Rheology control is crucial for developing optimum cosmetic formulations, and if rheology modifiers can offer additional benefits so much the better. Silicas could be considered the next generation in rheology control
In today’s evolving personal care market, consumers look for a sophisticated combination of performance and aesthetics in their skin care products. Some are drawn to a matte appearance, others to a light, non-greasy feel, while some prefer products that provide therapeutic ingredients or a pleasing, long-lasting fragrance.
In response, personal care chemists must create highly differentiated formulations to gain a competitive edge, and they increasingly look to novel rheology effects as a critical component of innovation. Other issues are important too. Broader use of actives requires appropriate delivery systems that ensure stability and longevity. On the practical side, raw materials that allow cold processing are a plus in terms of faster development, easier manufacturing and cost efficiency. With competing trends and needs, multifunctional ingredients that provide a number of formulating solutions present an advantage.
The number of commercially available ingredients described as thickening agents, opacifying agents, abrasives and absorbents demonstrates the abundant use and application benefits of silica in cosmetic preparations. Many varieties of silica exist, including materials with surface modification or those combined with other ingredients. Surface treatments of silica result in a wide range of performance benefits for various applications. Among these materials, silica silylates are a category of hydrophobically treated silica with a variety of traditional uses as anti-foaming agents, bulking agents and skin care ingredients.
Silica silylate aerogels are a new class of hydrophobic silica particles whose applications extend beyond skin care to sun care, colour cosmetics, fragrance and underarm products. They are efficient oil phase thickeners at concentrations of 1 to 15% and can absorb sebum at levels as low as 0.5 to 1.5%, in both cases providing improved performance at lower concentrations than traditional hydrophobically modified materials. Figure 1 compares the aerogels to other materials used in similar applications.
Distinctive & versatile
The manufacturing process for the silica silylate aerogels is responsible for a number of unique properties. The proprietary process prevents shrinkage and produces aerogel particles with greater than 90% porosity. Individual particles have a significant surface area, averaging 600 to 800m2/g. The quality of the trimethylsilylated hydrophobic coverage of the silica surface (ie complete coverage with strong chemical bonds to the silica) allows for permanent hydrophobicity and stability against attack by water and hydrolysis over a broad pH range. The aerogels are available as 1mm beads or fine particles between 5 and 15µm.
The properties of these aerogels can be adapted to a number of personal care applications.
High oil absorption rate - A silica silylate aerogel can absorb more than ten times its weight in low molecular weight oils such as cyclomethicone or fragrance. Absorption of higher molecular weight oils such as vitamins, organic oils or silicone oils can reach seven to eight times the weight of the powder.
High sebum absorption - The aerogels have demonstrated effective and long-lasting sebum absorption properties compared to other sebum-absorbing powders promoted for this purpose. Figure 2 compares this property for five powders after two, four and six hours. When delivered from a formulation, results show the long-term ability of the aerogel to reduce sebum compared to the other powders. Formulation 1 illustrates the use of the aerogel fine particles to absorb sebum from the skin.
Effective thickening of the oil phase - The silica silylate aerogels are efficient oil phase thickeners at concentrations of 1 to 15%. Figure 3 shows the thickening effect of the silica silylate aerogel (fine particles) and a typical hydrophobic silica with a lower surface area (225m2/g). Dimethicone represents the oil phase in this example. A similar effect is observed with mineral oil and other oils.
Improved retention of fragrance - Another important characteristic of the silica silylate aerogels is their ability to reduce formulation volatility, a property that can aid the delivery of fragrance or active ingredients. Samples of the aerogel fine particles and the silica silylate with lower surface area were loaded with fragrance, overcoated with various siloxane waxes and evaluated for fragrance retention. Figure 4 summarises volatility assessments as recorded by weight loss. Volatility rate decreased when the silica silylate aerogel was encapsulated within a siloxane wax. Materials evaluated included several alkylmethylsiloxane waxes and one polyether functional wax, the latter providing the best performance.
Reduced syneresis and whitening - The addition of only 1% silica silylate aerogel in antiperspirant formulations can significantly reduce syneresis from antiperspirant formulations (Figure 5). The aerogel does not increase whitening caused by the antiperspirant salt. Formulation 2 illustrates the use of the silica silylate aerogel in an antiperspirant, where it provides thickening while reducing syneresis and whitening.
Sensory enhancement - Used alone in formulations, silica silylates can have a slightly rough feel on the skin. However, when combined with other silicones, the overall sensory profile is significantly improved while maintaining the sebum absorption performance. In Figure 6, a hydrogel containing 0.5% aerogel fine particles and 6% cyclomethicone (the control) was compared to the same formulation with the addition of 4% cyclo-pentasiloxane (and) dimethiconol. Sensory panellists noted that the second formulation felt less wet and tacky and left less film residue on the skin. It had a feeling of greater slipperiness and smoothness and spread more easily on the skin.
Opportunity for novel product forms - The distinctive form and properties of the silica silylate aerogels allow formulators to create sophisticated and innovative applications for this technology. For example, using any oil including fragrance, vitamins and silicones, it is possible to prepare a formulation with caviar-like beads. In this case, up to 90% of the oil can be absorbed by 10% of the aerogel beads (Formulation 3).
Silica silylate aerogels are a new class of hydrophobic silica particles that act as rheology modifiers while offering additional formulation benefits. As consumer demand grows for innovative and aesthetically distinctive products, personal care chemists are challenged to create highly differentiated formulations. The aerogels can be a useful component of new product design, allowing stable thickening, broader use of active ingredients and fragrances with greater stability and longevity. It also allows cold processing for faster development, easier manufacturing and cost efficiency.