This enhancement in the fiber dyeability would be attributed
to the presence of amidoxime groups present in the modified
samples that render the fiber more hydrophilic and less
crystalline [26].
Effect of Dye Bath pH
Figures 5 and 6 show that the pH values of the dye bath
have a considerable effect on the dyeability of acrylic fabrics
with natural dyes. For madder dye, the effect of pH (Figure
5) reveals that increasing the pH decreases the dyeability of
the fabric.
The effect of dye bath pH can be attributed to the
correlation between dye structure and the fibers used. Acidic
pH is expected to reduce the negative charge of the surface
of all acrylic samples in the dye bath and as a consequence
the dyeability increases. This effect is further manifested for
samples A and B due to their content of amidoxime groups
which become protonated at acidic pH and the extent of dye
adsorption mirrored the extent of protonation of the terminal
amino groups in these fibers. It is an equilibrium process of
protonation and deprotonation as depicted in Scheme 1 and
therefore, the protonation of the amino groups will increase
with decreasing the pH of the dye bath.
On the other hand, curcumin dye reveals (Figure 6)
different results in which decreasing the pH less than 4 leads
to lower dyeability. This result is mainly related to the effect
of pH on the keto-enol tautomersim of curcumin (Figure 1).