Fixation Stage
Chemical bonding of reactive dyes to cellulose fibres relies on specific shifts in pH to generate permanent colour links within the polymer matrix. Within alkaline fixation, the introduction of sodium carbonate or similar bases triggers the conversion of hydroxyl groups into reactive cellulosate ions. The process operates between pH 10.5 and 11.5 to ensure the yield of the covalent bond between dye and substrate.
It stops applying to the bath once the dye molecules have successfully exhausted from the liquor and formed the desired chemical union.
Reaction Mechanism
Gradual introduction of the alkali manages the strike rate to ensure even distribution of colour across the fabric surface. If the pH rise occurs too quickly, dye reacts with water molecules instead of the fibre. This side reaction produces hydrolyzed dye that lacks the ability to bond, creating potential for shade variance and poor fastness.
Concentration Threshold
Lower liquor ratios demand precise titration of alkali to maintain sufficient chemical potential. Bath volume influences the quantity of base required to reach the reactive environment.
Verification Requirement
Rinsing sequences remove latent chemical agents to return the textile to a neutral state after completion. Laboratory extraction tests verify the removal of residual alkali to prevent shade changes during downstream heat setting.
