In today’s competitive skin care market, consumers look for a range of benefits, superior aesthetics and cost effectiveness. In response, formulators strive to create products that are multifunctional, easy to use and effective in small quantities. At the same time, the most successful products deliver benefits that stand out from those of the competition. Creams, lotions and color cosmetics must be easy to spread and comfortable to wear, without feeling tacky or greasy.
Silicones have been popular ingredients in skin care products for more than fifty years. These versatile materials are known for their emolliency, wetting and spreading characteristics and ability to provide unique aesthetics. As technology continues to evolve, silicones increasingly offer multifunctional, high performance properties. This article reviews some of the newest silicone technologies available for the formulation of innovative skin care products.
Silicone elastomers are among the newest performance-enhancing silicone ingredients. Their distinctive skin feel has been described as dry, smooth, silky and powdery. In skin care applications, the aesthetics of silicone elastomers and their ability to absorb various oils and sebum are two of the most important properties. Some of the first silicone elastomers for personal care applications were elastomer pastes composed of silicone elastomers swollen by solvents such as cyclopentasiloxane or low viscosity dimethicone (1). Since their introduction more than 10 years ago, the product offering for silicone elastomers has expanded to include anhydrous gels and powders, in pure form or delivered via aqueous suspensions (1).
In each product form, the flexibility of silicone elastomers can be limited only by the imagination of the bench chemist: elastomer powders can be hard or soft, and elastic or free flowing via the addition of other powder types, thus allowing their use in compact products. The composition of elastomer gels can be tailored to fit a specified sensory profile by blending volatile and nonvolatile carriers and other emollients such as petrolatum, as well as adapting the level of elastomer. Last, but not least, silicone elastomer suspensions allow easy use of the elastomer in a broad range of aqueous formulations such as oil-in-water foundations. Suspensions of this type can also be modified by post addition of different ingredients.
Because of the absorption characteristics of silicone elastomers, various actives can be entrapped within their structures, allowing them to act as delivery systems for actives, including fragrances. Also, the gelling properties of these elastomers allow the suspension within anhydrous systems of materials such as propylene glycol, which are good solvents for several actives. The active is released when the gel is rubbed onto the skin, with the important benefits of providing the unique powdery feel of the elastomer, which helps mask the less desirable feel of propylene glycol. This is also an elegant way to deliver pure glycerin, a highly efficient humectant, to the skin (2).
Figure 1. Larger silicone elastomer spheres are coated with smaller silica particles, which have a lighter appearance in the photo. (SEM image courtesy of and copyright by LVMH Labs.)
Among these materials is a silicone elastomer cosmetic powder consisting of silicone elastomer spheres (approximately 1-10 µm in diameter) coated with silica. Figure 1 illustrates the spheres and their silica coating.
In addition to its ability to absorb sebum, the elastomer powder can be used to help mask wrinkles. In evaluations with 21 panelists, there was an improvement in the appearance of wrinkles for 75% of the subjects, with a significant total mean decrease of 28% (p<0.01) (3).
The silica-coated elastomer is appropriate for applications in creams, lotions, gels, face masks and products that exfoliate or offer sun protection, whitening or anti-aging benefits. Numerous potential applications exist in color cosmetics, including pressed and loose foundation powders and makeup, eye shadow, blushers, liquid foundations, lipsticks and bronzers.
The small amount of silica coating transforms these spherical elastomer particles into a free-flowing powder without agglomeration, so handling and formulation are easy. The powder does not require predispersion, even in the presence of pigments, and it can be incorporated into a range of skin care and color cosmetic formulations, including hydrogels, without using sophisticated processes or equipment. Sensory benefits and novel textures remain cost effective because low use levels of the elastomer are required.
The silicone elastomer powder has a shear-thinning effect on thick creams or gels, transforming them to a fluid-like texture when rubbed onto the skin. This property helps ensure easy spreading for good coverage. It also is important in foundation products to achieve an even, uniform tone.
Elastomer powders can also be coated with other materials such as alumina, lauryl lysine and micronized titanium dioxide, further enhancing their benefits. For example, coating with alumina optimizes the soft-focus effect of the powder. Coating with lauryl lysine further enhances sebum absorption. Another advantage of the coating (e.g., with titanium dioxide) is the improvement of the compaction property, making these powders very easy to incorporate into compact powders. All these powders maintain their unique, silky and powdery feel, but those coated with alumina, lauryl lysine and titanium dioxide add nuances to the sensory profile, allowing for a broader range of effects.
