Silicone elastomers have claimed a strong position in the realm of skin care and color cosmetics as consumers continue to focus on luxurious aesthetics, distinctive textures and comfortable wear. First-generation silicone elastomers introduced a dry, powdery sensation that left a unique after-feel on the skin.1 As the technology advanced, these cross-linked silicone polymers expanded beyond carrier fluids such as dimethicone and cyclomethicone, to other organic volatile ingredients that extended their formulating possibilities2 and allowed broader capabilities with organic materials.
A recent development takes the versatility of silicone elastomers a step further by combining an alkyl methyl siloxane carrier fluid of moderate volatility with a high molecular weight polyglycol-modified silicone elastomer, referred to as a hydro elastomer blend. The result is a material that meets the difficult formulating challenge of combining organic compatibility with the ability to incorporate water and/or polar solvents with polar actives, as will be shown. Note that for ease of reading, in this article, the following nomenclature is used: silicone elastomer blend, where INCI = Dimethicone Crosspolymer; silicone organic elastomer blend, where INCI = Dimethicone/bis-Isobutyl PPG-20 Crosspolymer; and hydro elastomer blend, where INCI = PEG-12 Dimethicone/PPG-20 Crosspolymer.
Hydro Elastomer Blend Caprylyl Methicone (and) PEG-12 Dimethicone/PPG-20 Crosspolymera is a newly developed mixture of high molecular weight polyglycol-modified silicone elastomer in caprylyl methicone. ">Figure 1 illustrates the general structure of the material, with cross-linked silicone chains and hydrophilic polyether functionality. Specifically, hydrophilicity is obtained by the addition of the PEG-12 chains to the cross-linked silicone chains, which allows the incorporation of water or polar actives inside the silicone elastomer gel. The PPG-20 chain used as the cross-linker provides compatibility with organic materials such as organic sunscreens.
Materials and Methods
The benefits of the hydro elastomer were compared with existing silicone elastomer blends for properties including texture, rheology, organic compatibility and sensory characteristics. ">Figure 2 illustrates the dilution viscosities measured when a silicone elastomer blendb, a silicone organic elastomer blendc and the hydro elastomer blend were combined with 25% of three organic materials, caprylic capric triglycerides, ethylhexylmethoxycinnamate and ethanol, which were selected due to their common use in cosmetic formulations. With these three materials, the hydro elastomer blend formed blends with similar or slightly higher viscosity than the starting elastomer gel, demonstrating that the texturing properties are not affected by the addition of other ingredients. This was less the case for the silicone organic elastomer blend, and even less so for the silicone elastomer blend, which was not compatible with caprylic/capric triglyceride or ethanol at 25% loading, resulting in two separate phases.
For the bench chemist, compatibility with other ingredients in a formulation is an important parameter for selecting a raw material. While this is a drawback for most silicone elastomer blends, improved compatibility was observed and maintained with the hydro silicone elastomer blend, as demonstrated in ">Figure 3. Here, compatibility was evaluated based on visual clarity using a 75:25 blend of elastomer and several organic materials selected for their typical use in personal care formulations and range of polarity. Note that although the squalane and mineral oil samples are nearly transparent, they are not compatible with the hydro elastomer blend; the materials apparently have close refractive indices but there is a layer of free fluid on top of both samples. In comparison with the remaining materials, the samples based on the hydro elastomer blend showed overall greater clarity than those based on the silicone organic elastomer blend. In the bottom row, samples based on the silicone elastomer blend show incompatibility across the range of materials tested.
Combining this improved compatibility with organic sunscreens, the ability to incorporate polar materials such as water and glycerin, and a formulation technique called refractive index matching,3 formulators can create clear gel systems. To achieve this clarity/translucence, the refractive index of the water phase is modified to match the refractive index of the oil phase using glycerin.
Consumers have come to expect novel, even surprising sensory and textural characteristics from new personal care products that appear on the market. Compared to first-generation silicone elastomers, which indeed offered consumers a new feel, the hydro elastomer blend provides more smoothness, spreadability and slipperiness; less tackiness; and increased wetness with a resulting refreshing feel due to its ability to incorporate water. These capabilities were validated via several panel tests using multiple paired comparisons, described hereafter.
Throughout the studies, sensory characteristics were evaluated by 18 panelists who ranked the materials along a 10-cm line representing the least to most of an attribute, and using multiple paired comparisons under controlled humidity (50% + 5%) and temperature (22°C + 3°C). Statistical analysis of data was performed via 2-sample t-test for normal distribution data or Mann-Whitney test for abnormal distribution data, when comparing two samples; and via one-way ANOVA for normal distribution data or Kruskal-Wallis for abnormal distribution data, when comparing three samples.
