Kinetics of the changes imparted to the main structural components of human hair by thermal treatment

Thermal straightening of hair is a wide-spread consumer practice, which will impart specific hair damage. For practically relevant, cumulative conditions of thermal treatment (straightening iron, 200 oC, 100 - 800 s) untreated and oxidized (bleached) hair were investigated by DSC in water to determine the time-dependent changes of protein denaturation enthalpy (ΔHD) and temperature (TD). Assuming the standard two-phase morphology, the parameters are associated in specific ways with the α-helical proteins (ΔHD) in the intermediate filaments (IF) and their associated matrix (TD), respectively. Both parameters show systematic decreases with treatment time with synergistic effects of oxidation. The decrease can in all cases be described by a 1st-order type kinetic model. These predict that ΔHD, and thus the contents of α-helical material in the IFs, will approach zero for longer times of thermal treatment. The half-life time for the process is consistently about 20-25 min. A two-level, 1st-order approach shows that TD approaches limiting lower values with comparatively short half-times (untreated: ̴ 5 min, oxidized: ̴ 1 min). The approaches thus succeed to provide specific kinetic models for the thermal degradation in intermediate filaments (IFs) and matrix, including the synergistic effects of bleaching. The kinetic approaches are expected to be useful in the context of a range of further analytical investigations of thermal hair treatments.