SILICONES
region, differential absorption by the proteins causes uneven swelling, creating a frizzy appearance.1
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
P =S·D
where P = Permeability S = Solubility coefficient D = Diffusion coefficient
Equation 1: Rate of moisture permeability. Medulla Prior to treatment, all samples were Cortex
washed with Sodium Lauryl Sulfate (SLS) and allowed to air dry. Next, swatches to be treated were submerged in 25% w/w solution of polymer in volatile siloxane and again were allowed to dry. Afterwards, treated samples were combed to distribute the treatment. Following these steps, all samples were exposed to 30˚C and 90% RI and each sample was weighed prior to exposure and at three hours; six hours; and, finally, 24 hours of exposure.4 When reviewing Figure 7, two important
Cuticle Figure 3: Anatomy of hair shaft.
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
Treated vs. untreated hair Now that we see a correlation between an increase in mol % phenyl on a polymer and a decrease in moisture permeability, the next step is to treat hair with the various silicone materials and test the hair’s moisture absorption. In the study, swatches 1” wide by 6” long and medium brown #4 in colour were used for evaluation. Two swatches were treated with diphenyl substituted siloxane polymers, two swatches were treated with dimethyl siloxane polymers and two swatches were left untreated as a control for comparison.
1.56 1.54 1.52 1.50 1.48 1.46 1.44 1.42 1.40
0 10 20 30 % Phenyl (W/W) Figure 4: Refractive index versus % Phenyl (W/W) on silicone polymer. March 2012 PERSONAL CARE 63 40 50 60
observations should be considered. First, when compared to the untreated control, both silicone treatments were able to decrease the amount of water absorbed by the hair. Second, as was seen in prior testing when comparing phenyl substituted silicones of various percentages to non- substituted methyl silicones, the phenyl substituted polymer allowed the least moisture to penetrate into the hair. Revisiting what was discussed above, it is the absorption of water in the cortex that causes frizz. Thus, by reducing water absorption, the aesthetically displeasing effect of frizz is also reduced.
Conclusion When considering materials to use for personal care products, top on the list of requirements is safety. For more than 30 years, the Silicones Environmental, Health and Safety Council (SEHSC) has coordinated and organised scientific research on health, safety and environmental issues. Silicone has been found to be biologically inert, non- toxic/non-allergenic and stable.5
In fact,
the material attributes and benefits of silicone are rooted in their fundamental physical and chemical properties. Also beneficial are the many forms silicone can take, ranging from a liquid with a similar viscosity as water all the way to a high- viscosity sheared silicone system. This variety of forms allows silicone to be
Refractive index
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