New Formulating Options with Silicone Emulsifiers

Transcription

1 New Formulating Options with Silicone Emulsifiers Isabelle Van Reeth Marilena Morè Dow Corning Europe Robin Hickerson Dow Corning USA

2 In today s highly competitive skin care and underarm markets, multifunctional, high performance products have the best chance of success. Consumers expect convenience and superior aesthetics. They want long-lasting, highly efficient moisturizers; effective antiaging and anti-wrinkle creams; durable, wash-off resistant, protective color cosmetics; and underarm products that go on smoothly, without tackiness or residue. Formulators strive for all that and more: cost-effectiveness, formulating flexibility and easy processing. In many cases, the solution to fulfilling this broad range of requirements points to evolving emulsion technologies. More than 80 percent of emulsions in today s personal care market are oil-in-water systems. Emulsions of this type are favored for their stability, flexibility, high water content (and hence, lower cost), their nongreasy and nonoily feel, and their ability to form an extensive array of proven systems with predictable stability. Compared to oil-in-water emulsions, water-in-oil systems are recognized for a different range of benefits. The external oil phase typically spreads more easily on skin, formulations generally are longer lasting with improved emolliency and wash-off resistance, and they exhibit enhanced film barrier properties. Despite these characteristics, conventional waterin-oil systems can be perceived as imparting a greasy, oily feel, and lacking in formulation flexibility. In addition, they tend to be less cost effective and more difficult to produce, typically requiring a high shear finishing step. Silicone Emulsifiers for the Best All-Around Silicone emulsifiers can help bridge the gap between the two systems, providing the best characteristics of both: they can aid in the formulation of stable, aesthetically pleasing cosmetic emulsions with high water levels, while requiring no heat during processing to provide a positive impact on overall cost. Some silicone emulsifiers such as cyclopentasiloxane (and) PEG/PPG- 18/18 dimethicone a already have earned a reputation in personal care by making possible new product forms, particularly clear gels for antiperspirant applications. These materials belong to the dimethicone copolyol family of silicones, which is broadly used in personal care products. Combined U.S. and West European consumption of dimethicone copolyols for skin care applications was estimated at between 2,500 metric tons and 3,000 metric tons in The U.S. accounts for around 65 percent of this total, driven by the large consumption of dimethicone copolyols in antiperspirants and deodorants, estimated at more than 85 percent of total U.S. dimethicone copolyol consumption for skin care applications. In contrast, antiperspirant and deodorant applications account for only 55 percent of West European dimethicone copolyol consumption for skin care applications (1). Silicone emulsifier technology is based on the ability of these materials to function differently from organic emulsifiers. To perform properly, a silicone emulsifier must satisfy three requirements: it must be able to migrate to the interface between the two phases, stay at that interface, and stabilize the repulsion forces of the two phases. While typical organic emulsifiers are amphiphilic molecules of type AB, the action of the silicone emulsifier is the Interfacial Tension (mn/m) result of functional groups alongside the silicone backbone, which result in lower interfacial tension and a more robust, flexible film at the interface. This characteristic is exemplified by dimethicone copolyol emulsifiers such as the previously mentioned material, cyclomethicone (and) PEG/PPG-18/18 dimethicone, and also by lauryl PEG/PPG-18/18 methicone. b The graph in Figure 1 compares the difference in interfacial tension at the water and oil boundary for two organic emulsifiers and a silicone emulsifier. Notice that a lower concentration of lauryl PEG/PPG-18/18 methicone is required to reduce interfacial tension compared to the organic materials. Finally, because of