Studies of the ζ-Potential of Natural and Synthetic Fibers in Dye Solutions. XVIII. ζ-Potential and Surface Dyeability of Heat-Set Nylon 6 Fiber in an Acid Dye Solution

Abstract

Abstract In order to ascertain the relation between the change in the fibrous fine structure and the dyeability of nylon 6 fiber caused by its heat-setting, the ζ-potential, the surface dye adsorption, and the total dye adsorption of the heat-set nylon 6 fiber in an acidic solution (pH 3.0) of acid dye—Orange II were determined. The crystallinity and the birefringence of the fiber increase in the order of the untreated, the wet-heat-treated, and the dry-heat-treated fiber. The isoelectric points of the untreated, the wet-heat-treated, and the dry-heat-treated fiber are pH 5.4, pH 4.6, and pH 2.8 respectively. With the increase in the dye concentration, the sign of the ζ-potential of the untreated and the wet-heat-treated fiber in a dye solution change from positive to negative because of the electrostatic bonds between the NH3+ of the fiber and the dye anion, RSO3−, while the ζ-potential of the dry-heat-treated fiber increases its negative value because of the van der Waals’ forces between the fiber and the dye. The surface dye adsorption, as calculated from the difference between the surface charge density, Δσ, of the system with dye and that without dye, increases in any case as the dye concentration increases. The total dye adsorption increases as the dye concentration increases and attained the plateau regions at higher dye concentrations. The surface dye adsorptions become smaller in the order of the untreated, the dry-heat-treated, and the wet-heat-treated fiber. The total dye adsorptions become smaller in the order of the wet-heat-treated, the untreated, and the dry-heat-treated fiber. The differences in behavior among the fibers may be attributed to the adsorption of the dye to the loosely-packed portions in the amorphous region of the wet-heat-treated fiber, in addition to the differences in crystallinity and birefringence among the fibers. Moreover, the surface areas covered by adsorbed dye molecules, which were calculated from the reciprocal of the slope of the surface dye adsorption vs. the total dye adsorption curve of each fiber, were 1.8, 4.0, and 0.6×106cm2/g fiber in the order of the untreated, the wet-heat-treated, and the dry-heattreated fiber.