Silicon technology: at home in paints and cosmetics.

At first glance, paints and cosmetics seem to be two very different types of products that should contain vastly different ingredients. Upon closer inspection, however, it becomes apparent that cosmetics are designed to coat a surface, just as paints do. The difference between the two, of course, is the surfaces involved--skin, hair, and lips vs. metal, wood, and plastic. Not surprisingly, many of the technologies used for paints and coatings also find value in cosmetics formulations. Many of the surfactants, pigments, dispersants, and other materials used as additives for paints, although processed and delivered differently to meet safety and health standards, can also be found in cosmetic products.

Technology transfer across industry segments has made it possible for Dow Corning to expand the application of its silicon resins into many apparently different markets such as paints and cosmetics. "Our development groups focus on a type of technology, rather than a specific market segment," explains Lori Stark-Kasley, senior development specialist with Dow Corning's Resins and Fluids Development Group. She adds that when a market need is identified, each technology is considered. As a result, the scientists support a number of different application sectors within each group. "This arrangement facilitates the transfer of technology from one application to another."

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Silicon technology has been widely utilized by both market sectors for many years. In the past, cosmetic formulations have incorporated more linear siloxanes. There is now a growing interest in more highly oxygenated siloxanes. "The number of patents indicating the use of more branched silicon resins in personal care applications has blossomed in the past five years," says Stark-Kasley. "These resins are cited as having several beneficial properties as compared to their straight chain analogs. The top indications include formation of a more substantive film with improved feel, increased color durability, improved stability, comfort, and moisturizing ability."

Silicon, like carbon, can form four single bonds to substituents. Dow Corning classifies siloxane structures based on the number of Si-O bonds present (see Figure 1). The "M" (mono) structure has one Si-O bond and typically three Si-C bonds. Those with two Si-O bonds are labeled "D" (di) structures, those with three Si-O bonds are "T" (tri) structures, and siloxanes with four Si-O bonds are "Q" (quaternary) structures. Resins formed from M and D structures tend to be more linear, while those that contain T and Q structures are more branched.

Phenyl trimethicone is an MT structure and has been widely used in cosmetics for a long time. However, it is not typically thought of as a resin as it is highly end-capped, which results in a low molecular weight. Trimethylsiloxy-silicate is one of the first traditional resins based on an MQ structure to be heavily used in cosmetics, according to Stark-Kasley. Methyl silsesquioxanes and propyl silses-quioxanes, both of which are T structures, have been experiencing an increase in use as well.

All of these resins have been incorporated into paints and coatings for many years. MQ resins are found in water-repellent coatings. They increase the hydrophobicity of the surface on which the coating is applied. Methyl silsesquioxanes are also used in water repellent coatings, but the propyl derivatives can have a tackier feel. In fact, while MQ resins and methyl silsesquioxanes can often serve as the base for a coating, propyl silsesquioxanes are typically incorporated as additives.

The T resins are also used as pigment treatments in coatings manufactured for industrial and institutional (I & I) applications. They increase the hydrophobicity of the pigment surface so incorporation of a pigment into a dispersion will be improved.

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In cosmetics applications, trimethylsiloxysilicate is used in lipsticks and foundations for its transfer resistance properties. The addition of these resins helps prevent make-up from rubbing off when touched. Silsesquioxanes serve a similar purpose, with the MQ structure providing a grainier feel and the T structure a softer one.

Phenyl trimethicone is used in hair care and color cosmetics products to improve gloss. The phenyl group has a high refractive index, which imparts a higher gloss value. Aminopropyl phenyl trimethicone has been used to improve the adherence of the resin to human hair, thus providing a longer lasting, higher gloss product.

In paints and coatings, silicone polyethers provide wettability and leveling as well as mar and slip resistance. In roll-on deodorants, foundation, and suncare products they are excellent emulsifiers for water-in-oil emulsions. In styling products, they act as a plasticizer for the hair resins in the formulation.

Si-H materials, also known as reactive silicones, are another type of pigment treatment that is used for the same purpose in coatings and personal care products. "Inorganic pigments like iron oxide are naturally hydrophilic. Treatment with Si-H materials makes them hydrophobic so they disperse better into the oil phase of color cosmetics," explains Isabelle Van Reeth, senior industry specialist for Dow Corning Life Science Industry.

In both paints and cosmetics, silicon resins are used to improve the appearance and/or performance of products. The branched structure of silicon resins provides similar benefits to both formulation types. "The branched nature of the resins relates to both water repellency and transfer resistance," states Stark-Kasley. "Steric hindrance caused by the branching also keeps some silanol moieties from being end-capped. The reactive sites improve the adherence and transfer resistance of the resins as well. The presence of organic substituents also provides good water repellency, both for skin and for concrete."

Pigment dispersion is also critical for both paints and cosmetics. "Both types of products need to level well so they look good. Even color distribution is of critical importance. To achieve even color distribution, the formulation must have excellent pigment dispersion. Silicon resins and Si-H materials are ideal ingredients for treating pigments in order to achieve the necessary level of dispersion," notes Van Reeth.

It is also interesting to note that much of the same analytical instrumentation is used when evaluating paints and cosmetics. Typical measurements include gloss, color, rub resistance, and oil and water resistance. All of these values are frequently determined for both coatings and personal care formulations.

It is important to stress, though, that while the silicon technology may be the same, the use of these materials in paint and personal care products is achieved through very different processing methods. "The processing of the active ingredients and the make-up of the final formulations--how the ingredient is delivered--is vastly different for these two types of applications," says Stark-Kasley. "All of our ingredients produced for the personal care market are manufactured according to strict health and safety guidelines for these types of products. Of course, our resins for paints and coatings are manufactured in accordance with all applicable regulatory requirements as well."

They may follow different guidelines and delivery methods for ingredients, but both paints and cosmetics products seek the same result--to protect and beautify.

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