Assessing the properties of thermally treated human hair by tensile testing and DSC: Are they complementary or equivalent methods?

Tensile testing and thermal analysis by DSC are important tools to assess structural changes and damage imparted to human hair by cosmetic processes and products. The methods target specifically properties of the two main morphological components of hair, assuming a simplified two-phase, filament/matrix structure. From DSC in water denaturation temperature (TD) and enthalpy (ΔHD) are obtained, which reflect the thermal properties of the partly alpha-helical filaments and their associated matrix, respectively. Non-destructive tensile testing in water yields the elastic modulus (Ew), as a joint measure of the components’ performance with a strong bias towards filament properties. In view of the assignment of the parameters to specific morphological components it is of academic as well as practical interest to investigate potential relationships between DSC and tensile parameter values. These relationships have been assessed for hairs with a complex chemical and physical history. Untreated and bleached European hair tresses were subjected to high temperature (200oC), in line with practical conditions of hair straightening, for times between 100 s and 800 s. The results show significant differences in the overall relationships of the parameter values between hair types, including pronounced non-linearity, namely for bleached hair. However, 2D-projections, that is, specific considerations of Ew vs ΔHD and Ew vs TD, respectively, show in all cases good linear relationships, again with pronounced differences between untreated and bleached hair. The correlations are discussed on the basis of the two-phase structure of hair and the ‘latent damage’ hypothesis for alpha-helical structures in bleached hair. For long treatment times hair approaches the properties of an amorphous, cross-linked, duro-plastic material. The results indicate a strong overall equivalence of tensile and DSC-parameters for assessing changes of hair. The practical advantage of DSC is that it is a collective rather than a single fibre test. It furthermore gives a differential view of the properties of the main morphological components of hair, while standard tensile testing only provides a summary assessment.