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Chemical Reaction: Skin Care: The Importance of Feel

Steve Herman

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).
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

The ratings depend on a trained panel and the elimination of as many variables as possible.

Once a product is created, a panel can evaluate its consumer acceptability.

A favored way to treat the data from a variety of responses is a spider graph. It can display a large number of characteristics together, including fragrance. It is well known that odor can profoundly affect opinions, including unrelated performance values, and spider graphs reflect this. For example, the same shampoo formula can get different evaluations of foaming and cleansing when only the fragrance is different.

Using systematic methods to create the proper feel and appearance is a prerequisite for a new formulation. A combination of optimal sensory properties combined with functionality will result in a superior product. Attention to economics, packaging and marketing is then needed to assure commercial success. Only when every aspect of the product’s promise and performance is aligned will the consumer respond with total satisfaction.

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