Silicones in Shower Gels: How to Achieve After-Feel from a Rinse-Off Product

Silicones in Shower Gels: How to Achieve After-Feel

from a Rinse-Off Product

Isabelle Van Reeth, Virginie Caprasse, Dow Corning SA Belgium

Introduction

Silicones have been used in personal care applications for more than 40 years and have penetrated almost all product types from hair conditioners to lipsticks. These materials also have a long history of use in hair care rinse-off applications. A major breakthrough was their introduction in 2-in-1 shampoos, where they provide a consumer-perceivable difference, especially on dry hair. Silicones are also well known for their aesthetic properties in leave-on skin care products, and their use across cleansing products has previously been described.1 However, to date, a real consumer-perceivable breakthrough in sensory benefits from a rinse-off skin care product has not occurred. This is due to either a lack of deposition of the silicone on the skin or a difficulty of incorporation into the formulation.

Although shower gels are a growing category, the use of silicone in this application has been much less extensive than in shampoos. This is likely due to two factors: existing silicones could not bring perceivable benefits, and the positioning of shower gels has not focused on after-feel. However, this trend is changing. The purpose of this study was to develop a silicone product that could deliver after-feel from shower gels, and to identify the necessary formulation parameters for this silicone in order to optimize its performance.

Materials

A new emulsion polymerization process patented by Dow Corning2 is based on an in situ polymerization. The

polymerization takes place inside the emulsion droplet and leads to the synthesis of an ultra high molecular weight linear polymer in emulsion. The material has the following structure:

Vi (Me)2Si - [OSi(Me)2]n - [CH2 - CH2 - Si(Me)2 -O- ]mSi(Me)2Vi

Due to the flexibility of this technology, a wide range of emulsions can be prepared: - Anionic, cationic and nonionic

- From small (< 0.15 micron) to large particle size (> 40 microns)

- Internal polymer dynamic viscosity as high as 300 million cPs at 0.01 Hz and 25ºC, while maintaining a linear structure. The INCI name of the silicone polymer present in this emulsion is divinyldimethicone/dimethicone copolymer.

Methods

1) Sensory methods

This method uses multiple parameters, with paired comparison on the following: - During washing: foam generation, foam quality, foam quantity, cushiony feel. - After rinsing: wet feel

- After padding: tackiness

- After complete drying: smoothness, film presence, suppleness Testing procedure:

Each panelist tests two products, one after one another randomly, and compares the first one against the second one, usually the shower gel containing the silicone against the control (no silicone).

- Wet the hands for 5 sec with tap water, 37 ºC

- Apply 3 ml of the product with a syringe into the hands of the panelist - Wash the hands for 20 sec and evaluate foam generation, quality and quantity - Rinse for 20 sec under tap water 37 ºC

- Evaluate the wet feel

- Remove excess water with a paper towel - Evaluate the tackiness during drying

- Wait for complete drying (1 to 2 min) and evaluate smoothness, film presence and tightness/dryness. Testing formulation:

Sodium laureth sulfate 10 %

Decyl polyglucoside 2.5 %

Cocamidopropylbetaine 4 %

Laureth-4 2 %

Stabilising agent 2 %

Silicone active (in emulsion) 5 %

Water up to 100 %

pH 6-7

Preservative q.s.

A triangular sensory test was used to determine the sensory threshold of the emulsion technology described in the material section. Three sites are drawn on the panelist’s forearm. Two sites are treated with the same material and the third one with another material in a random way. The panelist is asked to find the different one. Depending on the number of positive responses, the two products are found to be significantly different or not.

2) Silicone deposition measurement method

This approach uses a Fourier transform infrared spectrophotometer equipped with an attenuated total reflectance skin analyzer (crystal ZnSe).3

A shower gel is applied following a specific procedure on the forearm of a panelist, then measured using FTIR. The quantity of silicone deposited on the skin is proportional to the following ratio:

Surface under the Si-CH3 peak at 1260 nm-1

Surface under the amide II peak at 1540 nm-1

The amide II peak is used as an internal standard to eliminate variation due to the pressure of the panelist’s arm on the crystal. For quantitative measurement, calibration needs to be made using the product that will be incorporated into the shower gel.

3) Protection method4

This method helps evaluate the potential of silicone to form a protective film on the skin; more specifically, against waterborne irritant using a equipment called a wash-off simulator. This device allows skin treated with different materials to be washed in a reproducible way with a ink solution. This step is followed by evaluation of the skin color: the lighter the color, the less ink uptake by the skin, and the better the film barrier property of the material applied onto the skin.