their branched structures and high molecular weights, each silicone molecule packs very tightly at the oil and water interface. Hydrophilic and lipophilic portions of the molecule are tightly aligned by the flexible silicone backbone to provide highly effective steric repulsion. A New Option: Low Shear Processing In general, most silicone water-in-oil emulsifiers require high shear to make emulsions with optimized stability. Recent developments in silicone emulsifier technology have resulted in still Log Emulsifier Concentration ( µ mol/1) Polyglycerin-3 Diisostearate Sorbitan Oleate Lauryl PEG/ PPG-18/18 Methicone Figure 1. Comparison of the difference in interfacial tension at the water-oil boundary as a function of the log emulsifier concentration. a Dow Corning 5225C Formulation Aid b Dow Corning 5200 Formulation Aid

3 newer materials that offer expanded performance along with cost effectiveness and formulating ease. For nextgeneration water-in-oil emulsions, a new silicone emulsifier based on silicone elastomer technology has been developed. This material allows formulators to develop stable water-in-silicone emulsions without the need for high shear processing equipment. Given the INCI designation cyclopentasiloxane (and) PEG-12 dimethicone crosspolymer c, the emulsifier provides: A broad variety of sensory profiles and textures to offer a sensory experience ranging from very light to rich, depending on the oil used. Enhanced aesthetics over dimethicone copolyol emulsifiers. Formulation flexibility for forming clear emulsions with stability at low to high viscosities, and accommodating a wide range of oil-phase ingredients. Processing flexibility for low- to high-shear processing and the option for cold mixing. Cost effectiveness, with the option for using high water content (up to 82 percent water in the internal phase), low emulsifier levels and cold processing. Stability for long shelf life and consistent performance. Improved active suspension in underarm products. Using the new emulsion, it is also possible to create novel product forms such as anhydrous systems or multiple emulsions with two distinct aqueous phases (water-in-oil-in-water). The latter approach results in aesthetics that are different from those associated with typical water-in-silicone emulsions; the external phase is now aqueous, making the first impression on the skin very light. This method also suggests potential use for delivery of active ingredients such as emollients, moisturizers, sunscreens, pigments, vitamins and antiperspirant salts. The ability of this emulsifier to make stable anhydrous propylene glycol-in-silicone emulsions allows formulators to develop systems where vitamin C is not degraded during the shelf life of the product. This emul- c Dow Corning 9011 Silicone Elastomer Blend sifier is outside current clear antiperspirant patents and therefore allows freedom to practice. Formulation 1 illustrates the use of the new silicone emulsifier in a clear antiperspirant gel made with a low shear process. Formulation 2 demonstrates the use of the silicone emulsifier in a water-insilicone foundation. Formulation 1 Clear Antiperspirant Gel Ingredient Wt. % Trade Name/Supplier Phase A 1. Cyclopentasiloxane (and) PEG Dow Corning 9011 Silicone Elastomer Blend dimethicone crosspolymer 2. Dimethicone 3.5 IAMETER PM-200 Silicone Fluid 10 cst 3. Phenyl trimethicone 0.5 Dow Corning 556 Cosmetic Grade Fluid 4. Cyclopentasiloxane 6.0 IAMETER PM-0245 Cyclopentasiloxane Phase B 5. Aluminum sesquichlorohydrate 42.0 Reach 301 Solution/Reheis Inc. 6. Deionized water Propylene glycol 12.5 Propylene glycol/the Dow Chemical Company 8. Glycerin 6.5 Glycerin/Fisher Chemical Company 9. Ethyl alcohol, 200 proof 1.5 Alcohol/Equistar Procedure Combine Phase A ingredients. In a separate container, combine Phase B ingredients. Match the refractive index of Phase A to that of Phase B. If the refractive index of Phase A is higher than that of Phase B, add water to the aqueous phase to match. If lower, add glycerin to match. With rapid mixing, add Phase B to Phase A very slowly, using a separatory funnel. (Use a 1000 ml tall beaker and 1376 rpm.) Formulation 2 Water-in-Silicone Foundation The Solution for Novel