<b>Dyeing Behavior of Keratin Fibers with Oxidative Dyeing System:Mechanism for the Respective Effects of Each EDTA and Ascorbic Acid</b>

Abstract

The effects of EDTA and ascorbic acid on the oxidative dyeing behavior of wool fibers dyed with the couple systems of p-phenilenediamine/p-amino-o-cresol (pPDA/pAOC) and p-aminophenol/p-amino-o-cresol (pAP/pAOC) were investigated by means of analyzing dyeing rate curve which was expressed in relation to the surface dye concentration (K/S) and the dyeing time. The effects of these additives on the dye uptake behavior were dependent on the reactivity of the oxidative dye intermediate, hydrogen peroxide concentration, and the additive concentration. In pAP/pAOC dyeing system, the amount of dye uptake in dying system with EDTA was reduced compared with that of non-additive dyeing system. This is due to the fact that oxidative coupling reaction is inhibited by chelating metal ions present on the fiber surface. However, the amount of dye uptake increased with increasing of the added amount of EDTA. This finding implies that there is another effect of EDTA. This effect was believed to arise from that the oxidative decomposition of the oxidation dye generated in the fiber-liquid interface is suppressed by the reduction in the perhydroxyl anion concentration. On the other hand, ascorbic acid exerted the effect in both dyeing system and brought on the action similar to EDTA; the inhibiting action of oxidative coupling reaction and the suppressing action of the oxidative decomposition of the generated dye. These actions are due to the fact that ascorbic acid preferentially reacts with the active oxygen species. The former is primarily caused by the decrease in the amount of hydroxyl radical, and the latter is due to the decrease in the perhydroxyl anion. In addition, ascorbic acid acted as a pro-oxidant and resulted in decomposition of the dye that adsorbed into the fiber.