Results and Discussion

The evaluations in this study helped determine the emulsion parameters having the highest impact on the sensory characteristics of the shower gels.

Figure 1 illustrates the effect of the internal silicone polymer viscosity on sensory properties. Results are expressed as the amplitude of the difference versus the control (shower gel without silicones polymer).

Figure 1. Impact of internal polymer viscosity.

The best performance was obtained with an emulsion with a polymer having a dynamic viscosity of 170 million cPs. The impact is on both the foam quality and the dry skin after-feel. The particle size was the same for all the emulsions, approximately 0.4 micron.

The same evaluation has been made for the impact of the particle size, and the type of surfactant has been evaluated in the same way. Particle size was found to have an important impact on sensory performance, the smaller particle size (0.4 micron) being the best. The impact of the type of surfactant was minimal.

To understand the reason behind this performance, in a first step, semiquantitative deposition measurements were conducted using the FTIR method.

Figure 2 shows the impact on deposition of particle size, explaining why the best sensory performances were obtained with small particle size.

A calibration curve was established with different concentrations of emulsions with the following parameters: - Dynamic viscosity : 170 million cPs

- Particle Size : 0.4 microns

The deposition level of silicone onto the skin from a shower gel containing 5 % silicone active was approximately 0.0023 mg/cm2.

Figure 2. Silicone deposition on the skin by FTIR (semi-quantitative).

Figure 3 illustrates the increase in deposition after several uses of a shower gel containing 3 % active level of silicone emulsion.

Foam quality Foam quantity

Cushiony feel W et feel Tackiness Smoothness Film presence

Suppleness 5 MM cps 50 MM cps 170 MM cps

Figure 3. Silicone deposition versus washes.

This graph demonstrates that the quantity of silicone deposited on the skin increases with the frequency of usage of the shower gel, and that the sensory performance may be perceived by the consumer only after several washes with the shower gel.

The next evaluation determined the impact of the internal polymer viscosity on the sensory threshold. The results are as follows:

170 million cPs nonionic 0.0040 mg/cm2

4 million cPs 0.045 mg/cm2

1 million cPs 0.045 mg/cm2

These results demonstrate that with a very high viscosity internal phase, only 1/10the amount of silicone must be deposited on the skin in order to be perceived.

The combination of a lower sensory threshold for high internal polymer viscosity and improved deposition on skin due to small particle size support the sensory performance of this new emulsion technology in shower gels. It can also be assumed that because silicone is deposited on the skin, a film is made on the skin with the potential to protect the skin. Further, with regular use, this film is continually renewed, even as it is gradually removed by the washing process. As this occurs, the skin is allowed to rebuild its own natural barrier.

This effect has been confirmed by a study using an ink solution delivered to the skin treated with the shower gel with the help of a wash-off simulator. The skin washed with the shower gel containing the high viscosity polymer emulsions had a lighter color than both the nontreated skin and the skin washed with a shower gel control, showing that silicone deposits on the skin, forming a film barrier.

Conclusion

A new emulsion technology developed and patented by Dow Corning has been evaluated for improving the sensory profile of shower gel products. The positive impacts are on both the foam quality and the dry skin after-feel. It has been found that the best emulsion has a low particle size and a very high internal linear polymer viscosity, a combination that can only be produced using this new technology.

The mechanism for this performance has been identified: an improved deposition with a low particle size combined with a lower sensory threshold due to a high internal polymer viscosity. The deposition level has been quantified to be approximately 0.0023 mg/cm2. In addition, it has been demonstrated that sensory performance is expected to be increasingly perceivable with use frequency of the shower gels, and also that the silicone deposited from the shower gel could potentially also protect the skin during the time necessary for rebuilding its natural barrier.

0 0.05 0.1 0.15 0.2 0.25 0.3

Temp 0 Wash 1 Wash 2 Wash 3 Wash 4 Wash 5

S I/ A m id e II p eak r a ti o DC2220 Control

References

1. Janet.M.Blakely, The Benefits of Silicones in Facial and Body Cleansing Products, Dow Corning Europe Article, Ref: 22-1549-01 (1994)

2. US 6,013,682 issued to Dow Corning. European patent application pending: EP0874017

3. Helen M.Klimish, Grish Chandra, Use of Fourier transform infrared spectroscopy with attenuated total reflectance of in vivo quantitation of polydimethylsiloxanes on human skin, J Soc Cosmet Chem 37 73-87 (1986)

4. Isabelle Van Reeth, Sabrina Marchioretto, Fatima Dahman, Anne Dupont, Anne Desmedt, Silicones: enhanced protection across personal care applications, Dow Corning Corporation Article, Ref: 22-1712-01 (1998)