Processing Solution
Industrial dyeing of cotton fabrics involves aqueous solutions containing dissolved colorants, salts, and alkaline agents. Preparation of a reactive dye bath requires careful sequencing of these components to achieve a uniform shade on the yarn. The colored molecules form covalent bonds with the cellulose fibers in the fabric.
This process terminates when all dye molecules have reacted. The composition of the solution must be adjusted based on the fiber weight.
Alkaline Fixation
Soda ash is added to the reactive dye bath to raise the pH and initiate the chemical reaction between dye and cotton. This chemical reaction occurs at high pH levels, typically between ten and eleven. If the alkalinity is too low, the dye will not bind to the fiber, leading to poor wash fastness.
Electrolyte Addition
Sodium chloride or sodium sulfate is dissolved in the liquid to reduce the negative charge on the surface of the cotton fibers. This electrolyte addition forces the dye molecules to migrate out of the liquid and onto the fabric. High salt concentrations are needed to achieve deep shades like navy or black.
Rinsing Requirement
Unreacted dye molecules must be washed out of the cotton fabric after the dyeing cycle is complete. This post-dyeing rinse prevents the dye from bleeding onto other clothes during home laundering. Fabric mills use multiple hot washes to ensure that only covalently bound dye remains on the yarn.
