Thread Content
The role of alkalis: Alkalis serve primarily as a catalyst that enables interaction between dyes and the materials to be dyed, facilitating the bonding of dye molecules with cellulose molecules. Therefore, what is important is not the type of base, but the average pH of the dye bath after the addition of the base. Under normal circumstances, the optimal pH value for the dye solution is 11.5; typically, adding 20 g/L of soda ash can bring the pH to around 11.5. However, some dyeing and finishing companies do use caustic soda (NaOH). Since it is a strong base, it is somewhat difficult to adjust the pH to 11.5; in other words, it is not easy to control, which is why it is used less often. Nevertheless, it can be utilized when a dosing system is available. To prevent the fabric from folding into rope-like shapes during dyeing, it is very effective to add softeners to the dye bath; when the water used for dyeing is hard, metal chelators can be added. Use of alkalis in reactive dye dyeing: When cellulose fibers are dyed with reactive dyes, alkalis act as catalysts for the reaction between the dyes and the fibers; therefore, alkalis must be used to facilitate this reaction. At this point, the type and amount of alkali are not the key factors; rather, attention must be paid to controlling the pH value of the dye bath. The optimal pH value for dyeing varies depending on the dyeing temperature; generally, for medium-temperature dyeing at 60°C, the pH value is set around 11.5, while for high-temperature dyeing it is maintained between 10 and 11, and for low-temperature dyeing it is around 12.5. Generally, adding 20 G/L of sodium carbonate is sufficient to achieve a pH of 11.5. If a higher pH value is required, sodium phosphate or sodium hydroxide can be used; whereas for a lower pH value, sodium bicarbonate can be utilized. Sodium hydroxide is a strong base whose pH value is difficult to control, so it is not used frequently.