In order to investigate the diffusion behavior of reducing agents into hair keratin fibers, crosssectional samples of virgin white human hair treated with thioglycolic acid (TG), thiolactic acid (TL), and Lcysteine (CYS), which have a carboxyl group in their molecule, were prepared. After the treatment, the crosssectioned hair samples were dyed with methylene blue and the cross-sectional intensity spectra were measured at a wavelength of 664 nm (λ _(max) of methylene blue) with a microspectrophotometer. The three different diffusion patterns from the three reducing agents (TG, TL, and CYS) were obtained. The diffusion pattern of TL, which appeared as a sharp boundary line by using an optical microscope showed a combination of Fickian (TG) and Non-Fickian (CYS) types. Also, the diffusion pattern of thioglycerol (TGR), cysteamine hydrochloride, butyrolactonethiol, and glyceryl monothioglycolate, which don’t have any carboxyl group in their molecule showed Fickian type characteristics. In addition, it was found that the different alkali agents influenced the diffusion behavior of TGR and TG into virgin white human hair, and especially the waving efficiency of the waved hair treated with TGR. From these experiments, it has been concluded that the diffusion patterns of the reducing agents depended on the electrostatic interaction between the human hair and the reducing agents, and the chemical structure of the reducing agents.
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