For dyeing with reactive dyes, the choice of the three primary colors of dyes must be paid attention to by dyeing workers. Among them, the ratio shift value (Rf) of the dye is a very important parameter. The Rf value represents the affinity of the dye. During dyeing, it determines the instantaneous dye uptake rate, which in turn affects the stability and reproducibility of dyeing. Therefore, in order to make dyeing stability good, it is necessary to fully understand the ratio shift value of the dye, especially the ratio shift value of the three primary colors of the dye.
Prepare 2g/l, 20g/l, 30g/l three different concentrations of dye solution, cut the filter paper into 2cm×12cm, draw a pencil line at the bottom 0.5cm, hang it vertically, and place a container under it. Pour the dye into the container so that the dye solution just immerses the pencil line. At this time, the dye liquor starts to climb along the filter paper, and at the same time, a water line without dye will rise upward. Since the affinity of reactive dyes to filter paper made of cellulose is greater than that of water to filter paper, the water line will always surpass the dye line.
At 20 minutes, the dye line and the water line have basically reached equilibrium, and the rise is no longer extended. Remove the filter paper, dry, and measure the heights of the dye solution line and the water line respectively (h2 is the climb of the dye, which is lower than the water; h1 is the climb of the water, which is higher than the dye). The Rf value of the dye can be measured by Rf=h2/h1. A large Rf value means that the dye affinity is low, the dye uptake rate is low, and the absorption is slow; on the contrary, the Rf value is small, which means the dye affinity is high, and the dye uptake rate is high, and the absorption is faster.
When selecting the three primary colors, the closer the Rf value is, the closer the simultaneous dyeing rate is, and the better the dyeing stability. Even if there are some fluctuations in the process, the shade will change, but the hue will not change much. If the Rf value differs greatly, the simultaneous dyeing rate must be different, the dyeing stability is poor, the sensitivity to process changes is also greater, and the hue is difficult to control.
The specific shift value of reactive dyes is not constant:
As the concentration increases, it increases, and as the concentration decreases, it decreases. Generally speaking, the lower the concentration, the more obvious the decrease; the higher the concentration, the slower the increase. When the concentration of most dyes exceeds 20g/l, the specific shift value gradually balances; when sodium sulfate is added to the dye solution, its Rf value also decreases sharply, the dyeing affinity increases rapidly, and the dye migration property is significantly reduced!