Silicone – smooth operators

Formulators and consumers are always looking for innovation, differentiation and performance. Isabelle Van Reeth, Kevin Fang, Erin Lacher & Ingrid Vervier investigate advanced silicone technologies for colour cosmetics

With the global colour cosmetics market reaching US$42.6bn in 2009,^[1] the challenge for innovation, demand for high performance and competition for differentiated personal care products continue to lead formulators into new territory and create a need for materials that push technology boundaries. The objective is to offer a combination of pleasing and even unique sensory effects with multifunctional performance.

Today’s colour cosmetics can be bold or understated, offering a realm of choices to suit mood, occasion and personal style. Performance is a must, with consumers looking for long lasting, comfortable wear, combined with colour intensity and a fresh look. Convenience and easy application are important, along with novel textures and sensory effects. A desire for UV protection and an eye to safety are growing concerns, with increasing focus on material testing.

Multifunctional needs demand multifunctional formulating materials. Many of the essential features of colour cosmetics can be traced to the use of speciality silicones, versatile ingredients that allow formulators to create high performance, innovative colour cosmetics with novel textures and sensory effects. In 2009, a global average of 72.6% of colour cosmetics introduced contained at least one silicone material. This article reviews some of the most important properties and applications of the new silicone materials for colour cosmetics.

Silicone resins

Materials such as trimethylsiloxysilicate (MQ type) and silsesquioxane (T type) silicone resins have been used in personal care applications for several years, especially for their non-transfer and wash-off resistance properties. To date, these two resin types were available as single species, delivered either as pure solid in powder form or from a solvent such as cyclopentasiloxane.

Silicone resin technology continues to evolve, offering formulators the means to give lipsticks, foundations, eyeliners and other colour cosmetics longer, more comfortable wear without transfer. With a new process for producing resins in flake form, a blend of trimethylsiloxysilicate and propyl silsesquioxane was developed (Dow Corning MQ-1640 Flake Resin). The ratio between the two resin types was optimised to deliver the best combination for performance: excellent wash-off and non-transfer properties, with improved film flexibility for more comfortable wear in foundations and lipsticks. The new material is easier to handle, with broad formulation versatility due to its solid form. In addition the presence of a propyl instead of a methyl group improves compatibility compared to most currently available silicone resins. Rub-off resistance of the film formed by the new blend is improved compared to that of the MQ resin, yet the film is still firm. Another new silicone resin forms firmer films for applications that require similar properties but a more rigid structure (Dow Corning MQ-1600 Solid Resin).

Flexibility of the film formed on skin by a cosmetic is an important factor in wearing comfort. The film-forming properties of the resins were evaluated in a foundation formulation (containing 4.25% solid resin), using a silicone rubber belt as a substrate.

Figure 2 Film-forming properties of resins in a prototype foundation, after washing

After stretching (figure 1), the foundation with the MQ resin showed cracks and was easy to peel from the silicone substrate. The films containing the MQ/T propyl resin blend and the T propyl resin were still homogeneous.

Wash-off resistance is an important property if a cosmetic is to be long lasting on the skin. After the stretching phase, the samples were washed by gentle sponging with a 0.5% solution of sodium lauryl ether sulphate, then rinsed for 15 seconds with water (figure 2). No change was apparent for the MQ/T or the T propyl resin. This series of experiments demonstrated that the MQ/T propyl resin had the best balance of performance with regard to low tackiness, film flexibility and wash-off resistance in the foundation.

To evaluate wash-off of mascara from the consumer perspective, a simple test was developed using the feather of a shuttlecock. Mascara was applied and subsequently washed with water as shown in figure 3.

Figure 3 Comparison of mascara performance with three resins

The MQ/propyl and T propyl resins showed the best performance, where almost none of the mascara was washed away.

Tests using the Payne cup method^[2] showed that the silicone resins form a permeable film on skin. The higher the level of T propyl resin, the less permeable the film. However, even the least permeable films were far from being occlusive, so they should not interfere with the skin’s natural breathing function.

Formulation 1 illustrates a long lasting foundation that applies evenly on the skin. The addition of a silicone elastomer gives a silky, powdery feel.

Silicone resins are recognised for their ability to reduce transfer from colour cosmetics such as lipsticks. A solid lipstick containing 10% MQ/T propyl resin was evaluated by paired comparison sensory testing against a lipstick with the same level of MQ resin. The lipstick containing the MQ/T resin was shinier and had better colour intensity than the one containing the MQ resin, probably due to the presence of the T propyl functionality, which aids compatibility with the pigment and has a higher refractive index compared to the MQ, but also more transfer.

