The kinetics of moisture sorption by hair.

Objective: The rate process of moisture sorption by human hair has been analysed in order to hints for helping deepen knowledge on the hair structure and to explain the behaviour of hair in response to moisture.

Methods: The isotherms of moisture sorption by hair were recorded, via dynamic vapour sorption (DVS), for untreated (virgin) and treated (heat-shaped and bleached) hair, as well as for separated cuticle and cortex.

Results: By considering that, during moisture uptake, the hair fibres also swell, it is possible to introduce a time-dependent rate constant for describing the kinetics of the moisture sorption. This model allows for clearly separating the moisture sorption processes occurring in Cuticle and in Cortex and for proposing a role of chain entanglement in the two main compartments of the fibre. It may also provide some hints on the structural changes occurring in the fibre after different cosmetic treatments. The influence of the weight of the sample on the kinetics of the sorption process has also been noted and quantified. The analysis pointed to a transition occurring at around 30% relative humidity, assigned to the opening of the hair inner structure, and accommodation of more water molecules. This allowed for an estimate of the value of the activation energy of the water molecules reacting with the active sites, which was found to be in good agreement with results published in the literature.

Conclusion: The analysis of the kinetics of moisture sorption by hair was shown not only to provide information on the chain entanglement inside the fibre and the effect of cosmetic treatments but also to demonstrate and quantify the influence of fibre density on the sorption process. It is thus suggested that, along with examination of the hysteresis, the analysis of sorption kinetics helps reveal a more complete picture of hair moisture management.