Cold pad batch dyeing usually uses vinyl sulfone dyes. This is because the reactive groups of commonly used vinyl sulfone reactive dyes are very sensitive to alkali. In an alkaline bath, an elimination reaction can occur quickly to generate a highly active vinyl sulfone group (-SO2CH=CH2). This results in strong responsiveness.
It is worth noting that in cold pad-batch dyeing, the dye liquor is too alkaline (pH =12~13), far exceeding the optimal fixation pH range of medium-temperature reactive dyes (10.5-11). Therefore, even though dyeing is performed at room temperature (5~30°C), the hydrolysis rate of the dye is still too fast.
The reason is that in the dye bath with pH=11, the cellulose fiber has the highest concentration of Cell-O- and the lowest concentration of OHF. At this time, the fixation reaction between the dye and the fiber is much faster than the hydrolysis reaction between the dye and water. Therefore, the fixation rate of the dye is high, the hydrolysis rate is low, and the color is deep. In a dye bath with pH>1, the stronger the alkalinity, the higher the OH- concentration and the lower the Cell-O- concentration. Therefore, the fixation reaction between dye and fiber is relatively slow, and the hydrolysis reaction between dye and water is relatively fast. Therefore, vinyl sulfone reactive dyes have poor hydrolysis stability in cold dyeing liquors. It is especially sensitive to the dyeing temperature (referring to the temperature of the padding solution and the reaction temperature of the cloth roll). Even if the temperature fluctuates within the range of room temperature (5~30℃), the hydrolysis rate of the dye will change significantly. For example, if reactive brilliant blue 50g/L pH=13.26, placed at 15℃ for 10 minutes, the hydrolysis rate will be 0.94%. After being placed at 30°C for 10 minutes, the hydrolysis rate was 7.82%. Remazol yellow RGB50g/L, pH=13.5, placed at 15℃ for 10min, hydrolysis rate is 4.34%. After being placed at 30°C for 10 minutes, the hydrolysis rate was 32.72%.
It can be seen that in cold dyeing, the dyeing temperature is also a key factor affecting the depth of color. Moreover, padding and stacking at a lower temperature are most beneficial to improving the color yield and reproducibility of dyeing. It is not appropriate to deliberately increase the dyeing temperature. The reason is:
Using natural room temperature for padding and stacking, due to the small temperature difference between the inside and outside of the cloth rolls or between the cloth rolls , the color is more consistent and the dyeing quality is the most stable.
When dyeing at a lower temperature, the pH value of the dye bath should be appropriately increased, and the stacking reaction time should be appropriately extended. Because, dyeing temperature, dye bath pH value, and stacking time are the three decisive factors for cold pad-batch dyeing. There is an extremely close interdependent and balanced relationship between the three. That is, if the dyeing temperature is low, the pH value should be high, and the stacking time should be long; if the dyeing temperature is high, the pH value should be low, and the stacking time should be short. As long as the balance between the three is achieved and the best complementary effect is achieved, the best dyeing effect can be obtained. </p
