Effects of thermal, non-permanent straightening on the morphological components of human hair

The established practice of treating slightly curly or frizzy European hair with flat, heated ‘irons’ provides an effective grooming tool to achieve straight hair. The effect is reasonably stable until exposure to humid conditions or washing. Due to the fact that in practice high temperatures are employed (≥190oC), which for some devices (~235oC) even approach pyrolysis temperatures (>240oC), as well as frequently repeated treatments, there is a firm expectation for substantial and cumulative hair damage. Due to the relevance of this grooming practice in the market place, the expert working group ‘Hair Care Products’ of the DGK currently conducts a comprehensive 2-phase study to contribute to our understanding of the objective as well as subjective changes of European hair through thermal straightening. During the first phase of this study brown, untreated as well as 2x bleached Caucasian hair tresses were treated at 200oC for cumulative times of 100s, 300s, and 800s, estimated to represent 2, 6, and 16 months of grooming practice, respectively. As a first part, results are reported for two analytical methods, which target two quite different morphological components of hair, namely, the intermediate filament/matrix complex in the cortex on the one hand and the cuticle as protective sheath, on the other. After the heat treatment the tresses were investigated with respect to their denaturation performance by Modulated DSC in water. On the basis of denaturation temperature TD and enthalpy ΔHD the changes of matrix and intermediate filaments (IF) in the structure of the cortex were assessed. The results show that both parameters follow consistently 1st order kinetics with treatment time and between them show strong correlations for both untreated and bleached hair. A superposition procedure was developed to determine specific bleaching ‘damage factors’ for IFs and matrix. The success of this approach implicitly leads to the conclusion, that heat damage is independent of the specific previous chemical history of hair.