Properties of Hair- Part 2

Though it may not be that obvious, water is an important component of hair. Its presence helps with the elasticity or pliability of hair. The water content of hair is between 15-35%. The high variability is due to the hairtype and health of the hair fibre. Hair that is damaged is less likely to hold onto water. ‘

Water has the potential to increase not only the weight of the hair fibre but also the diameter. This is referred to as the swelling potential of hair. It can increase to nearly 14% in diameter, but less than 2% in length. Other reagents such as sodium lauryl sulfate, formic acid, and thioglycolic acid have a greater effect on the swelling potential of hair, hence the reason for these products to potentially damage hair.

The more the hair absorbs is the more damaged the hair, indicating that the hair shaft is more porous. This is what is  termed as “porosity”.  Hair that absorbs more water has decreased tensile strength i.e. it easily breaks. The porosity of the hair shaft is about 20%, allowing a weight increase of 12–18% when soaked in water. The absorption rate is very rapid, with 75% of the maximum absorbable water entering the hair shaft within 4 minutes.

However, as quickly as the hair absorbs water, it can quickly lose it. Those with quite porous hair can attest to this. Water has been shown to stay in the hair shaft better if attached to some highly osmotic molecule like sorbitol, or some other hygroscopic (water-loving) substance such as glycerin. These molecules allow water from the atmosphere to stay longer in contact with the hair shaft. This increases the moisture content of the hair shaft and thus allows the hair to not only feel better but to look better as well.

References

Syed AN, Ayoub H. Correlating Porosity and Tensile Strength of Chemically Modified Hair. Cosmet Toil 2002; 117: 57-64.

Draelos MD, Zoe Diana (2007-04-17). Hair Care (Kindle Locations 331-334). Informa Healthcare. Kindle Edition.

Egawa, M., Hagihara, M. and Yanai, M. (2013), Near-infrared imaging of water in human hair. Skin Research and Technology, 19: 35–41