In the excitement over peptides, neurocosmetics, antiglycation endpoints and prebiotics, it is easy to forget the importance of product look and feel. Once upon a time—before virtually every product had to repair DNA, stimulate collagen synthesis and send a wellness signal to the limbic system—key attributes were more mundane properties, such as spreading, cohesiveness, shine, tack and afterfeel. It is important to remember that without acceptable tactile properties, the greatest antiaging technology can fail to please the consumer. Additionally, odor and color can influence the perception of functional performance beyond what would seem logical.
It is necessary to formulate a product for pleasing sensory attributes and to confirm the results with meaningful tests. Emollient properties and rheology are obviously important formulation parameters for skin application. Some results can be obtained with instruments, but expert panels and consumer testing can be crucial to determining the market requirements for commercial success.
While kinesthetic attributes are perceptions gained without touching, such as coolness and warmth, tactile attributes are registered by touching: oiliness, tackiness, dryness, smoothness or silkiness. This column will consider tactile attributes for skin and hair products.
Esters form the largest family of emollient oils, and they have some predictable properties. As chain length increases, they become less irritating, have a heavier feel, and are harder to emulsify. An increase in branching raises the dry feel and lowers the viscosity. Unsaturation increases skin penetration and makes emulsification more difficult. Hydroxyl groups make them more water-soluble and easier to emulsify.
Several companies have proposed methods for systematically adjusting the skin feel of emulsions. The “Cascading Emollients” approach developed by Henkel’s chemical products business (now part of Cognis) is based on blends of high, medium and low spreading emollients. One or two emollients cannot provide the complete profile of an elegant product. The proper combination of three or more emollients is necessary.
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).
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