Structural changes of human hair through bleaching as shown by Differential Scanning Calorimetry and Scanning Electron Microscopy

Results are presented from investigations by Differential Scanning Calorimetry (DSC) in water of virgin and bleached human hair samples for commercial use. The two primary results of a DSC investigation are denaturation temperature, TD, which measures the thermal stability of the matrix in human hair, and denaturation enthalpy, ∆HD, which gives the amount and structural integrity of the α-helical material in the intermediate filaments (IFs). Using this analytical approach, the relationship between TD and ∆HD for hair samples at various levels of bleaching was investigated. The relationship between heating rate, β (as ln β) and 1/TD, according to the Arrhenius-equation, was shown to be linear giving for virgin human hair an activation energy of 262 J/mol, while for bleached hair (2h, 9% commercial bleach), a somewhat higher result of 295 J/mol was observed. The effects of thermal denaturation of hair in water for various heating rates were visually assessed by Scanning Electron Microscopy (SEM).