Moisture permeability study

Studies have revealed that high humidity leads to higher moisture levels in the hair. The resulting negative perception of the feel and appearance of hair exposed to humidity prompted a study to evaluate the efficacy of silicone to control moisture absorption by hair. In this study, the water vapour permeability of silicones with varying degrees of phenyl substitution was measured. Moreover, hair tresses treated with the same silicones were subjected to high humidity levels and characterised.

Phenyl vs. methyl

Before testing a particular silicone on hair, it was first determined what silicone would be best to protect hair from moisture. To establish this, several silicone samples of varying degrees of phenyl loading were tested – 100% dimethyl; 100% phenylmethyl; 60, 40, 25, 10 w/w % diphenyl substituted dimethyl siloxanes – by Mocon Testing Service using the Mocon Permatran-W 3/33 Water Vapor Permeability instrument. To confirm these percentages, RI (refractive index) spectrometry was utilised. As seen in Figure 4, a direct linear relationship exists between RI and the w/w % phenyl (Ø) on silicone polymer. As we increase the amount of phenyl on the polymer backbone, the RI increases accordingly. A 1.40 RI is typical of a standard dimethyl silicone, while a 1.56 RI polymer is representative of a highly substituted mol % diphenyl silicone. All testing was performed on samples of nominally 0.075” thickness, and rates were measured at 40.0°C, 90% RH (relative humidity) and 760.0 mmHg barometric pressure using Equation 1 (see below). With Equation 1, several factors influence moisture permeability rates in polymeric materials. First, a complex relationship exists between diffusion and solubility of moisture through silicone materials. Also, permeability rates depend on material thickness and environmental factors such as temperature, per cent relative humidity and pressure. Ultimately, however, the silicone’s chemical characteristics and bulk physical properties influence the rate moisture absorbs onto the material’s surface, dissolves through the material and desorbs as it exits. Figure 5 relates data gathered from these tests; it is clear that after exposure to 90% RH and a temperature of 40°C, the addition of phenyl noticeably decreases moisture permeability across the silicone barrier. To accompany this information, Figure 6 compares the performance of a dimethyl silicone to various mol % of diphenyl silicones.3


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