Impact of ultraviolet radiation‐induced damage to hair fiber integrity: A multi‐technique physicochemical characterization of surface and cortex properties

The combination of chemist techniques is a powerful tool on hair's structure observation and its behaviour when submitted to different process, like chemical damages as bleaching and ultraviolet (UV) radiation exposition, or physical damages, like successive brushing. This study mimicked solar radiation damage suffered by hair fibres during Brazilian summer (3 months of intense sun), and compared this kind of damage with the ones caused by bleaching process using different techniques. Natural hair after radiation exposition showed cuticular damages compared to the effects caused by bleaching. The increase of cystic components oxidation and hydrophobicity decrease were observed when natural hair was exposed to 96 and 192 h of UV radiation. Additionally, keratin's degradation enthalpy of damaged hair showed a decrease of mean values in comparison to natural healthy hair. These changes can be explained due to the protein and lipidic oxidative damages caused by UV radiation, causing alterations on cuticular level. Regarding cortical damage, natural hair tresses exposed to UV radiation for 192 h exhibited a significant decrease in mechanical parameters-specifically, force at 20% elongation and elongation at break-when compared with the initial condition of both bleached and double-bleached hair. This suggests that UV exposure exerts a more pronounced impact on these mechanical properties than chemical oxidation alone. In contrast, the enthalpy measurements show that the chemical degradation caused by bleaching is comparable to the structural alterations observed after 192 h of UV exposure. Moreover, double bleaching results in substantially more severe degradation of structural proteins than that induced by UV radiation.