Product Forms For formulators seeking ways to create highly differentiated product forms, lauryl PEG/PPG-18/18 methicone, a dimethicone copolyol emulsifier previously mentioned, offers unique potential in skin care applications. Ingredient Wt. % Trade Name/Supplier Phase A 1. Dextrin palmitate 2.10 Rheopearl KL/Miyoshi Kasel 2. PEG-12 dimethicone 1.90 IAMETER OF-0193 Fluid 3. Tricaprylin 5.00 Trivent OC-G/Trivent 4. Cyclopentasiloxane (and) PEG Dow Corning 9011 Silicone Elastomer Blend dimethicone crosspolymer Phase B 5. Cyclopentasiloxane & Cyclohexasiloxane IAMETER PM-0345 Cyclosiloxane Blend Phase C 6. C.I , Dimethicone 2.75 SAT-T /US Cosmetics 7. C.I , Dimethicone 0.43 SAT-Y /US Cosmetics 8. C.I , Dimethicone 0.32 SAT-R /US Cosmetics 9. C.I , Dimethicone 0.01 SAT-B-33134/US Cosmetics Phase D 10. Deionized water to Sodium chloride 1.00 Sodium chloride/merck Procedure Heat Phase A to 80 C, ensuring that ingredient 1 is fully dissolved. Cool to 50 C. Combine ingredients of Phase C. Add Phase C to Phase B and mix. Add Phase B to Phase A with mixing. Combine ingredients of Phase D. Add Phase D to Phase A with high shear mixing. (Formulation developed by S Black Ltd. UK)

4 This material allows: Very stable water-in-oil systems without the addition of waxes and up to approximately 80 percent water phase. The ability to accommodate a low to medium polarity oil phase including a high level of silicone oils. Long-lasting moisturizing, wash-off resistance and superior aesthetics. A flexible sensory profile from light and nongreasy to nourishing and richly emollient. The option for cold processing. Novel product forms such as waterin-wax emulsions for potential use in foundation sticks and lipsticks. For example, a prototype water-inwax base stick containing 60 percent water imparts a feel of freshness to skin compared to the expected feel from an anhydrous stick. In addition to a surprising feel, the presence of water offers the possibility of adding water-soluble actives to the stick, as well as reducing the overall cost of the formulation. Figure 2 illustrates the moisturizing performance of a water-in-oil emulsion based on lauryl PEG/PPG-18/18 methicone. These data show that an increase of 40 percent in the hydration level of the skin can be maintained for longer than six hours. Although this formulation contains 10 percent glycerin, the cream has a silky, nongreasy and nontacky feel as shown in Figure 3, where it is compared to an oil-in-water emulsion. Based on paired comparisons, the spider diagram of Figure 3 compares the sensory profiles of an oil-in-water formulation and a water-in-oil formulation made with the lauryl PEG/PPG-18/18 methicone emulsifier. Except for the wetness and spreadability parameters, the waterin-oil formulation demonstrates either equivalent or improved sensory performance over the oil-in-water system. For an equivalent performance on greasiness (a parameter that is usually a disadvantage of water-in-oil systems), the waterin-oil formulation with the silicone emulsifier has a silky, nontacky and slippery feel on the skin. Percentage increase moisturization versus neat skin Tackiness after absorption 99.9% Silkiness 99.9% An Added Advantage: Durability and Resistance to Wash-Off With their external oil phase, water-inoil systems can form an immediate continuous and homogeneous film on the skin to optimize the film-barrier properties. In addition, since the emulsifier is water insoluble, wash-off resistance will be improved because the oil phase cannot be re-emulsified by water. The degree of film forming properties and wash-off resistance will be dependent on the types of ingredients present in the oil phase. Another benefit of waterin-oil emulsions is the protection of hydrosoluble actives such as vitamin C, which are sensitive to oxidation. Moisturization with Corneometer. 