Silicones have a long history of use in skin care products

Tag : Gums & Stabilizers
One of the dominant trends in today’s dynamic skin caremarket is the quest for products that effectively prevent and treataging skin. The importance of antiaging benefits is driven bydemographics and the desire of most consumers to maintain ayouthful appearance. The population in the leading personal caremarkets including the United States, Western Europe and Japan, isshifting toward older consumers as birthrates decline. At the sametime, the rise in living standards in emerging markets allows agingconsumers to purchase more sophisticated skin products thatincorporate the latest developments in antiaging technology. Theglobal skin care industry has responded by developing productsintended to prevent premature aging, reverse specific aspects ofaged skin and improve the appearance of skin.
Dimethicone – the Silicone Foundation for Skin Care
Silicones have a long history of use in skin care products,beginning in the 1950s. The first applications involved basicsilicone fluids (INCI: Dimethicone). These linear polymers areliquid over a wide range of molecular weights. Dimethicones remainimportant for their emollient properties and their ability toimprove the skin feel of many types of skin care formulations. Inthe late 1970s, another important class of silicones was introducedto the industry. Cyclomethicones are volatile, low-viscositysilicone fluids that act as cosmetic solvents. They areparticularly suited for use with other silicones and as deliveryvehicles for a variety of active ingredients.
Starting in the 1980s, the increasing popularity of silicones inskin care applications prompted silicone manufacturers to develop avariety of new materials, which led to even broader use. Many ofthe new silicones were derivatives of dimethicones, where specificfunctional groups were added to the backbone of the siliconepolymer. For example, grafting hydrophilic polyethylene oxidechains to the dimethicone backbone produces non-ionic siliconesurfactants that are useful as emulsifiers, foam stabilizers andwetting agents. Another family of silicones was created byintroducing phenyl groups onto the silicone backbone to producefluids with a higher refractive index and increased compatibilitywith cosmetic waxes. These phenyl silicones are useful for colorcosmetics such as lipsticks, where the goal is to produce a highgloss coating on the lips.
One of the newest and most rapidly growing classes of siliconesused in skin care applications is silicone elastomers, which aremade by cross-linking dimethicone polymers to produce elastomericsolids that have properties quite different from dimethiconefluids. As the degree of cross-linking increases, the siliconenetwork becomes more rigid. Although they are produced by adifferent process, silicone resins can be thought of asrepresenting the most extreme examples of cross-linked siliconepolymers. Silicone resins have a tight, three-dimensional structurethat results in rigid materials that can form hard, durable films. Figure 1 illustrates variations in polydimethylsiloxane morphology based onincreasing molecular weight and cross-linking.


Figure 1. Morphology of various polydimethylsiloxanes.
Silicone Applications in Cosmetics
The signs of skin aging include a number of undesirable changes inits appearance, such as the development of wrinkles, loss ofelasticity and uneven pigmentation. One of the most popularapproaches used by skin care product manufacturers to address theseproblems is to develop cosmetics, which are products thattemporarily improve skin appearance. Cosmetic products represent alarge fraction of the skin care market because they work. Theyessentially provide instant gratification because consumers noticean improvement as soon as the product is applied. This is incontrast to products intended to provide a therapeutic effect,which may require 6 to12 weeks before improvement becomes apparent.
The oldest approach to enhancing skin appearance involves theapplication of inorganic pigments, principally iron oxides andtitanium dioxide. This practice is still used in modern cosmeticproducts such as foundations and blushes. By choosing the properratio of the three forms of iron oxide (black, red, and yellow) andtitanium dioxide (white), formulators can design products thatcover imperfections and provide an even skin tone for virtually anyskin type.
The key to achieving uniform pigment films is to deliver them in avehicle that has good spreading properties, a function for whichsilicones are ideally suited. This is especially true forcyclomethicone, which helps spread pigments over the skin and thenevaporates to leave the pigment behind. Cyclomethicones areavailable with different rates of evaporation so formulators cancontrol the “play time,” or the time it takes for theliquid cosmetic to dry on the skin. The use of cyclomethicone andother silicone fluids became more widespread with the introductionof surface-treated pigments in the early 1980s. The treated pigmentparticles have a hydrophobic coating (often silicone or silane)that makes them more compatible with silicone fluids and thereforemore 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 othersurfaces. This allows application of makeup in the morning thatmaintains good skin appearance all day. Silicone resins such astrimethylsiloxysilicate and polymethylsilsesquioxane arefilm-formers that improve adhesion of pigments to the skin andtransfer resistance. These resins are soluble in cyclomethicone andmany other silicone fluids so they are easy to incorporate intocosmetic products. They can also be used in lipsticks to providelong-lasting color with greatly reduced transfer properties. In lipproducts, a high gloss film as well as attractive color isdesirable. Phenyl silicones such as phenyl trimethicone are popularin this application. In addition to imparting gloss, phenylsilicones are more soluble in cosmetic waxes used in lipstick, aproperty that reduces the problem of syneresis, or bleeding of oilfrom the lipstick.
