Analysis of Volatile Dimethicones in Cosmetics

Volatile linear silicones are low molecular weight fluids (ranging between 0.65 cSt and 2 cSt) used as transparent, odorless softening agents in personal care products. In addition to being volatile on the skin, they stand out for their low surface tension, rapid evaporation, special lubricity, easy spreading and easy miscibility with numerous raw materials (see Table 3). As a result, they are used in many different product groups such as skin care, deodorants, antiperspirants, sun care, hair care and color cosmetics to impart a non-greasy silky feel. For example, in color cosmetics they are used as volatile carriers in pigment dispersions, as substitutes for organic solvents in aerosols, and as active solvents in sun care and as aids in dispersing micronized oxides. Upon evaporation, volatile linear silicones provide a soft, dry, velvety feel without leaving residue or buildup. They reduce tackiness, thus preventing spray nozzle clogging in sprayable formulations. In stick formulations, they reduce sticky feel, improve spreadability; thereby emulsions acquire a soft, smooth and unique feel. To produce low-viscosity dimethicones, BRB uses a special process to polymerize siloxanes with very low D4 content. In this process, with minimal carbon footprint, BRB maintains its commitment to developing and using environmentally conscious manufacturing processes.

In addition to polymerization, the product is processed through many long continuous distillation columns to achieve high purity. Superior Feel

As shown in Table 3, super low-viscosity dimethicones are more compatible with most cosmetic raw materials compared to low-viscosity classic dimethicones (5 cSt–20 cSt). A wide palette of volatile linear silicone fluids is available on the market to achieve superior feel. Figure 2 shows the volatility of various low-viscosity fluids. Due to their low evaporation heat, they do not produce a cooling effect in application. Compared to ethanol, they do not dry the skin; rather, they provide a unique luxury feel.

Volatile dimethicones provide silky, protective, breathable moisturization while helping reduce dry-down time without imparting a greasy feel. Environmental Degradability

Today, there is a high degree of misunderstanding regarding the source and performance of silicones. There are a number of blogs and popular publications that associate the source of silicone with petroleum. In reality, this is completely wrong, because silicones are derived from quartz (sand). Silicon is the second most abundant element in the Earth's crust. Silicon production chemistry encompasses the production of polydimethylsiloxanes following synthesis, distillation, hydrolysis and polymerization steps, which result in different silicones. Polydimethylsiloxane fluids are not water-soluble. Therefore, when these silicones are used in rinse-off products, they pass into wastewater, mix with municipal sewage, and are rarely detected in rivers and lakes. When they enter soil, they readily dissolve upon contact with clay components and degrade to silica and CO2. Numerous scientific studies indicate that polydimethylsiloxane does not bioaccumulate in living organisms because its molecular size is too large to cross biological membranes. When considering low-viscosity linear fluids, they evaporate and pass into the air without transitioning to other environmental areas such as water and soil. In the air, with the aid of sunlight, they degrade to silica, water and CO2.

Non-Occlusive Agents

One of the fundamental physicochemical properties of PDMS fluids is their permeability to gases and water vapor. Although they are resistant to liquid water, they provide spread on the skin, creating a breathable and non-comedogenic barrier. Numerous TEWL (transepidermal water loss) tests have been conducted to demonstrate the breathable and non-occlusive properties of silicones (Table 4). This test was performed to compare the time-dependent TEWL (transepidermal water loss) values of various silicones, a w/Si emulsion, and petrolatum, which is known to be occlusive, over time. Statistical analysis results show that there was no significant difference in TEWL values between the control, silicone 1, silicone 2 and w/Si emulsions, with only petrolatum showing a significant decrease in this value. In fact, petrolatum creates a layer on the skin that minimizes water vapor transmission, preventing the skin from breathing. Moreover, silicones and w/Si emulsions do not reduce epidermal transmission and therefore allow the skin to breathe normally. As described above, low-viscosity dimethicones are synthesized fluids. Therefore, they do not cause any irritation or allergic reactions on the skin. Despite being volatile, they do not dry the skin. As we have seen before, they do not reduce transepidermal water loss; rather, they create a protective, breathable barrier on the skin.

For this reason, they are frequently used in products formulated for sensitive and atopic skin. Formulations

Dual-phase micellar waters (Formulation 1) are used as makeup cleansing lotions consisting of two semi-phases. Shaking is recommended before use to obtain a homogeneous product. A few minutes after shaking, the dual-phase micellar water gradually begins to separate and returns within one hour to its initial state consisting of two completely separate transparent phases. Dual-phase micellar water (formulated here with BRB DM 55) cleanses facial and eye makeup and leaves the skin with a clean and smooth feel. Invisible deodorant roll-on are deodorants specially formulated to protect the skin and prevent staining on clothing. They do not contain aluminum salts, leave a soft and smooth feel on the skin, dry quickly and spread easily. This formulation uses BRB 423 (oil-based emulsifier), BRB DM 55 (light softening effect), BRB Caprylyl Methicone (spreading and lubricity aid), BRB DM 0.65 (dry smooth feel aid), BRB SG 516 (silky and powdery feel) and BRB 1736 (silky feel aid and non-occlusive barrier formation) products. Today, we see that formulators increasingly prefer linear silicones over cyclic fluids. BRB DM 55 and BRB DM 66 dimethicones can be considered equivalent to D5 and D6 silicones in terms of viscosity, volatility and feel, and can be used as direct replacements in formulations. A feel comparison between deodorants containing BRB CM 50 (cyclopentasiloxane) and its linear alternative BRB DM 55 (dimethicone) is shown in Figure 3. Volatile dimethicones can be considered an ideal solution that meets the most demanding requirements of formulators and consumers. Despite continuous coverage in the popular press, volatile linear silicones are not occlusive and protect the epidermis with a breathable silky barrier. Because they are inert fluids, they do not cause allergies and reactions. Additionally, their high degree of volatility does not dry the skin and does not affect transepidermal water loss. As a result, they find widespread use in sensitive skin products. Low-viscosity dimethicones do not bioaccumulate because they are highly volatile. They evaporate in the air, degrading to silica, water and CO2. Their usage percentages in cosmetics and low C molecular ratio mean they are not considered VOCs (volatile organic compounds) and therefore are not harmful to the ozone layer. Volatile dimethicones have many benefits including excellent spreading, unique non-greasy lubricity, superior compatibility with other cosmetic products, tackiness reduction and silky, luxurious touch. Volatile dimethicones can be considered a good alternative for cyclic silicones and next-generation moisturizing agents. Author: Clara Gil Navarro Technical Sales Manager BRB International, The Netherlands           Translator: Özge Yüksel Özel Technical Business Development Manager Azelis TR