Tensile Properties

 

Stylists and clients evaluate the cosmetic properties of hair extensions according to combing ease, strength and elasticity.

We translate these attributes into single fiber evaluations of friction and tensile properties.

Human hair is an elastic substance that is subject to strain (deformation) when stress is applied.

The usual procedure for evaluating the strength and elasticity properties of human hair is via tensile strain/stress tests.

 

How it's done:

 

1. A fiber of known length and diameter is stretched at a fixed rate and a fixed relative humidity and temperature on an automated instrument.

2. The procedures for testing the strength and elasticity involves setting the temperature in the testing room to approximately 21 degrees Celsius (70 degrees Fahrenheit) and the humidity between 40%-63%.

3. The higher the humidity the stronger the hair will appear, which can guarantee results that do not represent reality.

4. A length of hair will be mounted on clamps that will be zeroed, meaning we have to make sure that there is no tension or slack on the hair. Our software performs this for us.

5. A motor attached to the tensile strain stand will move the clamps apart at a very low speed.

6. While activating the motor, we will simultaneously activate the software that will transfer the information, analyze the numbers and plot it onto a graph.

7. After few minutes the strand of hair will break apart and the software will mark this exact point on the graph.

8. The diameter of the strand must be measured prior to the test and entered for the software to generate corresponding strength and elasticity figures.

9. In addition, the software will analyze the quality of bonds coming from 3 different regions of the hair.

 

The performance of the tensile strain/stress tests mimics the act of combing the hair and imparts important information for the value-minded cosmetic manufacturer.

 

              

 

Cortical Properties

 

Tensile properties have less to do with surface properties (like cuticles), and more to do with cortical properties (like where the stretching occurs).

The cuticle does not contribute to the tensile properties, but can be damaged by excessive stretching as the scales lift and separate from the fiber.

Cuticle damage is the first to occur.

 

Much like stretching a rubber band that has printing on the surface. When you stretch it beyond the normal relaxation point, the ink will crack and eventually shed off the surface long before the rubber band will break.

 

The cortex is a major part of the fiber mass. Within the cortex are micro fibrils that make up a large part of the cortical cells. And these consist of intermediate filaments and the matrix.

During tensile strain testing, we strain the fiber to extend further into the post yield regions where a transformation occurs in the intermediate filaments.

This causes a loss of structure that is usually recovered on relaxation.

Unfortunately, recovery occurs with normal healthy hair extensions and not over-processed or non-cuticle hair extensions and replacements.

 

Combing and Friction

 

Both combing and friction are considered the most perilous factors when discussing the maintenance of human hair.

While combing our hair, we apply a significant amount of force upon the hair fibers and it is stretched in various degrees. The force applied during combing also creates friction that results in cuticles breaking and detaching from the strand, which increases the hair's vulnerability.

 

Each time we run into a small tangle in our hair, we have to remember to be more patient.

 

Essentially, when we apply combing and stretch our hair, the hair is supposed to bounce back.

In scientific values, if we stretch the hair 5%-10% beyond it's original length, the hair will usually perform a complete recovery and will bounce back to the Zero-point.

If we stretch the hair 14%-17% beyond its original length, the percentage of relaxation (the mechanism of bouncing back) will not recover to zero. And there is now irreversible damage, although, we may not feel it.

Stretching the hair beyond 17% will cause irreversible damage that we will notice.

The hair will snap back into a curl, almost like a snake recoiling.

With processed hair extensions, the percentage values are much less as the cortex has suffered massive chemical damage and many of the bonds have been broken and cannot withstand applications of high force.

And the processed strand will not snap back when stretched beyond the recovery point.

Usually it will either fracture or immediately break or the hair will lay lifeless because it didn't even have the elasticity to stretch in 5%-10%.

 

Caring for the Hair Extensions

 

What can you do to prevent further loss of tensile properties?

Understand the limitations of the hair type or hair extensions you purchase and maintain it accordingly.

· Avoid excessive combing and styling, especially when the hair is wet.

· If the hair is tangled, use a warm conditioner bath to relax the hair. It will greatly reduce friction and allow you to slowly work out the tangles.

· Do not scrub wet hair with a towel, pat and gently squeeze to remove the excess moisture.

· If you have had no choice but to tackle tangled dry hair with nothing but a brush and determination, wet it down when you are finished and let it air dry. As the water evaporates, the fibers will have a greater chance to recover and reform as much as possible to its original structure. Allowing the hair time to recover is highly important.