Exploring the Potential of the Three-Point Cantilever Bending Test

The three-point-bending test is, at present, a very seldom-used measurement in the hair category. Dr. Xuzi Kang, at TRI Princeton, has taken this method out of the drawer and looked at it afresh. Her studies have shown that the three-point bending test can measure the effects of damage on hair softness, and open new opportunities for damage repair claims. Her work has also shown that the technique can measure the degree of set delivered by different styling products and their resistance to humidity.

Illustration of the technique and the machine setting.

Test description

The 3-point cantilever bending technique is a test used to measure the strength and flexibility of the hair. It involves applying a force at a specific point between two supports and measuring the amount of deflection at the middle point. The amount of deflection measured can be used to calculate various properties of the hair, such as its stiffness, toughness, etc. [1].

Pre-treated hair tresses are mounted to the 3-point cantilever bending machine, and the probe goes 1 cm into each hair tress to form the bending. The peak force of the 1st cycle (F1) is used to determine the stiffness of the tress. The area under the curve (AUC) evaluates the toughness.

On the left is an example of a force vs. distance plot generated from the 3-point cantilever bending test. The brown curves correspond to a 9% bleached hair tress, the blue curves correspond to a 9% under-bleached hair tress, and the yellow and black curves correspond to 6% bleached and virgin hair tresses. On the right is an example of the force vs. time plot for virgin medium brown (MB) hair, showing the F1 and AUC.

Effects of hair damage.

Recent work, using the three-point bending technique, has revealed that bleach damage makes bundles of hair less easy to bend when gently pressed. These experimental data match what consumers say about their damaged hair feeling stiffer and drier. The question now is, can we make damaged hair softer again with cosmetic treatments? Watch this space!

The stiffness and toughness of the hair tresses are subjected to different strengths of bleaching. Compared with virgin hair and 6% bleached hair, 9% bleached hair is stiffer and tougher. The results align with the hair tress measurement of cysteic acid, a widely recognized method for assessing hair damage.

The degree of hair damage after different bleaching treatments as measured by ATR-FTIR

Degree of styling hold and effects of humidity

The degree of hold from styling polymers is often measured using the curl drop test, however recent studies suggest that the three-point bending test might be equally effective. The degree of hold measured by the three-point bending is also sensitive to moisture, opening opportunities for humidity resistance claims.

The stiffness and toughness of two different hair spray products under different humidity. Both displayed significant change under humid conditions. High humidity increases the stiffness and toughness of product 1 (weak-hold dry shampoo) and decreases that of product 2 (strong-hold hair spray). A possible explanation is that when the starch in the weak-hold dry shampoo absorbs water, the hair tresses become stiffer and tougher. Thus, in high humidity, the tresses become stiffer and tougher when treated with product 1.

Conclusions

The 3-point cantilever bending technique is a powerful tool specifically for testing hair care products such as conditioners, fixatives (spray, gel, mousse, etc.), and bleaching products. By providing an objective assessment of the mechanical properties of hair, this technique enables researchers to test claims about the performance of their hair products under different humidity mimicking real life. With its ability to measure multiple parameters, the 3-point bending technique is a sensitive method for hair care product developers to make claims about the benefits of their products.

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Reference

1. Development of a three-point cantilever bending technique to study the mechanical properties of hair styling ingredients. RogerL.McMullen, TimGillece. DOI:10.1111/srt.13256.