivalry
Competition between the fibre and the water molecules for the reactive site of the dye determines the final efficiency of the dyeing process. When hydroxyl ion hydrolysis occurs at a high rate, the fixation yield drops, requiring the mill to use more dye to achieve the target shade. Temperature and alkalinity are the primary drivers of this unwanted chemical pathway.
Alkaline Condition
Precise control of the pH level is necessary to minimize the destruction of the dye while still providing enough activation for the cellulose. Most reactive dyeing processes use a staged alkali addition.
Resource Loss
Economic and environmental costs increase when a considerable portion of the dye remains unfixed due to water reactions. High levels of hydrolyzed dye in the effluent require more treatment and increase the volume of water needed for the final rinsing stages.