Correlation between Mechanical and Thermal Properties of Human Hair

Hair strength depends on integrity of protein structure, aging, and chemical effects and its exposure to mechanical (combing and curling) and thermal (hair drying and straightening) stimuli.

The aim of this study is to correlate the mechanical properties such as tensile yield stress and tensile modulus of single hair fibers with thermal properties such as melting enthalpy and melting point obtained by differential scanning calorimetry.

Single hair fibers covering seven decades of age were cut 2 cm above the scalp. Tensile specimens were prepared using single fibers with gauge length of 20 mm, and test was carried out with speed of 20 mm/min. For thermal analysis, 2-3 mg of hair samples were cut to 0.5 mm length and loaded in aluminum pan and heated at a rate of 10°C/min from 30°C to 280°C.

Tensile yield stress (determined at the onset of stress constancy at ~3% strain) and tensile modulus were determined using Origin software. The same software was used to plot the thermal data as heat flow (W/g)-temperature diagrams and to determine the peak temperature and peak area (melting enthalpy) by creating a sigmoidal baseline.

The typical values of tensile modulus, yield stress, and maximum stress obtained were 5.1 ± 0.5 GPa, 109 ± 9 MPa, and 161 ± 24 MPa, respectively. Further, typical values of melting enthalpy and peak temperature determined were 6.29 ± 0.20 J/g and 235°C ± 0.6°C, respectively.

Mechanical yield stress and melting enthalpy were compared, and it was found that a good nonlinear (sigmoidal) correlation was obtained. The nonlinear correlation seems to provide an accurate representation of a composite keratin structure wherein the % crystallinity exhibits a range, and also, crystalline and amorphous regions are intertwined and intricate in nature.