Impairment of hair mechanical properties by sun exposure and bleaching treatments

The aim of this study was to compare the effects of sun exposure and bleaching treatments on hair mechanical properties. Tresses of virgin dark-brown hair were initially washed with lauryl sodium sulfate solution. Sun irradiation was simulated with xenon (full radiation) or mercury-vapor (full or UVB-filtered radiation) lamps. Hair was bleached using a commercial persulfate-peroxide solution. Stress/strain data were obtained from 20 fibers of each sample using a universal test machine. Breaking elongation and breaking strength were the main parameters affected by both treatments. Reduction in the breaking strength was observed for the photo-oxidized and bleached hair (compared to untreated samples). A reduction in the breaking elongation was also observed for the photo-oxidized samples. On the other hand, an increased tendency in this parameter was observed for the bleached hair. In relation to the radiation source, similar results were achieved with much less exposure time to a xenon lamp compared to a mercury-vapor lamp. When UVB radiation was filtered, no breaking strength reduction was observed, indicating that changes in hair mechanical properties are mainly related to that range of the solar spectrum.