Because of their ability to swell when mixed with solvents, silicone elastomers are recognized as effective oil-phase thickeners. Compared to other organic powders, the silicone elastomer powder shows superior viscosity enhancement (based on the same use level of powder in the same formulation). For example, a water-in-silicone cream that incorporated 5% silicone elastomer powder showed viscosity enhancement of approximately 33% over a control formulation without the elastomer powder (3).
Silicone Acrylate Copolymers
Two new silicone acrylate copolymers offer enhanced durability and aesthetics coupled with easy formulation for skin care products and color cosmetics. A 40% silicone acrylate copolymer dispersed in isododecane was designed for applications that require the solvent to evaporate quickly, while leaving a high molecular weight silicone acrylate copolymer on the skin. The other material, a 30% silicone acrylate copolymer dispersed in cyclopentasiloxane is more suited for slower evaporation or other formulation requirements.
Figure 2. Lipcream durability and wearability; sensory evaluations by 12 panelists over 8 hr.
The transparent, water-like liquids form non-occlusive films that resist wash-off. This film-forming property is useful for long-lasting sunscreens, while in lipsticks it pairs with an excellent sensory profile, translating into improved non-transfer while maintaining very good wearing comfort. Figure 2 compares the sensory properties of a prototype lipcream formulated with silicone acrylate copolymer with a similar formulation containing an MQ resin and a leading commercial benchmark. The formulation with silicone acrylate copolymer showed significant improvements in shine, ease of application, nourished skin feel and comfort over eight hours of wear (4).
The non-transfer properties can also be applied in water-in-silicone foundations, where the silicone acrylate copolymers have demonstrated significant improvement versus silicone resins. Silicone acrylate copolymers also are permeable to water vapor, so they do not interfere with the skin exchange function, an important parameter for products meant for long-lasting wear. Finished formulations have minimal tackiness and greasiness, leaving skin smooth, without a dry after-feel.
These versatile copolymers have also sebum resistance properties, which help keep sebum from spreading on the skin and therefore mitigate the disruption of a sunscreen film. In the case of foundations, this same property helps maintain the even distribution of pigments to help ensure optimum appearance for a longer time. In non-pigmented products, it may also reduce the spreading of sebum, giving the impression of a less greasy and shiny skin.
For easy formulating, the silicone acrylate copolymers are compatible with a variety of common cosmetic ingredients, including pigments, organic sunscreens and triglycerides.
New Alkyl Silicone with Moderate Volatility
Caprylyl methicone is an alkylmethylsiloxane fluid of moderate volatility, which feels less dry on the skin than more volatile materials. The clear, colorless fluid spreads easily with a light, silky, and smooth feel.
The popularity of “natural” products remains strong with consumers, but formulating with vegetable oils can present aesthetic challenges, such as skin residue or a tacky and greasy feel. With caprylyl methicone, it’s possible to create products with ingredients of natural origin that also meet consumer needs for high performance and pleasing sensory properties (5).
Figure 3. Sensory improvement of natural oil in combination with caprylyl methicone (after absorption). Numbers in parentheses indicate level of confidence.
Figure 3 illustrates how the addition of caprylyl methicone to a vegetable oil blend can result in enhanced sensory properties.
Caprylyl methicone is useful in a variety of personal care applications:
- Skin care – moisturizers, gels, makeup removers, facial cleansers
- Sun protection – sunscreens, creams, UV blockers
- Color cosmetics – foundations, eye shadows, eye liners, lipsticks, blushes
- Hair care – leave-on conditioners, hair sprays, cuticle coats
The material is a versatile, easy-to-handle fluid because of its compatibility with a wide range of personal care ingredients. In addition to formulating it with vegetable oils for improved sensory properties, it can serve as a carrier for other ingredients, or as a dispersing medium for hydrophobic powders and pigments.
Water-in-Silicone Liquid Dispersion Polymer
A number of silicone materials are available as emulsifiers, and still others offer thickening as a secondary benefit. Among the newest multifunctional materials is a water-in-silicone emulsion of aqueous sodium polyacrylate in dimethicone. This versatile liquid dispersion polymer (LDP) can thicken and emulsify a variety of oils in aqueous formulations (gels, oil-in-water or silicone-in-water), while imparting recognized silicone properties such as smoothness without a greasy or tacky feel (6).
At concentrations as low as 1%, the thickening agent is a useful solution for formulating with elastomers, gum blends and other difficult-to-emulsify silicones. It is compatible with a wide range of mineral and organic oils, esters, sunscreens and vitamins and can be used in formulations with high polar solvent content (up to 30% ethanol, isopropyl alcohol or acetone) or up to 50% glycerin or propylene glycol. Compared to powder thickeners that can clump, cause excessive dusting, or build up a static electrical charge, it was designed for cold processing by simply adding water and mixing. No neutralization or heating are required.
