Abstract
The equilibrium absorptions of C. I. Direct Yellow 12, C. I. Direct Red 1 and C. I. Direct Red 2 by Vinylon have been investigated at 90°C. The salt-dye ratio p and the substantivity ratio q were calculated from the results. For C. I. Direct Red 1 and C. I. Direct Red 2, a logarithmic p, q plot is represented by an approximate single line, and this result is found in agreement with the theoretical deduction that the isothermal relation between the substantivity ratio and the salt-dye ratio is independent of the composition of the equilibrium solution.
On the other hand, for C. I. Direct Yellow 12, the result does not agree with the prediction of the direct dyeing theory, and the substantivity ratio increases with increase of the salt concentration in the dye-bath.
To explain this abnormal dyeing behavior, it is assumed that in the case of C. I. Direct Yellow 12, the dye molecules in the solution are easily dehydrated by added NaCl even at the dyeing temperature and the degree of hydration of the dye molecules decreases with increase of NaCl concentration in the dye-bath. Such a dehydration of the dye molecule increases the chemical potential of dye in the solution and decreases the chemical potential of dye in the fibre, so the substantivity ratio increases with an increase of the salt concentration in the dye-bath.
