Effects of cosmetic treatments on the morphology, biotribology and sensorial properties of a single human hair fiber

Human hair has a complex mull-scale structure that is extremely sensitive to several external factors. Hair fiber has three compartments: cuticle (scales), cortex and medulla. In the everyday life, hair experience important mechanical and chemical solicitations, such as smoothing, combing, drying and even washing, which can induce cracks in the cuticle. These damages diminish the mechanical properties of the cortex and contribute to a decrease in the brightness and softness of the hair. In the present article, the effects of different cosmetic treatments: bleaching, coloring and smoothing (with different temperatures) have been explored. Firstly, the morphology of hair was measured by interferometry. A mull-scale roughness characterization was performed by applying continuous wavelet transform. This method quantified the arithmetic mean of the surface topography at each wave length and permitted observing the evolution of the cuticle morphology before and after treatment. Secondly, a patented haptic tribometer system was used to measure the friction coefficient and acoustic vibrations generated during the tribological test. Previous results have shown that vibrations are related to the softness of the hair surface. The results show that all the treatments change the morphology and biotribology of hair by damaging the hair cuticle. These changes impact the sensorial properties of hair. In particular, the morphology of colored and bleached hair, compared to virgin one, is strongly modified: the scales are less defined, and the roughness is increased. These morphological modifications are correlated with an increase of the friction coefficient of around 50%, and acoustic vibratory level of around 2%, indicated a decrease of the sensorial properties. The hair fiber becomes rougher, dull and less soft. On the contrary, thermomechanical treatments of previously damaged hair, impact the morphology and biotribology of hair differently. The cuticle scales are flattened and the values of the friction coefficient and acoustic vibratory level decrease to reach the virgin hair values. Thus, the hair fiber seems to be smoother, brighter and softer. In conclusion, these results are essential for understanding the impact of different solicitations on the physical and sensorial properties of hair.