The following Application Highlight addresses the thermal conductivity measurement of human hair using the Modified Transient Plane Source (MTPS) method.
To achieve a desired style, hair can be subjected to various thermal, mechanical and chemical treatments. These can include straightening or curling via thermal tools, detangling via brushing or changing colors using various chemical dyes. While fashionable, these treatments can have damaging effects on the hair structure, primarily through attack of the protective outer most layer (cuticle). While various products can be used to repair and/or prevent hair damage, ultimately these treatments can cause harm, especially if frequently applied. From a thermal perspective, heat damage occurs through the conversion of α-keratin (fibrous protein that makes up >95% of the hair follicle) to β-keratin (structurally weaker form) resulting in a loss of elasticity and making it more easily prone to subsequent damage. To better understand the negative influence heat treatments can have on hair and how to combat them, thermal properties such as thermal conductivity and heat capacity are often highlighted as key attributes of interest.
Figure 1. Left) Depiction of damaged vs healthy hair. Right) Different types of hair damage.
Thermal conductivity is important as it relates to the ability of the hair to handle the heat generated from various thermal applications. Measuring thermal conductivity of hair can be challenging due to the small size of the individual fibers (in the range of 70 µm diameter). Thermal conductivity instruments can vary with regards to sample size requirements, however in all cases, samples of this size are well below what is required for commercially available instrumentation. While direct measurements of individual strands are not currently possible, bundling of hair into a macro structure (think ponytail) can be used to better understand the effective thermal behaviour. Variables of the hair bundle such as hair diameter, density, assembly pattern, etc. can influence results, similar to how textiles and fabrics have been shown to behave. The following is an example dataset collected on a human hair extension piece using C-Therm’s MTPS method. Results are the average of 5 consecutive measurements, repeated in triplicate. The hair was bundled together and tested in the though-thickness direction under a 500gF compressive load under ambient conditions.
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