The thickening agent is made by polymerizing a concentrated aqueous solution of the monomer that has been dispersed into the silicone oil (dimethicone and cyclopentasiloxane) and stabilized with the aid of a silicone emulsifier. An inverting agent (nonionic surfactant) is added at the end of the process. When formulating, the emulsion of sodium polyacrylate is mixed with water, and the inverting surfactant helps bring water in contact with the polymer. The polymer “blooms” into the aqueous phase, allowing both emulsification and thickening from the same ingredient, and providing a smooth, nongreasy and nonsticky product.
The thickener is effective over a pH range of 5.5 to 11, making it suitable for use in high pH products. It is not recommended for use in acidic systems due to poor thickening efficacy at low pH.
In formulations, the thickening agent exhibits a shear-thinning effect. The thickener maintains high viscosity at a low shear rate, while at a high shear rate viscosity drops. This property allows formulation of thick, stable creams or gels that spread easily to uniform films on the skin as a result of increased shear during application.
Figure 4. Sensory characteristics of the new thickening agent and polyacrylamide/C13-14 isoparaffin/laureth-7 before absorption.
Paired-comparison tests of sensory attributes were conducted with 18 experienced panelists (6). A formulation based on a current commercial benchmark contained 3% thickener, 3% glycerin, 0.3% biocide and water (to 100%). Figures 4 and 5 illustrate the sensory properties before and after absorption when the silicone emulsion was compared to a commercial liquid thickening polymer, polyacrylamide (and) C13-14 isoparaffin (and) laureth-7. (The degree of “absorption” was based on panelists’ perceptions, not on actual skin absorption.) Note that before absorption, the formulation containing the silicone thickening agent showed faster absorption and reduced tackiness compared to the isoparaffin-based thickener. After absorption, the silicone-containing product was significantly less tacky, and was rated as less slippery and greasy, with less film residue. The numbers in parenthesis indicate a statistically significant difference, with the confidence level given by the numerical value.
Figure 5. Sensory characteristics of the new thickening agent and polyacrylamide/C13-14 isoparaffin/laureth-7 after absorption.
The prototype formulation in Table 1 demonstrates how the thickening agent allows easy formulation when combining specialty silicone materials such as gum blends or elastomer blends with natural oils or butters.
Table 1. Body Cream with Moisturizing and Nourishing Feel
|Cetearyl wheat straw glycosides (and) cetearyl alcohol (Xyliance/ Soliance)
|Dow Corning® 9546 Silicone Elastomer Blend
|Dow Corning® FZ-3196
|Potassium Cetyl Phosphate (Amphisol K / DSM Nutritional Products)
|Dow Corning® CB-1556 Fluid
|Water (and) propylene glycol (and) Paullinia cupana seed extract (Uniplant Guarana EG / Chemy Union)
|Porphyridium cruentum extract (Porphyraline / Soliance)
|Dow Corning® RM 2051 Thickening Agent
|Preservative and Fragrance
New silicone technologies offer formulators opportunities to create highly differentiated and innovative skin care products. In addition to offering improved aesthetics and novel textures, these performance-enhancing specialty chemicals can offer a versatile range of benefits to skin creams and lotions, color cosmetics and sun care applications.
- Dimethicone/vinyl dimethicone crosspolymer (and) silica
- Isododecane (and) acrylates/polytrimethylsiloxymethacrylate copolymer)
- Cyclopentasiloxane (and) acrylates/polytrimethylsiloxymethacrylate copolymer
- Sodium polyacrylate (and) dimethicone (and) cyclopentasiloxane (and) trideceth-6 (and) PEG/PPG-18/18 dimethicone
- Starch, M., “New developments in silicone elastomers for skin care,” Dow Corning white paper, 27-1060B-01 (2002).
- Newton, J. et al., “Silicone Elastomer Delivery Systems” Cosmet Toiletries (Bench and Beyond), 119:12, 24-31 (2004).
- Vervier, I. and Courel, B., “Masking wrinkles and enhancing skin feel with silicone elastomer powder,” Cosmet Toiletries, 121:11, 65-74 (2006).
- Postiaux, S., Iimura, T., Kondo, H., Schirosi, E., “Novel silicone acrylate copolymers for applications in color cosmetics,” Presented at 24th IFSCC Congress, Osaka, Japan (Oct. 16-19, 2006).
- Girboux A.L., Courbon E. Enhancing the Feel of Vegetable Oils with Silicone, Cosmet Toiletries, 123:07, 65-74 (2006).
- Starch, M., Ostergaard, T., Griboux, A-L. and Postiaux, S., “New thickening and emulsifying systems for aqueous and nonaqueous systems,” Dow Corning white paper, 27-1255-01 (2006).