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The Creation of Luxury Cosmetics with Viscosity Technology11th Installment

The Creation of Luxury Cosmetics with Viscosity Technology11th Installment

"Creamy in texture but dry to the touch when applied"
"Spreads smoothly like an emulsion when sponged, ideal for dry skin"

So goes the hype for a certain cosmetic product. Such references to emulsions and thixotropy are commonplace in advertisements and online mail order sites but may be somewhat startling to male readers who may have had less exposure to the world of cosmetics.
In this column we would like to examine cosmetics, not from the standpoint of their use, but from the perspective of the design and manufacture of such products within a rheological context.

Quantification of "sensory computers"

Cosmetic products are designed to integrate a variety of attributes, including essential functions (e.g. helping the skin appear vibrant, providing moisture retention), taste preferences (e.g. fragrance, color), usability, and so forth. How do rheological properties relate to such aspects? Typical examples are the use of viscosity measurements to control factors associated with achieving desired application comfort for creams and obtaining good coating results with manicures and lipsticks (including temperature characteristics, etc.).
Prerequisite for this type of product is the great influence individual preferences have on product characteristics. Here testers with their "sensory computers" play a key role in determining whether the final composition of a product is good or bad. Of course this type of evaluation is not limited to cosmetics. Another example is beer tasting where smoothness and other attributes are a reflection not only of taste tests involving many test subjects, the results of which are statistically analyzed, but also the input of professional taste specialists. (As we delved into in a previous column, the viscosity of beer is an important factor which determines the product’s smoothness. Viscosity in the case of this product is characterized in terms of crispness to describe the throat sensation as the beer is consumed at a constant rate.)
Modern methods of product design and manufacturing attempt to quantify such "sensory computer" judgments in order to achieve standardization in product development and production. Measured rheological values including viscosity are convenient and usable indicators for this purpose.
As polymers are used extensively in cosmetics there are naturally many opportunities for the involvement of rheology technology. One interesting example is lipstick.
Long wearing lipstick has been around for several years. Formerly, lipstick was generally made of oils with color pigments mixed in. Such lipsticks were designed to be easy to apply but came with a downside as they tended to leave lipstick traces on surfaces where lips had touched. Since then lipstick manufacturers have each devised various methods to create long lasting lipstick. One technique employs a polymer called supramacromolecular alginic acid. This ingredient is derived from seaweed and reacts well with water to form a surface film. When used in lipstick, the substance reacts readily with moisture -- from the lips or breath -- to coat the surface of the lipstick to provide long wearing characteristics.

Application sensation and yield value of skin creams

Viscosity control is especially effective in product control related to the application sensation of creams and emulsions.
Emulsions are liquids which contain water droplets dispersed in oil or oil droplets dispersed in water and are a typical type of substance from the rheological point of view. Creams are a category of emulsions with special properties where the application of a certain amount of force results in a rapid fall in viscosity. What this means is that when the cream is applied to the skin, it becomes more spreadable. The normal high viscosity of the cream changes instantly to a lower viscosity when subjected to a force which allows it to be spread easily while providing a sensation of smoothness. "Thixotropy" as used in the product description refers to this product attribute.
"Yield value" refers to the point at which viscosity starts to fall rapidly. In the case of a cream, the smaller the yield value, the less force is required to make the product easier to apply. However top of the line creams are perceived to be products which offer smoothness but without dripping. Yield value becomes crucial here and the control of this factor takes us into the realm of rheology technology. Controlling yield values helps you to determine, for example, how much force is required to squeeze cream from a tube and how smoothly lipstick can be applied.
Viscosity not only has implications for the product (such as shampoo) itself, but may also be an important factor in the design of its container as well. The viscosity of a shampoo may be an important consideration in determining the ideal container throat diameter to insure proper shampoo dispersal, for example.
There are many other rheological elements in addition to application smoothness which are employed to differentiate cream cosmetics. Should the cream feel dry or remain moist when it is applied? How should thixotropic recovery (speed) be controlled? If recovery to the highly viscous state occurs too rapidly, proper leveling cannot be achieved. Too slow a recovery on the other hand, may adversely affect the refreshing image of the product that the manufacturer wishes to convey. Rheological properties are thus manipulated to fit the product’s purpose and intended use. If the cream is to be applied thickly in a confined area, it is better not to have leveling. In this case, you would want to build in to your product high yield value properties with fast return to the high viscosity state.
The use of viscosity thickening agents, etc., to control the viscosity of a substance was alluded to in a previous column. Let us explore this further.

Role of viscosity thickeners and dispersion stability

Creams, as we have mentioned, are a type of emulsion. It may be helpful in describing such products by digressing a bit to look at another everyday product − milk.
Milk is a blend of milk fat and other substances. Why then, don’t these substances separate in the liquid? The components of milk do not separate because milk undergoes a process called homogenization. Homogenization involves the breakdown of fat globules under pressure to achieve a homogeneous state in which the globules of fat are finely and evenly distributed throughout the liquid. These substances remain stable in the liquid and do not aggregate. The homogenization of milk is thus one type of dispersion stabilizing technique. With non-homogenized milk, the water and fat tends to separate over time resulting in a layer of fat (cream).
Homogenized milk is smooth to the taste and has good absorption characteristics but there are milk producers as well as devoted milk drinkers who feel that homogenization detracts from the flavor and insist on milk that is non-homogenized.
Returning to the subject of creams, luxury cosmetic creams are characterized by component particles that are very fine and uniform. In order to maintain uniformity of oil-based components which provide heat retention and other functions in a water medium, dispersion stabilizers are employed. Unless dispersion stability is skillfully accomplished however, the cream cosmetic may separate into its oil and water components in its container, not unlike non-homogenized milk, which may cast an impression of "cheapness" with regard to the product.
Such dispersion stabilizers are in fact considered to be an aspect of thickening agents. Thickening agents employ additives which work to increase viscosity and by raising viscosity, you can promote stability of components in a medium. From this, it can be discerned that viscosity thickeners function as dispersion stabilizing agents. Thus not only do these substances provide stable uniform dispersion of the oil-based components in a medium but by utilizing their properties as viscosity thickeners, you can control the spreadability of the product.
Based on these aspects, we may understand that the difference between top quality cosmetic creams and general cosmetic creams lies not so much with the difference in the product’s functional components but more with differences in their rheological properties (there are of course, high end cosmetic creams which include medicinal components). All of this being said however, it would certainly be wise and more effective for a man making a gift of a cosmetic cream to a woman to stick to a pricy and well-known brand product.

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