The same lipstick without resin was softer and broke easily, making the evaluation impossible. This behaviour indicates that the silicone resins also add structure to the lipstick.

In another test of non-transfer, MQ and MQ/T propyl silicone resins were tested in an eyeliner formulation and compared to a commercial benchmark. The MQ/T propyl resin was the best performer overall, with improved non-transfer and retention of other performance qualities.

Formulation 2 illustrates the creamy eyeliner used in the evaluations. The eyeliner is easy to apply, with enhanced gloss and colour intensity. In addition to its non-transfer properties, it features resistance to rub-off and wash-off.

Formulation 1: Comfort stay foundation

Preparation: Premix pigments of B in a pigment grinder until uniform. Prepare B by dispersing pigments into the silicone, using high shear to ensure uniformity. Combine A ingredients and mix until the flake resin is completely dissolved. Add A to B, mixing until homogeneous. In a separate vessel, combine C ingredients in order. Add C very slowly to AB under strong agitation. When all the water phase is added, continue mixing for 15 mins at 1000 rpm. Add D ingredients in order listed with stirring. Homogenise until uniform.

Formulation 2: Long wearing, smooth gel eyeliner

Preparation: Combine A ingredients and heat to 75°C. Combine B ingredients and mix until completely dissolved. Add B to phase A and mix until homogeneous. Keep covered with aluminum foil when not mixing to avoid loss due to heating. Pour into suitable containers while hot.

Stable, novel emulsions

Emulsions are at the heart of a variety of colour cosmetics, providing a base for the delivery of pigments, moisturisers, sunscreens and other important components in a formulation. Among the most potentially sophisticated emulsions in terms of delivery and sensory effects, water-in-silicone emulsions are perceived to be complicated systems, probably because this formulation type is not so well known as oil-in-water systems. There is limited published literature on the fundamental understanding of the stabilisation of emulsions by silicone polyethers. Data from these studies demonstrate the uniqueness of silicone polyethers as water-in-oil emulsifiers.^[3,4]

These systems have seen their use steadily increase, especially in colour cosmetics but also in facial care, underarm applications and sun care. In all cases, the unique consumer benefits provided by an external silicone phase include superior spreading, long lasting wear, wash-off resistance and a non-occlusive film.

Specific practical approaches and knowledge are needed to formulate water-in-silicone systems. Factors such as process conditions and the addition of an electrolyte to improve stability are well known. However, some concepts may still be unfamiliar. The primary parameters identified to affect formulation viscosity and stability (eg water to silicone/oil ratio, mixing process, emulsifier level, nature of the external phase and the presence or lack of a coemulsifier) resulted in a number of recommendations, summarised in figure 4. Recent studies describe these parameters in detail, such as the potential impact of the carrier, emulsifier molecular weight or oil phase density on particle size.^[3]

Figure 4 Flowchart for evaluating the water-in-silicone emulsion process

The newest generation of silicone emulsifiers can be the basis for novel formulation concepts. Although considerable recent progress has been made, formulation of a stable liquid foundation with high pigment levels and without the need for high rheological additive levels is still a challenge for cosmetic chemists. PEG-10 dimethicone (Dow Corning ES-5612 Formulation Aid) combines very good pigment dispersion properties as well as the ability to make low viscosity stable emulsions. In formulation 3, silicone emulsifiers are used to create a stable, low viscosity foundation with a high pigment load.

Another new silicone emulsifier, PEG/PPG-19/19 dimethicone (and) C13-16 isoparaffin (and) C10-13 isoparaffin (Dow Corning BY 25-337), gives a creamier, gel-type foundation, but still with good spreading for optimum coverage and a light, smooth feel.

Formulation 3: Low viscosity fluid foundation

Preparation: Prepare a pigment premix by blending C ingredients and homogenise with a three-roll mill until colour is uniform. Combine ingredients of A and C, mixing until homogeneous. Combine ingredients of B and mix. Slowly add B to AC, while slowly increasing mixing speed (800 to 1100 rpm). When all of B is added, add D with mixing. Mix for an additional 5 mins at 2000 rpm.

Sensory benefits

Another way to expand formulating options is through the use of materials that offer greater compatibility with other cosmetic ingredients. Silicone organic elastomer blends (SOEBs) feature organic functionality cross-linked within the elastomer structure, as well as organic carrier fluids. The organic components greatly improve the ability of SOEBs to be combined with organic ingredients and actives.

Traditional silicone elastomers are recognised for distinctive textures as well as their sensory effects – a dry, smooth and powdery feel. However, incompatibility with organic ingredients ranging from solvents to sunscreens poses formulating challenges. In response, newly developed materials feature improved organic compatibility while maintaining characteristic silicone elastomer texture and aesthetics.

