Plant colorants for natural hair coloration: Dyeing optimization and photostability assessment

There is a growing interest in using naturally-occurring plant colorants as alternative to the toxic synthetic colors for hair coloration. However, poor color reproducibility and color fastness on the human hair remain a constant problem for natural hair dyeing. This study presents a systematic investigation into the dyeing performances of dye extracts of four hair dye plants (henna, walnut husks, sappanwood and Chinese gallnuts) for hair coloration with the focus on their photostability on the dyed human hair and the underlying mechanisms. First, the optimal dyeing conditions for each of the four plant colorants were obtained on yak hairs by assessing the effects of different dyeing variables (i.e. metallic mordants, mordant concentrations, mordanting methods and pHs) on color strength and CIE coordinates. Then, the plant colorants with ferrous mordant as the most influential metallic mordant to produce dark colors were investigated in terms of dyeability and photostability on human bleached hairs and natural gray hairs. The results demonstrated that the photosensitivity of naturally dyed human hairs is related to the nature of both hair dye colorant and the dyeing substrate. ESR measurement and fluoresce probing of hydroxyl radicals in hair fibers revealed that dyeing human hairs with natural plant colorants can inhibit, to varying extents, the photogeneration of oxidative radicals in human hairs, which may in turn provide the dyed human hairs with certain photo-protecting effects.