Today’s “state of the art” liquid foundationsoften are formulated as w/o emulsions. Unlike conventionalemulsions where the oily (water-insoluble) ingredients aredispersed as droplets in water, w/o emulsions are droplets of waterdispersed in the other ingredients (e.g., oils or silicones). For afoundation that incorporates silicones with hydrophobic pigments,w/s emulsions are ideal vehicles. Such formulations have pleasantaesthetics, excellent spreading characteristics and optimize thefilm-forming properties of silicone resins. To prepare stable w/semulsions, special polymeric silicone emulsifiers are required.These emulsifiers are made by grafting hydrophilic groups such aspolyethers onto a dimethicone backbone. Common examples arePEG/PPG-18/18 dimethicone, bis-PEG/PPG-14/14 dimethicone and cetylPEG/PPG-10/1 dimethicone.
Approaches to Hiding Wrinkles
Applying inorganic pigments to the skin is an effective way tocover uneven skin pigmentation, but it is less effective incovering wrinkles. Wrinkles are essentially permanent indentationsin the skin, and to fill them requires such a thick coating ofpigment that it creates an unnatural appearance many consumersreject. In addition, most wrinkles occur in areas of the face thatare stretched or compressed with changing facial expressions. Thesemovements tend to form creases in thick pigment films. A betterapproach to concealing wrinkles is to apply materials with opticalproperties that allow them to reflect and scatter light in such away as to reduce the visibility of wrinkles and other skinimperfections. This phenomenon is often referred to as the“soft focus” effect because it is similar to whathappens when skin is photographed with a camera that is out offocus; skin features such as wrinkles are “blurred” bythe effect of the treatment.
The most effective soft focus materials are various transparentparticles. The transparency allows the skin color to pass throughthe particles, giving a more natural skin appearance. Opticaleffects are influenced by the size, shape, and refractive index ofthe particles. Spherical particles are popular because they spreadwell, have a pleasant feel and act as tiny lenses that bend lightto create the soft focus effect. Silicone fluids offer the samebenefits for delivering soft focus particles to the skin as they dofor pigment particles. The refractive index of the silicone can beadjusted by choosing a dimethicone fluid or a phenyl silicone thathas a higher refractive index. Spherical silicone elastomer andsilicone resin particles are gaining in popularity for face careproducts. These are useful for soft focus applications because theyhave a different refractive index than organic cosmetic oils. Lightscattering typically increases when the oils and the particles inthe film have different refractive indexes. Apart from theiraesthetic and optical properties, silicone elastomer particlesoffer the added benefit of absorbing sebum.
Prevention of Skin Aging
Many skin care products make claims for the prevention of skinaging by protecting the skin against the damaging effects ofpollution, cigarette smoke, free radicals and other environmentalinsults. These claims have some scientific basis, but by far themost important factor in skin aging is exposure to the ultravioletradiation in sunlight. Countless studies have shown thatultraviolet radiation causes cumulative skin damage that leads toloss of elasticity, uneven pigmentation and other signs of agedskin. The most effective prevention is regular use of sunscreen,which protects the skin against exposure to ultraviolet radiation.
Recognition of this fact has led to rapid expansion of productsthat contain sunscreen ingredients, particularly for use on theface and other areas of the body that are exposed to the sun on adaily basis. These active ingredients fall into two broadcategories: organic and inorganic. Organic sunscreens are usuallyoily liquids that are added to a formulation in concentrationsranging from 3%-15%. Inorganic sunscreens consist of particles oftitanium dioxide or zinc oxide that are used alone or incombination with organic sunscreens. When used alone, inorganicsunscreen concentrations range from about 10% to as high as 25%.When used to supplement the protection form organic sunscreens,inorganic sunscreens are typically included at 2-8%.
For both organic and inorganic sunscreens, the goal is aformulation that delivers the sunscreens in a uniform film tomaximize protection against ultraviolet radiation. The film shouldremain uniform over time and help keep the sunscreens in place sothey are not removed when the skin is brushed against othersurfaces. For sunscreen products intended for use at the beach orduring exercise, the sunscreen film must also be resistant towash-off.