Two new SOEBs are delivered in volatile and non-volatile organic carrier fluids, respectively:

• Isododecane (and) dimethicone/bis-isobutyl PPG-20 crosspolymer (Dow Corning EL-8050 ID Silicone Organic Elastomer Blend)
• Isodecyl neopentanoate (and) dimethicone/bis-isobutyl PPG-20 crosspolymer (Dow Corning EL-8051 IN Silicone Organic Elastomer Blend )

This new technology makes it possible to offer formulation flexibility and form transparent systems with organics and lipophilic active ingredients including common sunscreens, vitamins, esters, triglycerides and alcohols. SOEBs enhance the aesthetics of both anhydrous and water-based formulations, and they can be used to provide thickening benefits in alcohol-based systems.

The differences based on volatility of the carriers can be translated to specific formulations, where not only are sensory properties important but also colour intensity, uniformity and non-transfer characteristics. For example, the two SOEBs were compared in a tinted make-up base.

Figure 5 Sensory profile comparison of two SOEBs in tinted foundation (Numbers in parentheses indicate levels of confidence)

Upon application and before absorption, the SOEB with the volatile carrier enhanced drying time, as might be expected. As figure 5 indicates, after absorption, the silicone organic elastomer blend with the volatile organic carrier demonstrated significantly better transfer resistance, while the silicone organic elastomer blend with the non-volatile carrier delivered superior smoothness and slipperiness and a powdery feel.

Formulation 4 is a combination lipgloss and cheek bronzer in which the silicone organic elastomer blend enhances transfer resistance. The formulation has an aerated, mousse-like texture and spreads easily on the skin, leaving a smooth afterfeel.

For comparison, the formulation was also prepared with a traditional silicone elastomer blend. Figure 6 illustrates how the SOEB with the volatile isododecane carrier improved the non-transfer properties of the formulation and enhanced its long lasting wear.

Figure 6 Comparison of SOEB and traditional silicone elastomer blend in prototype lip and cheek illuminator

In formulation 5 for an innovative concealer, the silicone organic elastomer blend gives a distinctive soft and silky feel, along with colour intensity and uniformity. The SOEB improves coverage and demonstrates excellent compatibility with the organic ingredients in the formulation.

When the SOEB and a traditional silicone elastomer blend, dimethicone (and) dimethicone crosspolymer were tested in this formulation, the SOEB showed statistically significant improvements in coverage, colour intensity, colour uniformity, slipperiness and smoothness (figure 7). For purposes of comparison, a silicone elastomer blend with a non-volatile carrier was chosen for evaluation with this SOEB, which also has a non-volatile carrier.

Figure 7 Comparison of a traditional silicone elastomer blend and SOEB in a prototype concealer

Fashion forward

Successful colour cosmetics not only meet fashion trends but also respond to changing lifestyles, growing health and environmental awareness and specific demographics, whether in an ageing population or emerging regional markets.

Next generation speciality silicones such as silicone resins, emulsifiers and silicone organic elastomer blends can respond to changes in the global colour cosmetic market, offering formulators flexibility for creating a range of innovative, high performance products.

Formulation 4: Cherry cheer lip and cheek illuminator

Preparation: Add A to main vessel and heat with mixing to 80°C. As a safety precaution, use a closed mixing vessel and nitrogen gas to make the head space of the mixer inert. In a separate vessel, combine and heat B to 80°C with gentle mixing. Slowly add B to A with mixing. Continue mixing until temperature drops to 45°C. Pour into appropriate container. Formulation developed by Azelis.

Formulation 5: Imperceptible concealer

Preparation: Combine A and mix until homogeneous. In a separate vessel combine B. Add A to B and heat to 85°C. Combine C and heat to 85°C. Add C to AB, mixing until uniform. Pour into containers while warm. Cool for viscosity increase. Formulation developed by Azelis.

Authors Isabelle Van Reeth & Kevin Fang, Dow Corning (China) Holding Co. Ltd
Erin Lacher, Dow Corning Corporation
Ingrid Vervier, Dow Corning Europe SA

Contact
Isabelle Van Reeth, Dow Corning (China)
e-mail i.van.reeth@dowcorning.com

References
1. Euromonitor (May 2010)
2. Van Reeth I & Wilson A, Understanding Factors which Influence Permeability of Silicones and Their Derivatives, Cosmet Toil 109, 7 (1994)
3. Durand B et al, New Formulation Capabilities Using Water-in-Silicone Emulsions, SCC Scientific Seminar, Chicago, IL, (June 2009)
4. Dimitrova T et al, Stabilization of Cosmetic Formulations by Silicone Polyethers, 25th IFSCC Congress, Barcelona (October 2008)