0 min 30 min 1 hour 2 hours 3 hours 4 hours 5 hours 6 hours Slipperiness 99.9% Greasiness T ime Figure 2. Corneometer measurements of moisturization effect from a water-inoil formulation containing lauryl PEG/PPG-18/18 methicone emulsifier. Figure 4 illustrates the wash-off resistance of a water-soluble sunscreen active in an oil-in-water formulation containing an organic emulsifier (cetyl phosphate) and a water-in-oil system formulated with lauryl PEG/PPG-18/18 methicone. The formulation containing Wetness 99.9% Film residue Spreadability 99.9% Gloss Tackiness before absorption Absorbancy Figure 3. Sensory profiles of oil-in-water and water-in-oil formulations, based on paired comparisons. O/W W/O the silicone emulsifier resists wash-off, while 53 percent of the sunscreen in the formulation containing the organic emulsifier is washed away. Forming Stable Emulsions Stable emulsions can exist only when the internal phase droplets remain separated from one another over a period of time. Several factors affect this state, such as particle size, distribution

5 of particle sizes, formulation components and phase ratio. A homogeneous particle size is key to developing a stable emulsion. A bi- or multi-modal particle size distribution can significantly reduce stability by increasing coalescence through the collisions of the particles among themselves. Uniformity is more critical to stability than size because large particles have a greater mass, and when collisions with smaller particles occur, large particles will incorporate the smaller particles. Drop in viscosity is often the first sign that coalescence is occurring. When a multi-modal particle size distribution occurs, both formulation and process should be investigated. Emulsifier level. Too much emulsifier can result in as much difficulty as too % sunscreen washed away % O/W O/W: Cetyl phosphate + 3 % Phenylbenzimidazole sulfonic acid* W/O: Laurylmethicone copolyol + 3 % Phenylbenzimidazole sulfonic acid *Parsol HS little. Excess emulsifier may surround itself or attract other droplets. For that reason, the recommended approach is to formulate with the minimum and maximum amount of emulsifier until instability is noted. Phase ratio. A phase ratio of 80:20 (water to oil) can be achieved with the three silicone emulsifiers discussed in this article. Increasing the internal phase increases viscosity. Phase ratios significantly below 65 percent internal phase may require either additional emulsifier, thickener or a reduced particle size to avoid settling. For example, the oil (external) phase can be thickened by the use of waxes, bentonite gels or silicone elastomers. Aqueous phase. In the formulation of underarm products, small changes in active ingredients (e.g., switching from ACH to AlZr salts) should have little impact on the system. However, any element in either phase that changes the solubility of the emulsifier will have an impact on the system. Some fragrance components may have this effect, as can high levels of antiperspirant salts in combination with lauryl PEG/PPG- 18/18 methicone. Oil Phase. Some emulsifiers are more effective in certain types of oil systems, such as cyclic versus linear siloxanes or low viscosity versus high viscosity. < 1 % W/O Figure 4. Comparison of wash-off resistance between a water-soluble sunscreen active in a water-in-oil formulation containing a silicone emulsifier, versus an oil-in-water formulation containing an organic emulsifier. In addition, the resulting polarity of the oil phase should be considered. Lauryl PEG/PPG-18/18 methicone is most efficient with low to medium polarity. It is important to thoroughly investigate the formulation on a laboratory scale prior to production. Inorganic electrolytes and polyols. The use of an inorganic electrolyte (e.g., sodium citrate, magnesium sulfate, sodium chloride or sodium tetraborate) at approximately 1-2 percent by weight has been shown to reduce even further the interfacial tension as well as improve freeze-thaw stability. The addition of small quantities of polyols with their humectancy properties decreases the water loss; however, high levels might destabilize the emulsion. Co-emulsifiers. The use of co-emulsifiers (e.g., polysorbate-20, C12-15 pareth-9, laureth-7) should be limited. These ingredients should be used at levels lower than their critical micelle concentration to avoid depletion flocculation. Table 1 compares the levels of formulation components for forming stable water-in-oil or water-in silicone systems with silicone emulsifiers. Processing. The following general processing