Characterisation of Human Hair and the Effects of Chemical Treatments

Abstract

Chemical treatments of hair, especially bleaching, affect the inner and outer hair structure due to oxidative damage reactions. Several aspects of these damage effects are investigated. An optical evaluation system is developed to characterise the surface properties of unbleached and bleached hair in an uncontrolled in vivo study by means of confocal laser scanning microscopy. Bleaching leads to deformation of the scale edges, distances between scale edges and other surface phenomena e.g. residues. A rougher surface of bleached hair compared to unbleached hair is observed by using the automatic measuring software of the confocal laser scanning microscope. The influence of natural hair colour, length of hair, as well as heat styling or hair care rituals on the contact angle and tensile properties of hair is determined as well. Particularly bleaching affects both the inner and outer properties.In a controlled in vitro study the influence of bleaching on the inner hair structure is analysed by means of DSC and tensile measurements. Ultra bleaching and medium bleaching of hair lead to a direct decrease in denaturation temperature (TD). By monitoring TD over a period of six months a significant increase is observed. While the denaturation enthalpy of hair decreases directly after ultra bleaching, the enthalpy after medium bleaching is slightly higher compared to virgin hair. Similar effects can be observed by performing tensile measurements, which also show a strengthening after six months. The DSC results as well as those of tensile properties show a continuous change of the inner hair structure after bleaching, presumably due to an ionic rearrangement mainly in the matrix. The hair regains thermo-stability as well as mechanical stiffness. Differences in swelling behaviour as well as surface shrinkage phenomena after DSC also indicate changes in the matrix depending on the storage time after bleaching. The low denaturation temperature directly after bleaching can be increased again by a 24-hour dialysis of hair, which eliminates remaining ions of the bleaching product. However, the denaturation temperatue of hair depends mainly on the pH-value of the treatment: the lower the pH-value the higher TD. Therefore TD of bleached hair can be increased by a chelating agent treatment at pH 4.6 as well. The positive impact of EDTA is presumably caused due to its buffer action, which stabilises an acidic adjusted pH-value equal to the isoelectric point of hair, which results furthermore in lower swelling of hair.

Date of Award	1 Aug 2013
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Franz Wortmann (Supervisor)