In ">Figure 4, paired comparisons of sensory characteristics were evaluated for the hydro elastomer blend with capryl methicone as the carrier, and a silicone elastomer blend with cyclopentasiloxane as the carrier. Overall, the hydro elastomer blend retained the main sensory characteristics of a silicone elastomer gel such as good spreadability, low tackiness, low gloss, smoothness and powdery feel, but demonstrated significantly more slipperiness and greasiness, as defined by the sensory terms. This is likely linked to the difference in carriers, as the caprylyl methicone is significantly less volatile than cyclopentasiloxane.
As shown in ">Figure 5, the hydro-elastomer blend impacts the sensory characteristics of glycerin-rich anhydrous formulations by making them less tacky. Here, the hydro elastomer blend was compared with formulations containing a silicone elastomer blendd and a silicone organic elastomer blende; the latter two blends also contained 2% silicone emulsifierf for stabilization, and all three contained 50% glycerin. Although such high levels of glycerin are not typical, this demonstrates the flexibility of the hydro silicone elastomer system. These high glycerin systems are also interesting for stabilizing actives such as vitamin C.4
The aesthetic benefits shown in ">Figure 5 are present without sacrificing moisturization performance, as shown in ">Figure 6. Here, a gel containing 1/3 each of hydro elastomer blend, glycerin and water had moisturization performance similar to a solution of 1/3 glycerin and 2/3 water. In addition, when combined with 50% water instead of glycerin, the hydro elastomer blend delivered a significantly different feel with enhanced spreading, compared with the two other silicone elastomers blended with water and an emulsifier (see ">Figure 7).
The hydro elastomer also supports formulas with high pigment as well as water levels. In ">Figure 8, the formulation tested contained 43% water and 12% pigments with either the pure hydro elastomer silicone blend, or mixture of the two other elastomers combined with 4% PEG-10 dimethicone w/s emulsifier for stability. A mousse texture with a wetter feel was obtained with the hydro elastomer silicone blend, differentiated from the formulations containing the mix of silicone elastomer and silicone emulsifiers, resulting in a visually creamier texture. Combining the hydro elastomer blend with an emulsifier also resulted in a creamier texture with a drier feel (see ">Figure 9).
In summary of ">Figure 8, the formulation containing only the hydro elastomer blend (A) had significantly more wetness, less gloss, less film residue, less tackiness, more powdery feel, less uniformity and less coverage, compared with a formulation combining the hydro elastomer blend and an emulsifier (B). By way of further comparison, the formulation containing only the hydro elastomer blend (A) had significantly more wetness, more spreadability, less gloss, more smoothness, less tackiness, more powdery feel, less uniformity and less coverage, compared with a formulation combining the silicone organic elastomer blend and an emulsifier (C).
A final distinctive visual and sensory effect is the ability to form emulsions that break quickly, with visible beads of water. This effect can be achieved by blending 75% water with the hydro elastomer blend.
Formulating With the Hydro Elastomer Blend
The described hydro elastomer blend can be used by simply combining it with water-soluble ingredients such as hyaluronic acid, glycerin, niacinamide, vitamin C, caffeine, alcohol, arbutin and Aloe vera, a well-known calming agent for the treatment of sensitive skin. The phase containing water-soluble ingredients is gradually added to the oil phase containing the hydro silicone elastomer using turbulent mixing. It also can be used with organic ingredients such as ethylhexylmethoxy cinnamate, ethylhexyl salicylate, sunflower oil, C12–15 alkyl benzoate and caprylic/capric triglyceride. As noted, this material is compatible in systems with high water and pigment levels as well as glycerin-rich formulations, and can incorporate up to 75% water and up to 40% glycerin while retaining a gel-like structure. Finally, the hydro elastomer blend can be formulated into o/w emulsions, water-in-silicone emulsions and anhydrous products by addition to the oil phase or silicone phase in an emulsion formulation.
Although the use level of the hydro elastomer blend is broad, if used alone, a minimum of 25% is necessary to allow the incorporation of high levels of water, i.e., above 50%. The hydro elastomer blend also can be mixed with different organic materials including organic sunscreens. Although it is preferable to use compatible ingredients (see Figure 4), incompatible ingredients such as vegetable oils can be included in the formulation. No heating is required to incorporate the material.
By adding a PEG-12 functionality to a silicone elastomer cross-linked with a PPG-20 chain, it became possible to incorporate a high level of water, with or without other polar ingredients, into an elastomer gel structure while maintaining the gel texture and sensory benefits. This additional sensory dimension imparts a wet and refreshing feel thanks to the presence of the aqueous phase.
Compatibility with organic materials such as organic sunscreens, triglycerides and alcohol was maintained, allowing for flexibility on the composition of the oil phase as well. This material extends the possibilities for the bench chemist to develop formulations around silicone elastomers for skin care, color cosmetics and sun care. In addition, the hydro elastomer blend enables the incorporation of polar ingredients such as polyols, and does not require heating during the process, allowing for the incorporation of fragile actives.
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