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Chemical Reaction: Skin Care: The Importance of Feel
By: Steve Herman
Posted: December 10, 2007, from the December 2007 issue of GCI Magazine.
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The subjective feeling on the skin can be correlated and objectified within the physiochemical parameters of the spreading of the oils on the skin, as expounded by U. Zeidler. The spreading properties can be determined by a simple test. A 4 mg dose of oil is placed on the dorsal forearm in an environment of 23C and 60% relative humidity. The spreading area is, over time, typically affected by molecular structure, consistency and molecular weight. According to Zeidler, cosmetic oils can be classified as low spreading (below 300 mm2/10 min), medium spreading (around 300–1000 mm2/10 min) and high spreading (above 1000 mm2/10 min).
Croda created the Emollient Skin Spreading Factor as a sensory tool.3 The area covered by a 5μl sample of emollient originally contained in a 2 cm diameter circle is determined by the application of a dye (a 1% solution of FD&C Blue #1). The area not stained by the dye shows the spreading of the oil. The spreading factor is the final area divided by the original area. Croda then factors in an Emollient Viscosity Index (EVI). EVI is a measure of the change in viscosity of the oil at varying temperatures, which in turn influences spreading behavior.
Rheological properties are important in personal care products, but there is no simple correlation between rheology and sensory parameters. Wortel4 concluded that while no single rheological aspect satisfactorily relates to a sensory effect, multivariant methods could yield useful data. Cohesiveness was the focus of Wortel, evaluated by compressing the product between thumb and index finger and then pulling apart. A high stringy effect is a sign of high cohesiveness. Wortel analyzed this property successfully with a combination of yield stress and dynamic viscosity.
Sensory characteristics have been given 12 parameters by an ASTM Committee. A version from Dow Corning5 is available in full online. Each parameter can be given a value from zero to 10 by a panelist. For example, a zero rating for gloss implies the material is dull; 10 that it is shiny. The evaluation process has four categories: appearance, pick-up, rub-out, and residual feel and appearance. The temperature and humidity must be controlled for consistent results.
Hair products have very different performance characteristics. For a shampoo, flash foam, viscosity, color and fragrance are paramount. For hair fixatives, a valuable summary has been provided by Joe Dallal of ISP for the students in the FDU formulation lab.6 The rating is on a 1 to 10 scale. For static: 1 is total fly away, impossible to manage after the first brush stroke. A 10 has no static and no free hair movement.
The whole sequence for static:
10 — None
9 — Very, very slight
8 — Very slight
7 — Slight
6 — Slight to moderate fly away
5 — Moderate fly away
4 — Moderate to considerable fly away
3 — Considerable fly away
2 — Excessive fly away
1 — Total fly away