Multiple Bleaching of Hair: Fatigue Failure Investigations and Weibull Analyses

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Abstract
The fatigue failure (F/F) test is an important method for evaluating hair strength. In this study, we evaluate commercial European hair after up to four bleaches. F/F-data were directly fitted using a two-parameter cumulative Weibull distribution (CWD) function, described by lifetime index (λ) and shape factor (β). Fibre samples taken from treated tresses (T) exhibited stable F/F behaviour across multiple treatments, while treated single fibres (S) showed early breakage if treated more than twice. We attribute this effect mainly to the liquor ratio during treatment. In this context, we suggest a strategy to estimate a treatment-related apparent decay constant for λ. The constant for T-samples was determined as kn = 0.095. This is consistent with DSC results in the literature for the denaturation enthalpy. This may indicate an important role for intermediate filaments in fatigue failure. To possibly establish a link with hair breakage during combing, we investigated the properties of the hazard function for T-samples based on β = 0.92. The model calculations indicate that early failure risks are low but increase exponentially with repeated bleaches. The correspondence with the literature results leads to the hypothesis of a relationship between F/F and combing tests.

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