Mechanistic aspects of African hair relaxation

For wavy and curly hair, whether it is European, Asian, or African, permanent straightening is an important cosmetic process to improve hair manageability and appearance. For wavy hair types straightening technologies, based on reduction/reoxidation sometimes combined with heat treatments, are generally of satisfactory efficacy and stability. However, highly curly African hair is usually straightened by relaxer treatments, applying strong bases (pH 11-14). Straightening and relaxation are two fundamentally different processes, in that straightening by reduction/reoxidation stabilises an externally imposed deformation, while relaxation is a chemically induced, spontaneous loss of hair curvature towards the straight state. For this study selected and related aspects of relaxer treatments (lye, no-lye) were investigated to gain a better understanding of their mechanisms of operation, their efficacy, and of their effects on the chemistry and morphological structure of human hair. Combined relaxer/reductant treatments were assessed for synergistic potential. One important quality aspect of a relaxer product is the speed, with which it de-curls hair. To investigate this aspect, a method was devised to quantify for single hairs the time dependence and efficacy of relaxation. The data showed a time-lag for the onset of straightening and generally first-order type kinetics once the process started. Time-lag was of the order of minutes. Characteristic straightening times were comparable for both treatments. The time-lag of the processes is related to the diffusion characteristics of the actives into hair. Diffusion was studied on blonde, untreated, Caucasian hair, pre-treated with iodine to a black colour. Alkali and/or reductant lead to discoloration, thus enabling to monitor the diffusion front in parallel with overall fibre swelling in the light microscope. Fibre swelling increased significantly with alkaline pH and furthermore through the addition of a reductant. For alkali/reductant combinations dual diffusion fronts where observed, where the inner front is attributed to the alkali. Applying ‘Case II’-diffusion kinetics, the penetration characteristics of the agents were individually analysed. To study the effects of the relaxer treatments on the main morphological components of the cortex of hair, treated hair samples were investigated by Differential Scanning Calorimetry in water (HPDSC), determining denaturation –temperature and -enthalpy. Enthalpy, as a measure of the α-helix content of hair, was strongly affected by the relaxer treatment, dropping continuously towards zero at higher treatment times. The process of helix-damage was accelerated by the addition, namely, of a strong reductant. In parallel, the effects on denaturation temperature were rather limited and no synergistic effects of relaxer/reductant were observed. A strong correlation between helix-content and straightening effect was established. In view of the spontaneous nature of the straightening effect, this leads to the overall conclusion that the alkali-induced denaturation of the helical filaments in Afro-hair is the primary mechanistic component of the relaxation process.