Reactive dyes are the most commonly used dyes for cotton dyeing, and their consumption has been on the rise—a trend expected to continue in the coming years. Their popularity stems from moderate pricing, high color yield, and satisfactory color fastness. The only drawback, however, is the hydrolysis of the dyes.
Hydrolysis generally refers to the process where dyes fix onto cotton under alkaline conditions, while the alkalinity accelerates the reaction between dyes and water, leading to their inactivation. The deactivated dyes, known as hydrolyzed dyes, are unable to react with cotton, resulting in considerable dye loss. These hydrolyzed dyes adhere physically to the substrate and are removed only during the washing process, which consequently causes poor wash fastness. Additionally, hydrolyzed dyes discharge into wastewater, increasing the pollution load.
The dyeing of cellulosic fibers with reactive dyes involves three stages:
- Dye adsorption onto fibers in a neutral medium containing sodium chloride or sodium sulfate.
- Fixation of the adsorbed dyes upon the addition of alkali, forming covalent bonds between the dyes and fibers.
- Thorough removal of loosely adhering dyes on or within the fabric via washing.
The third stage mentioned above is indispensable. This is because, in the dye bath, most reactive dyes adsorb onto fibers, while a portion reacts with water under alkaline conditions, losing their reactivity toward fibers and thus exhibiting low affinity for the substrate. Improper execution of this step will result in poor wash fastness of dyed products due to the presence of hydrolyzed dyes, whereas dyes chemically bonded to fibers ensure high wash fastness.
The reaction between reactive dyes and water is not the sole factor affecting dyeing color yield. The application performance of dyes is also closely related to other factors, such as dye storage stability, the stability of padding liquors or printing pastes, and changes in reactive dye concentration during the hot dissolution of dye formulations.
Dye hydrolysis begins during production and continues through drying and grinding processes. The degree of dye hydrolysis depends on the reactive groups and chromophores in the dye molecules.
Hydrolyzed dyes are simply products in which unstable atoms or groups in the original reactive dyes are replaced by hydroxyl groups. Their diffusion and adsorption properties are very similar to those of the original reactive dyes, allowing them to easily diffuse into fibers and adsorb onto their surfaces. At the end of the alkaline dyeing stage, when all reactive dyes are completely consumed—either by reacting with water or fibers—a new equilibrium is established, namely the distribution equilibrium of hydrolyzed dyes between fibers and the dye bath, which follows a pattern similar to the distribution of reactive dyes in neutral solutions. At this stage, the fibers contain a large amount of chemically bonded dyes.
To obtain dyed products with high wet fastness, hydrolyzed dyes must be completely removed from the fibers during the final washing process. In reactive dyeing, an average of approximately 70% of the dyes fix onto fibers, while the remaining 30% or more are wasted due to hydrolysis. In actual reaction systems, numerous factors influence the hydrolysis rate of reactive dyes, including dye bath pH, temperature, dye concentration, and electrolyte concentration:
- Effect of dye bath pH
Studies on the relationship between medium pH and the degree of cellulose dissociation have shown that alkali concentration should not be increased to raise the solution pH above 10–11. This is because all accessible hydroxyl groups in cellulose become ionized at this pH range; any further increase in pH will only induce dye hydrolysis.
- Effect of temperature
Elevated temperatures lead to increased hydrolysis of reactive dyes.
- Effect of dye concentration
Higher reactive dye concentrations in the solution promote dye aggregation, thereby reducing the hydrolysis rate.
- Effect of electrolyte concentration
A higher electrolyte concentration in the dye bath results in greater amounts of hydrolyzed dyes.
Studies have found that the reactivity of reactive dyes toward water is also associated with the chromophores in the dye molecules, as chromophores affect the electron cloud density at the reaction centers. If anionic groups form in the chromophore system, the electron cloud density at the reaction centers increases, thereby reducing the reactivity of reactive dyes toward water.
Post time: Nov-25-2025