DVS Measurement of Human Hair Reveals Variations in Moisture Uptake
Research conducted by leading sorption solutions provider Surface Measurement Systems Ltd has revealed noticeable variations in the moisture uptake properties of damaged and undamaged human hair.
The method used in the research was Dynamic Vapour Sorption (DVS), which employs scientific instruments — known as DVS moisture sorption instruments — to measure how rapidly and how much of a solvent is absorbed by a sample.
Three types of human hair were subjected to DVS analysis in the study. These were classified by the researchers as;
- A — (undamaged Asian hair)
- Reference — (undamaged Caucasian hair)
- B — (damaged/bleached Caucasian hair).
The hair samples were taken from a few centimetres above the ends, rather than from the ends themselves, which might have been more severely damaged.
The samples were analysed at 25°C, using the following typical partial pressure profile: 95, 20, 30, 40, 50, 60, 70, 80, 90, 95, 90, 80, 70, 60, 50, 40, 30, 20,10, 0, 10, 20 and 30% RH (Relative Humidity).
For all the RH steps, the instrument was run in a dm/dt mode (mass variation over time variation). A fixed dm/dt value of 0.002% min-1 was selected.
This criterion permits the software in these analytical instruments to automatically determine when equilibrium has been reached and complete a relative humidity step. When the rate of change of mass falls below this threshold over a determined period of time, the humidity will proceed to the next programmed level.
A maximum stage time of 180 minutes and a minimum stage time of 60 minutes were selected for this experiment.
All data was normalised to the “dry” sample mass, as defined by the sample masses at the end of the 0% relative humidity step.
A number of differences in the samples were noted, as well as a number of similarities. Sample A (undamaged Asian hair) showed the highest moisture uptake at 95% RH. The Reference sample (undamaged Caucasian hair), gave an intermediate level of moisture uptake and sample B (damaged/bleached Caucasian hair) showed the lowest moisture uptake.
After all samples passed from 95% to 20% through a 0% RH drying stage, the percentage moisture uptake for the three samples coincided at 5.5% of the dry mass at the final 20% RH step.
Considering the uptakes at the initial and final 20% RH steps, the results showed that the 0% RH drying step had a greater effect on sample B resulting in 1.5% decrease in the moisture uptake, whereas Sample A and the Reference sample showed a decrease of 1% in uptake during the final 20% RH step.
The Reference sample and sample B showed a much higher level of sorption/desorption hysteresis (retardation of effect) at 50% RH compared to sample A.
The trend for decreasing hysteresis at 50% RH was as follows:
- Reference > Sample B > Sample A
This indicates that the moisture diffusion out of the bulk was more restricted for the undamaged Caucasian hair sample.
Surface Measurement Systems Ltd plans to do further work to investigate these differences.