Chemical Reactivity
The concentration of active nucleophilic groups in a chemical bath governs the speed and efficiency of covalent bonding during wet processing. In the dyeing of cellulosic fabrics, hydroxyl activity defines the effective concentration of ionized cellulosate groups or reactive ions available to interact with dye molecules. This activity depends heavily on the alkalinity of the solution and the temperature of the bath.
It determines the rate of dye fixation versus the rate of dye hydrolysis.
Fixation Control
Achieving high colorfastness in reactive dyeing requires careful monitoring of the chemical environment of the cotton fabric. Higher alkali concentration increases the hydroxyl activity of the cellulose fibres, which promotes covalent bonding with the reactive groups of the dye. However, excessive alkaline strength also increases the activity of water-borne hydroxyl ions, leading to unwanted hydrolysis.
This hydrolysis deactivates the dye molecules and prevents them from binding to the fibre.
Process Modification
Processors use specific salt concentrations and alkali dosing profiles to maintain the optimum balance of reactive species. Gradual addition of soda ash ensures that the hydroxyl activity rises slowly, allowing even distribution of dye throughout the yarn package or fabric roll before fixation occurs. This controlled dosing prevents surface-only dyeing and reduces the volume of unfixed dye that must be washed out during finishing.
Thermal Influence
High temperature accelerates both chemical reactions and molecular movement in the dye bath. Raising the bath temperature increases the hydroxyl activity but also speeds up dye hydrolysis, requiring precise thermal management during the process.
