32 HAIR CARE
Tensile The tensile properties of hair are predominantly governed by the integrity of the cortex. The more damaged the cortex, the lower the tensile strength of hair and the higher its propensity to break.
When evaluating strengthening actives,
it is common practice to damage the hair by bleaching. Bleaching breaks many types of bonds in the hair, the most important being the covalent disulphide bonds between cysteines. Hair is typically bleached for 40 minutes to reflect usual salon practices. Humidity and pH can significantly disrupt
tensile results. Performing tensile testing at high humidity levels can result in lower break force readings compared to tests conducted at lower humidity, which typically yield higher tensile strength results. Lower pH levels give more positive results in comparison to higher pH. This is because the natural pH of the hair is acidic between 3.5 and 4.5. When measuring the tensile strength of hair,
single fibres are stretched until they break. Due to the inherent variability of hair, the process is repeated at least thirty times. Specific tensile parameters are measured during the testing, such as break force, elastic modulus, total work done, etc. Elastic modulus is usually interpreted as a measure of hair’s softness, while total work done can be misleading if hair’s extensibility is affected. Break force is the most useful parameter to
use when evaluating a strengthening active as it is a measure of the extent to which new bonds have been formed within the damaged cortex by the active.
Differential scanning calorimetry A complimentary method to tensile testing is differential scanning calorimetry (DSC). This, again, is a measure of cortical integrity and strength. In this method, small hair snippets are heated up in a hermetically sealed aluminium pan and heat flow is compared to an empty control pan. The parameters obtained from this method
are denaturation enthalpy, and denaturation temperature. Generally, if these have increased after hair has been treated with an active, then hair is assumed to be stronger. However, many types of bonds are formed within the cortex after treatment with an active. Not all of these contribute to strength. In addition, severely damaged hair can also
make it difficult to obtain denaturation enthalpy and denaturation temperature from DSC, as the peak may become indistinguishable from the baseline due to excessive noise. Therefore, the DSC parameters need to be interpreted with care. When carrying out claims support testing for
hair strength, it is recommended not to rely on a single instrumental method. Only then, can one have confidence in the effectiveness of an active ingredient.
Fluorescence microscopy As strengthening active ingredients predominantly work in hair’s cortex, it is important to demonstrate the actives’ ability to penetrate into this layer. In fluorescence
PERSONAL CARE January 2025 Figure 2: Straightening treated hair
microscopy, an active ingredient is labelled with a fluorophore. Upon excitation at a specific wavelength of light, the fluorophore will re-emit light at a different wavelength (fluoresce). Hair is treated with the labelled active
ingredient. Treated hair fibres are then mounted in a matrix (usually paraffin) and thin cross sections (typically 5 μm) are made by using a microtome. The hair cross sections are finally viewed with a fluorescent microscope. Viewing the hair cross-section will show
whether the active has penetrated into the hair’s cortex as well as the depth to which it has penetrated. This is a very powerful visual tool that complements instrumental techniques for claims support.
Hair manageability Sometimes there is a difference between what a consumer perceives and what we understand is actually happening at JRSI. Damage to the cuticle layer leads to a decrease in the tactile perception of hair. Hair may become harder to brush or style and overall manageability is reduced. Increased breakages are observed leading to some consumers believing this is a loss of hair strength. While there may be some underlying cortical
damage, the loss of manageability is actually caused by the lifting up and swelling of the cuticle cell, breakages of the cells and loss of the lipid layer that provides fluidity. Most hair treatments target the cuticle and
aim to restore the fluidity of the hair by applying conditioning agents to replace the lost lipid layers or form barriers to restores cuticle function. Natural conditioning agents are increasing in popularity competing with long established synthetic alternatives. Conditioning performance of an active
ingredient is conveniently evaluated instrumentally using combing force and smoothness measurements. A good conditioning active will cause a reduction of combing force and an increase in smoothness.
Combing rig Treated hair tresses are mounted in a combing rig along with any controls or benchmark products. Hair tresses are combed multiple times under controlled conditions. Broken hairs are captured in trays and the performance of a treatment is evaluated by visually comparing the number of hairs broken versus a control or benchmark product. A lower number of broken hairs indicates
that the treatment is effective at increasing strength. Hair is subjected to many different kinds of
forces during combing or brushing. If an active ingredient improves lubricity of hair, then less of it will break. This is an alternative method of measuring hair strength as consumers perceive less hair fall as measure of increased hair strength.
Colour Hair colour maintenance is a challenging attribute to deliver. The effectiveness of an active material can be evaluated after multiple washing cycles or UV exposure by using colorimetry to quantifiably define the colour. Basic dyes are large molecules that adhere
to the surface of the cuticle, while direct dyes are smaller in size so some of the dye is able to penetrate into the hair. Basic and direct dyes are used for temporary and semi-permanent formulations such as conditioners and hair masks and only need a short application time for a vibrant colour result. Oxidative dye precursors are small molecules
that penetrate hair where they oxidize inside the hair forming larger coloured molecules that are trapped within the hair. Therefore, direct and basic dyes are more easily washed from hair as compared to oxidative dyes. To measure how effective an active
ingredient is in ensuring colour fastness, a colorimeter measures freshly dyed and rinsed hair tresses. A treatment is then applied to
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