Inside hair fibre testing

High curl type hair presents the wearer with unique styling and grooming challenges. Dia-Stron explains how hair fibre metrology can provide key insights into the effects of treatments, grooming regimes and products on the properties of the hair.

There are a variety of testing methods to choose from in the hair care industry, including salon studies, sensory panels and scientific instruments. The data from measuring physical hair fibre properties can be used to develop new hair care ingredients and technologies, as well as evaluate technical performance of new formulations and even advance hair fibre science.

Scientific tests using specialised instruments provide repeatable, unbiased results. The measurement process can be automated and the environment controlled, to expand the testing capabilities. Crucially, it also produces objective data for evidence-based claims support packages, substantiating claims referring to the efficacy, benefits or improvements in hair attributes as a result of using particular hair care products or appliances.

DIFFERENCES IN HAIR TYPE

Human hair can be categorised by curl type into 8 categories, with type 1 being straight or very loose waves, and type 8 being tightly coiled/crimped fibres.

Hair breakage is a major concern for all consumers, but particularly for those with high curl types or who use aggressive grooming regimes/styling practices. These individual fibres are highly curled and kinked, with an elliptical cross section and significant variation along the length of the fibre.

Premature breakage of high curl type fibres, occurring exclusively in the dry state at extensions often less than 20%, is attributed to the inherent structure in the twist of the fibre or flaws produced by grooming procedures. For this reason, some testing techniques are more suited to high curl type hair fibres.

TENSILE TESTING

Tensile studies are perfect for investigating and assessing the performance of ingredients, treatments and products that penetrate into the hair cortex or regulate/ control the moisture of the hair. Tensile data can be used to substantiate claims such as ‘strength’, ‘damage protection’ and ‘damage repair’.

The Dia-Stron Laboratory team conducted a study on hair of African origin with a curl type of 6/7.

Fibre dimensional measurements were recorded on the Dia-Stron fibre dimensional analysis system (FDAS770). Specific morphological attributes of African hair fibres can be quantified using this direct, noncontact and non-destructive technique: e.g. ellipticity ratio or cross-sectional changes along the fibre.

Fibre tensile data was acquired on a miniature tensile tester (MTT690), an automated system designed to measure tensile properties of single hair fibres by stretching them to a specific percentage or to failure, in the dry or wet states. Normalising tensile data with fibre cross-sectional area reduces data variability by up to 80%.

Comparing untreated (virgin) hair fibres and fibres relaxed twice using a commercially available no-lye relaxer treatment, the data indicated that hair became less stiff and more flexible as a result of applying the relaxer treatment, particularly in the wet state.

The relaxer also reduced the break stress considerably while increasing the break strain, i.e. fibres extending further before breaking but ultimately breaking at a lower force. The absence of a post-yield phase could also be observed on relaxed hair fibres.

Did you know?

Flaws on the hair surface could develop under fatigue as cracks, propagating and growing, most often longitudinally, until catastrophic failure occurs.

FATIGUE TESTING

During daily grooming practises and routines, hair fibres are exposed to differing amounts of stresses at levels much lower than required to break the hair. Over time, this leads to an accumulation of stress within the fibre, eventually culminating in failure due to microscopic cracks and flaws that propagate and grow along its substructures. Often cracks occur at stress concentrator locations such as at a change of twist along the fibre.

To study this, a repeated stress or strain can be applied to single fibres until they break – known as tensile fatigue analysis.

The Dia-Stron laboratory team undertook a second study on hair of South African origin with a curl type of 6/7, taking dimensional measurements with the Dia-Stron FDAS770 and fatigue measurements with the cyclic testing module (CYC802).

The CYC802 system measures the dynamic strength of hair fibres by subjecting them to repeated cyclic tensile deformations until failure, simulating repeated grooming. This is an ideal technique for evaluating the damage caused by treatments (such as heat or chemical damage) and for claims relating to ‘strengthening’ or ‘damage protection/prevention’.

Tests were conducted on hair in its natural form, relaxed using a commercially available no-lye relaxer system, and hair that had previously been braided. This was to ascertain how the additional torsional stresses caused by braiding impact the fatigue survivability compared to a chemical treatment.

The tensile elastic modulus, recorded on the first loading cycle of the fatigue test, demonstrated that the application of the relaxer significantly reduced the elastic modulus compared to the natural hair. However, the physical process of braiding did not appear to significantly impact the elastic modulus compared to the natural hair.

Plotting the number of cycles to break against the fibre survival probability showed the natural hair statistically survived for a higher number of cycles than both of the damaged hair groups. A likely cause of these results is the chemical damage from the highly alkaline relaxer and structural damage from the tight inter-weaving of the braided fibres.

CONCLUSION

The studies conducted by Dia-Stron using specialised instruments showed significant differences on the mechanical properties of the hair samples following both chemical treatments and styling practices.

There is scope for additional techniques to be used to provide insights into the unique properties of African hair. For example, highly elliptical fibres tend to become rounder in cross-section when fibres are immersed in water, which can be quantified using the DiaStron dynamic swelling module. Measuring fibre torsional properties has also shown interesting insights.

Dia-Stron is developing a new instrument to examine the extremely low forces required to extend curly fibres to a linear configuration, known as decrimping, opening new possibilities for the development of ingredient and formulation technologies tailored to the African hair market. •

A CLOSER LOOK AT THE COMPANY

Dia-Stron has been involved in measuring hair for over 30 years, connecting with the hair scientific community, collaborating with academics and interfacing with the hair care industry. Represented locally by Meganede, Dia-Stron is a leading manufacturer of measurement systems for single fibres and hair tresses, with an instrumentation range covering a wide array of testing techniques such as tensile strength, fatigue, dimensional, bending and torsional properties.

Fibre testing automation, introduced in the early 90s, has transformed the way hair studies are performed. Dia-Stron instruments are typically found in the R&D laboratories of research organisations, multinational corporations, local companies and universities.

FDAS770 loaded with a 30mm hair fibre sample

Four CYC802 fatigue modules on an automation platform