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Silicones' Benefits in Antiaging Skin Care
By: Michael S. Starch, Dow Corning Corp.
Posted: November 13, 2008
page 2 of 5
The oldest approach to enhancing skin appearance involves the application of inorganic pigments, principally iron oxides and titanium dioxide. This practice is still used in modern cosmetic products such as foundations and blushes. By choosing the proper ratio of the three forms of iron oxide (black, red, and yellow) and titanium dioxide (white), formulators can design products that cover imperfections and provide an even skin tone for virtually any skin type.
The key to achieving uniform pigment films is to deliver them in a vehicle that has good spreading properties, a function for which silicones are ideally suited. This is especially true for cyclomethicone, which helps spread pigments over the skin and then evaporates to leave the pigment behind. Cyclomethicones are available with different rates of evaporation so formulators can control the “play time,” or the time it takes for the liquid cosmetic to dry on the skin. The use of cyclomethicone and other silicone fluids became more widespread with the introduction of surface-treated pigments in the early 1980s. The treated pigment particles have a hydrophobic coating (often silicone or silane) that makes them more compatible with silicone fluids and therefore more effectively delivered from a silicone-based formulation.
Once the dried pigment film is on the skin, consumers want it to be “smudge proof” and resistant to transfer onto other surfaces. This allows application of makeup in the morning that maintains good skin appearance all day. Silicone resins such as trimethylsiloxysilicate and polymethylsilsesquioxane are film-formers that improve adhesion of pigments to the skin and transfer resistance. These resins are soluble in cyclomethicone and many other silicone fluids so they are easy to incorporate into cosmetic products. They can also be used in lipsticks to provide long-lasting color with greatly reduced transfer properties. In lip products, a high gloss film as well as attractive color is desirable. Phenyl silicones such as phenyl trimethicone are popular in this application. In addition to imparting gloss, phenyl silicones are more soluble in cosmetic waxes used in lipstick, a property that reduces the problem of syneresis, or bleeding of oil from the lipstick.
Today’s “state of the art” liquid foundations often are formulated as w/o emulsions. Unlike conventional emulsions where the oily (water-insoluble) ingredients are dispersed as droplets in water, w/o emulsions are droplets of water dispersed in the other ingredients (e.g., oils or silicones). For a foundation that incorporates silicones with hydrophobic pigments, w/s emulsions are ideal vehicles. Such formulations have pleasant aesthetics, excellent spreading characteristics and optimize the film-forming properties of silicone resins. To prepare stable w/s emulsions, special polymeric silicone emulsifiers are required. These emulsifiers are made by grafting hydrophilic groups such as polyethers onto a dimethicone backbone. Common examples are PEG/PPG-18/18 dimethicone, bis-PEG/PPG-14/14 dimethicone and cetyl PEG/PPG-10/1 dimethicone.
Approaches to Hiding Wrinkles
Applying inorganic pigments to the skin is an effective way to cover uneven skin pigmentation, but it is less effective in covering wrinkles. Wrinkles are essentially permanent indentations in the skin, and to fill them requires such a thick coating of pigment that it creates an unnatural appearance many consumers reject. In addition, most wrinkles occur in areas of the face that are stretched or compressed with changing facial expressions. These movements tend to form creases in thick pigment films. A better approach to concealing wrinkles is to apply materials with optical properties that allow them to reflect and scatter light in such a way as to reduce the visibility of wrinkles and other skin imperfections. This phenomenon is often referred to as the “soft focus” effect because it is similar to what happens when skin is photographed with a camera that is out of focus; skin features such as wrinkles are “blurred” by the effect of the treatment.