Franz J. Wortmann

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 k_n = 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.

Objectives: Machine‐based cyclic combing of hair tresses under dry conditions is a proven method for evaluating hair strength and the impact of treatments. Recent advancements in image analysis allow for a detailed review of hair fragment lengths and quantities produced after specific combing cycles. Our aim is to provide an in‐depth analysis of the kinetics of hair fragment formation. Methods: We analysed the combing performance of untreated and bleached European straight hair, assessing two conditioning treatments. Hair tresses underwent 5000 combing cycles, with the increasing number of fragments recorded. Results were fitted using a three‐parameter Voigt–Kelvin model. Results: The fragment counts were log‐normally distributed in all cases. The model uses ln( N ₀ ) to indicate early fragment release, showing higher numbers for bleached hair compared to untreated hair, which significantly drop with conditioner treatments. The ln( N _∞ ) parameter estimates the maximum expected fragments. The Failure Cycle Index (FCI) remains largely unchanged across materials according to its 95% confidence limits. Conclusions: The selected function's characteristics and the invariance of FCI indicate that friction primarily controls fragment formation in straight hair during combing. This suggest that there is no direct link between combing performance and tensile fatigue failure for this hair type. In contrast, textured hair will likely show more complex combing performance. Our analysis shows that the total number of fragments after many combing cycles can predict early failures and assess conditioning agents, thereby supporting product claims about ‘hair strength’.

Human hair is a fibrous protein composite, for which the thermal and mechanical properties change in specific ways with acidic and alkaline pH levels. To study these effects, DSC (wet) is a practical method for pH-equilibrated and unrinsed samples. When samples are exposed to deionised water in the DSC-pan, a secondary process of ion partitioning occurs. Based on the Freundlich sorption isotherms for H⁺ as well as OH⁻ -ions for hair, we found the effect to be small and constant. Furthermore, the isotherms provided estimates for the ion contents in hair, allowing us to study changes of the DSC-parameters based on ion content rather than pH. These changes are discussed with regard to the impacts of ion contents on the α-helic fraction of the IFs (denaturation enthalpy: ΔH _D) and on the matrix (denaturation temperature: T _D). T _D increases linearly with H⁺ -content by ≈8 °C down to ≈pH2 (≈ 800 μM g⁻¹ H⁺). However, for alkaline, pH changes are more complex. We observe two non-linear regions with OH⁻ -content and a discontinuous change around pH12. These observations are discussed on the basis of cross-link reformation in the matrix through lanthionine formation. The helical fraction in the IFs is found to be very stable against H⁺- as well as OH⁻ -contents up to ≈1500 μM g⁻¹. Any signs of helix denaturation for higher contents for both ions are attributed to the destabilisation of the apolar interactions, which stabilise helical dimers.

Objectives: Through the cooperation with an industrial partner, we gained a set of data for the tensile properties (wet) of human hair. The hair samples originated from a cross‐over study with two groups of individuals, using for a topical application sequentially two products (A and B). Each phase of the study lasted 6 months. The phases of the study by chance covered first largely the winter and then the summer period. Initially, tensile variables were chosen, which not only reflect the mechanical properties of hair (modulus, break strain, and break stress) but which are also considered to have a good connection to practice‐relevant hair properties. The initial analysis of the data showed that changes were observed for the variables due to the treatment phases. However, these were either small or difficult to interpret.Methods: Against this background and using two‐factor analysis of variance, we investigated the hypothesis that the tensile properties of hair (wet) may show significant seasonal changes. For this, we chose those two independent variables, which reflect the properties of the intermediate filaments (modulus) and the matrix (break strain) in the composite structure of the hair cortex.Results: The results support the ‘seasonal’ hypothesis and clearly show that the variables show significant changes from Winter to Summer (modulus: 10% increase; break strain: 3% decrease). The seasonal effect was thus a major reason, why the first stage of the analysis of the data was inconclusive.Conclusions: The tensile properties of the main morphological components of the cortex show distinct seasonal changes. Towards the summer, the hair becomes stiffer and more brittle. Furthermore, the results suggest that seasonal effects may need to be taken into account when conducting studies on lengths of hair grown during different seasons.

Objectives: This investigation focuses, 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 versus wet data show only a weak relationship forE, while coefficients of determination (R²) are quite high forBEandBS. Two‐factor ANOVA enables to quantify the various contributions to theTotal Sum‐of‐Squaresfor 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.