Comparing Hair Tensile Testing in the Wet and the Dry State: Possibilities and Limitations for Detecting Changes of Hair Properties due to Chemical and Physical Treatments

OBJECTIVES
This investigation focusses, first, on the question to which extent wet and dry tensile tests on human hair may be considered as leading to independent results. Second, we try to assess the sensitivities of wet and dry testing to detect changes of mechanical properties. Specifically we were interested in separating changes, which were induced by a combination of a chemical (oxidation/bleach) and a physical treatment (heat).

METHODS
The basis for our study are data for the tensile properties (wet and dry) of a set of untreated and bleached hair tresses, which were submitted to the same schedule of thermal treatments. As characteristic tensile parameters, we chose modulus (E), break extension (BE) and break stress (BS). First, parameters were analysed across treatments for the correlations between wet and dry data. Second, we applied two-factor Analysis of Variance to assess the effects of the factors and their potential interaction.

RESULTS
Correlations for the dry vs wet data show only a weak relationship for E, while coefficients of determination (R2) are quite high for BE and BS. Two-factor ANOVA enables to quantify the various contributions to the Total Sum-of-Squares for all three parameters. We show that, the parameters respond quite differently to the chemical and the thermal treatments as well as to testing conditions (wet or dry). It is of interest to note that, the interaction between the chemical and the physical treatment is generally quite weak. For the interpretation of the results, we use the concept of the humidity-dependent as well as strain-induced glass transition of the amorphous matrix.

CONCLUSIONS
The independence hypothesis for dry and wet tensile measurements only applies for modulus. Overall, we consider modulus (wet) as the best tensile measure of fibre damage when assessing chemical and/or physical treatments. Under ambient conditions (dry), break stress is shown to be a feasible alternative measure.