guidelines are recommended for formulating with silicone emulsifiers: Combine the ingredients of Phase A and Phase B in separate containers and mix each until uniform. Add the water Phase B to the silicone or oil Phase A very slowly, using a high turbulence mechanical blade mixer set at high speed (900 ft/min tip speed). This addition should take from 10 to 30 minutes. Because the aqueous phase typically has a higher density than the oil phase, it gravitates to the bottom of the mixing container. Adding the aqueous phase from the top enhances mixing efficiency. After addition is complete continue mixing for another 10 to 30 minutes. This step narrows the particle size distribution. To finish the emulsion, homogenize the system with a high shear mixing device. This step reduces the mean particle size and increases emulsion viscosity. The final high shear step is optional when using cyclopentasiloxane (and) PEG-12 dimethicone crosspolymer. Without the final shear step, particle diameters will typically be in the 1 to 3 micron range. With a final high shear pass, diameters will be less than 1 micron. Additional details related to processing and stability assessments are available in separate publications (2,3). Table 2 compares some of the physical and formulating characteristics that distinguish three silicone emulsifiers. By comparing the requirements and properties of various formulation types, it is possible to determine which silicone emulsifier is most appropriate for a particular application. Table 3 provides some

6 Level (%) Table 1. Critical Factors for Stable Emulsions Table 2. Overview of Silicone Emulsifier Characteristics INCI Name Emulsion Type % Actives Shear / Processing requirements Use Levels System Type Clear systems Table 3. Selection Criteria for Silicone Emulsifiers Multiple emulsions W/Si/W Multiple emulsions W/O/W Water-in-wax (W/W) systems Emulsions containing AP salts New product forms / new sensory Cyclopentasiloxane and PEG/PPG-18/18 Dimethicone (Dow Corning 5225C) Water-in-silicone, multiple emulsions 10% High shear 7-15% W/Si Systems W/O Systems Anhydrous Systems (PPG/S) Moisturizing, wash-off resistant, long lasting creams Dow Corning 5225C Cyclopentasiloxane and PEG-12 Dimethicone Crosspolymer (Dow Corning 9011) Water-in-silicone, multiple emulsions, anhydrous emulsions 12.5% Low to high shear 6-14% Water-in-Oil Emulsions (Dow Corning 5200*) Emulsifier Oil phase Water phase Electrolyte (NaCl preferred) Water-in-Silicone Emulsions (Dow Corning 5225C* and 9011) Co-emulsifier * Dispersion of approximately 10% active emulsifier Dow Corning 9011 Lauryl PEG/PPG- 18/18 Methicone (Dow Corning 5200) Water-in-oil, Water-in-wax 100% High shear 2-3% Dow Corning 5200 general guidelines. Additional details are available in a separate publication (4). Conclusions New developments in silicone emulsifier technology provide expanded options for creating stable water-in-oil and waterin-silicone emulsions with a broad range of sensory characteristics. In skin and underarm products, these ingredients give formulators flexibility for developing clear products with superior aesthetics and novel forms such as anhydrous systems, multiple emulsions or water-inwax sticks. In addition, benefits of waterin-oil systems such as good sensory profiles, improved wash-off resistance and excellent moisturization have been demonstrated. Silicone emulsifiers offer versatility for low or high shear options as well as cold processing, presenting new opportunities for cost-effective and highly innovative skin care and underarm products. References 1. Personal Communication, Gillian Morris, Group Director, Chemicals, Minerals, Polymers, Kline & Company (2003). 2. Kasprzak, K., A guide to formulating Water-in-Silicone emulsions with Dow Corning 3225C formulation aid, Dow Corning Internal Document, Form no (1995). 3. Dahms, G., and Zombeck, A., New formulation possibilities offered by silicone copolyols, Cosmetics & Toiletries, Vol 110 number 3, p 91+ (1995). Also available as Dow Corning Internal Document, Form no (1995). 4. Hickerson, R. and Van Reeth, I., Silicone Emulsifiers Guide, Dow Corning Internal Document, Form no (2002).

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