Thermal Induced Changes in Cuticle and Cortex to Chemically Treated Hair

The deterioration of the cuticle and cortex hair due to routine cosmetic practices has been identified as a primary factor contributing to undesirable changes in the esthetic qualities of hair. Chemical and physical treatments can cause significant damage to hair fibers. In this study, the damage induced by heating in chemically treated hair subjected to acid straightening, bleaching, and the combination of both treatments is investigated. Previous results published by our group have already clarified certain aspects of the mechanism of action of acid straightening on hair fibers. In this new work, we show other relevant aspects to the hair care area with the aim to respond: how can we assist cosmetic product developers and consumers in understanding the changes caused by the combined use of chemical transformation procedures (specifically acid straightening and bleaching) and exposure to heat? By examining the thermal behavior of chemically treated hair fibers, we shed light on key aspects of both external (through Fourier transform infrared spectroscopy-attenuated total reflectance [FTIR/ATR] and scanning electron microscopy [SEM]) and internal (through ultra-small angle x-ray scattering [USAXS], small angle x-ray scattering [SAXS], and wide-angle x-ray scattering [WAXS]) changes. SAXS and WAXS structural analyses provided information on the internal structure and hierarchical organization of hair samples. While these techniques have been widely used to evaluate hair fibers, the effects of heating on their structure have been less explored. We examined the changes in the hierarchical arrangements as the fibers were heated in situ during the x-ray scattering experiments. It was possible to evaluate the specific regions where heat causes damage to both the cortex and the cuticle of the hair fiber, and the extent of these damages. Furthermore, it was observed that the bleached and straightened fiber undergoes more changes with the use of heat, due to the loss of important surface components, as shown by FTIR and SEM measurements.