Hydroxyl Activation
Chemical conversion of neutral cellulose hydroxyl groups into highly reactive anions occurs in strongly alkaline environments. This cellulate ionization transforms the cotton fiber from a passive substrate into a nucleophile capable of bonding with reactive dyes. The process is necessary because neutral hydroxyl groups are poor nucleophiles that do not readily displace leaving groups on dye molecules.
Once the anion forms, the fiber can participate in the reaction.
Nucleophilic Attack
Negatively charged oxygen atoms on the ionized cellulose attack the reactive carbon centers of the dye molecules to create permanent chemical bonds. Through cellulate ionization, a covalent ether or ester linkage forms between the dye and the fiber polymer. This reaction provides the high color fastness associated with reactive dyeing on cotton garments.
Dyeing Efficiency
The ratio of ionized to unionized hydroxyl groups in the cellulose structure dictates the efficiency of the fixation process.
Bath Temperature
Alkaline bath temperature influences the equilibrium of the ionization process and the rate of subsequent dye fixation. During cellulate ionization, higher temperatures shift the chemical equilibrium, which can increase the reaction speed but also increases the risk of dye hydrolysis. Wet processing technicians balance this temperature effect by carefully adjusting both the chemical concentration and the bath dwell time to maximize the color yield of the fabric.