Silicone fluids are widely used in sunscreen formulations becauseof their ability to reduce the oily and sticky skin feel associatedwith high levels of organic sunscreen oils. Silicone fluids withphenyl groups have better solubility in sunscreen oils and this canincrease the effectiveness of the sunscreen, presumably byproducing a more uniform film on the skin. Silicone waxes in whichlong-chain alkyl groups have been grafted onto the siliconebackbone have been shown to increase the protective effect ofsunscreens. These silicone waxes are solids that affect therheology of the formulation; they increase the viscosity of thesunscreen film on the skin so that it has a greater tendency tostay in place.
A number of different polymers are used to increase the waterresistance of sunscreen formulations. Many are acrylate polymersand silicone manufacturers have produced hybrid silicone-acrylatecopolymers that offer the benefits of both. Other siliconefilm-formers such as silicone resins have been shown to increasethe water resistance of sunscreens.
Inorganic sunscreens are gaining in popularity because they canoffer more effective protection against ultraviolet radiation, butthese sunscreen actives also can present formulation challenges.Both titanium dioxide and zinc oxide are fine powders that aredifficult to handle in production due to dusting and concerns aboutinhalation by production workers. The small particle size of thepowders helps reduce their white appearance on the skin, but thenatural tendency of the particles to agglomerate negates thebenefits of small particle size.
For these reasons, inorganic sunscreens are often sold asdispersions in a fluid. The inorganic particles are subjected tohigh-shear processing during dispersion production, which breaks upthe agglomerated particles. Additives in the dispersion are used toprevent re-agglomeration. Cyclomethicone is a popular carrier fluidfor inorganic sunscreen dispersions, particularly when thesunscreen particles have a surface treatment to make them morehydrophobic. Silicone additives such as polyether-modifieddimethicones are included to prevent re-agglomeration.
Treatments for Aged Skin
Although protection from ultraviolet radiation is the mosteffective preventive measure for reducing skin aging, consumerswith signs of aging want products that can repair the damage andreturn skin to a more youthful appearance. This is the realm ofskin treatments that are designed to reverse the symptoms of agedskin. Such treatments represent a large part of the skin caremarket and utilize a variety of active ingredients. Some activeingredients have been used for many years, including tocopherol(vitamin E), retinol (vitamin A) and its derivatives, alpha-hydroxyacids (lactic acid, glycolic acid) and ingredients derived fromplants. Advances in biochemistry over the past 5-10 years haveproduced antiaging ingredients designed to stimulate enzymes in theskin that produce structural proteins and other molecules presentin younger skin. Many of these biochemical actives are peptides orpeptide derivatives.
One property common to many antiaging actives is that they arefragile molecules and can lose their effectiveness via interactionwith other ingredients in the formula. This problem has promptedthe development of various delivery systems designed to protect theactive ingredient in the formulation and make it available when theformulation is applied to the skin.
One of the simplest delivery systems that can effectively protectantiaging actives is an anhydrous formulation. This is becausewater and chemicals contained in the water (acids, bases, dissolvedoxygen, and metal ions) are often responsible for the degradationof the actives. Although this issue is eliminated by removing waterfrom the formulation, other problems are created. A formulationconsisting entirely of oils and oil-soluble ingredients is usuallyperceived as greasy or oily by the consumer. Another problem isthat most thickening agents used to control the consistency of theformulation must be used with water. So the challenge in ananhydrous formulation is to improve aesthetics and thicken theformulation so it is not too thin and does not separate over time.
Silicone fluids offer greatly improved aesthetics, and siliconeelastomers provide thickening while further improving aesthetics.Anhydrous skin treatment products based on silicones often are soldas small gelatin capsules that provide a single application whenthe capsule is broken. The protective effect of a siliconeelastomer delivery system for certain actives that are soluble insilicone (e.g., retinyl palmitate) has been shown for conventionalwater-based formulations. The active is first combined withcyclomethicone and silicone elastomers, and this mixture isemulsified into the formulation. The affinity between the activeand the silicone is apparently enough to keep the active in thesilicone droplets and protected from degradation.
Summary
The term silicone represents a large family of polymers that rangefrom low viscosity fluids, to viscous gums, to cross-linkedelastomers and hard resins. Their unique chemical and physicalproperties have made silicones important ingredients in antiagingskin care products. Although perhaps best known for their aestheticproperties, these versatile materials improve the performance ofmany cosmetics, sunscreens and skin treatment products. The helpdeliver pigments and other particles to the skin, enhanceprotection by sunscreens and improve the stability of antiagingingredients.
-Michael S. Starch
Personal Care Industry Scientist, Dow Corning Corporation
On behalf of the Silicones Environmental, Health and Safety Councilof North America