Skincare Suncare And Decorative Products Skin Feel Emolliency

The main reason that silicones are used in all types of skin care product is because of their sensory properties. Studies on the emollient properties of various materials have

shown that silicones deliver greater emolliency values than many commonly used cosmetic ingredients both, during and after application. They are described as smooth, velvety and non-greasy or oily and are able to impart this feel to cosmetic and toiletry formulations, improving the negative feel associated with other ingredients [5].

Cyclomethicones are used for transient effects giving slight lubricity, a light texture, fast spreading and good distribution of the product on application, whilst leaving no residual effects. They are often included in formulations to remove the greasy or oily feel of hydrocarbon-based emollients and are the basis for ''oil-free'' type claims [6]. They are used in light products for daily use such as facial cleansers, day creams or liquid foundations. Higher molecular weight silicones such as Dimethicone (and) Dimethiconol are used to give a more lubricious, longer lasting effect in richer, more nourishing skin treatment products such as night creams or after-sun products [7]. Silicone elastomers are used to give a dry, powdery feel to skincare formulations [8]. Silicones are also non-comedogenic/non-acnegenic unlike many occlusive, lipophilic fatty emollients which can promote comedone/acne formation on the skin [9].

Substantivity (Long-Lasting/Durability)

High molecular weight Dimethicones or Cyclomethicone (and) Dimethiconols form water-resistant films on the skin which can help prolong the effects of skin care, sun care or decorative products. This substantivity can be improved further by using Alkyl Dimethi-cones such as Cetyl Dimethicone or C30-45 Alkyl Methicone [7] (see Figure 2). The use of the substantivity of silicones to improve the substantivity of other ingredients in cosmetic and toiletry formulations has been demonstrated in sun care products. The addition of 2.5 wt% Cetyl Dimethicone to an oil-in-water sunscreen formulation shows excellent in vivo resistance to wash-off. The formulation has an in vivo SPF of 21.1 before immersion which reduces to 19.2 only, after immersion for 80 minutes [7] [10].

Cyclomethicones are the basis for long-lasting/non-transfer decorative products, especially lipsticks. They are used to disperse waxes and pigments, improve application and impart a pleasant skin feel, often replacing non-volatile hydrocarbon oils. When they evaporate, a uniform film of waxes and pigments remains which is resistant to transfer and wear [11].

Permeability/Controlled Moisturization/Protection Against Dehydration

Due to the flexibility of the Si—O—Si backbone, the majority of silicones are permeable to water vapour, producing ''breathable'' films. This is an important parameter for cleansing products or colour cosmetics to avoid clogging pores. The presence of an alkyl group in the chain, however, reduces this permeability, resulting in silicones which can give controlled moisturization, e.g., Stearyl Dimethicone or moisturization (occlusivity) similar to petrolatum e.g. C30-45 Alkyl Methicone [7] [12].

Enhanced Efficacy

Apart from improving the feel and long-lasting benefits of skincare products, silicones can also enhance the efficacy of other ingredients in the formulation. Studies carried out on suncare products have shown that the Alkylmethicones can enhance the in vitro SPF of products containing either organic or inorganic sunscreens. For inorganic sunscreens,

(b) % Left on skin after 3 washes with soap and water

Figure 2 Substantivity of different silicones, FTIR method.

10 IS 20 25 30 35 40

(b) % Left on skin after 3 washes with soap and water

Figure 2 Substantivity of different silicones, FTIR method.

a 100% increase in in vitro SPF was seen with an oil-in-water system containing 2 wt% Cetyl Dimethicone and a 75% increase in the in vitro SPF for a water-in-oil system containing C30-45 Alkyl Methicone [10] [12].

Protection

Dimethicone is listed in the FDA Monograph for Skin Protectant Drug Products for OTC Human Use in the United States [12]. Due to their hydrophobicity, silicones are used in protective hand creams to provide a water-resistant barrier against water-borne contami

nants. Recent studies indicate that Cyclomethicone and Dimethicone may also prevent irritation caused by sunscreen agents [13].

Cleansing

The excellent spreading characteristics, dry non-greasy/oily feel, and good solvency of Cyclomethicones make them ideal for use in skin cleansers to help lift and remove dirt without stinging. They can be used alone or in combination with ingredients such as mineral oil. Silicone emulsifiers allow Cyclomethicone to be present in the continuous phase as well as allowing the incorporation of polar ingredients such as water, glycerine etc. This makes the formulation of rinsible foaming facial washes possible [14].

Water-soluble and water-dispersible Dimethicone Copolyols have shown benefits in foaming facial washes. They provide a creamy, more dense foam as well as improving the foam volume. In liquid body cleansing products such as foam baths, shower gels and liquid soaps, they can improve foaming and foam stabilization. They have also been recognized as additives that reduce eye and skin irritation from anionic surfactants [14,15].

Rheology Modification/Structural Integrity (Sticks)

As well as improving the aesthetics of formulations, silicones can also act as rheology modifiers. This is particularly applicable to water-in-oil or water-in silicone-type systems. One such silicone rheology modifier is the C30-45 Alkyl Methicone where 149% and 93% increases in emulsion viscosity have been observed for water-in-silicone and water-in-oil emulsions respectively with 2 wt% of the wax [7]. Rheology modification using 24 wt% Stearyl Dimethicone is believed to be part of the reason for the success of this product in enhancing the SPF of sun care products containing organic sunscreens [10]. These waxes are also used to maintain the structural integrity of stick or soft solid products, improving their feel and application. Silicone elastomers can also be used to modify the rheology of skin care and antiperspirant formulations. Such elastomers have the capacity to absorb large amounts of solvents such as Cyclomethicone or low-viscosity Dimethicone without exhibiting any syneresis. It is this property which allows them to successfully thicken formulations. The ability of elastomers to significantly modify the rheology of a formulation combined with their unique powdery feel has led to their use in antiperspirant products.

Formulating Flexibility

Silicones can be used in all types of skin care products ranging from simple oil-in-water gels or emulsions to water-in-silicone and water-in-oil emulsions, from crystal clear to white in colour. Silicone emulsifiers increase this flexibility further. They allow silicones to be present in the continuous phase as well as allowing the incorporation of polar ingredients such as water, glycerine etc. Matching the refractive index of the water phase with the oil phase in such emulsions makes the formulation of clear gels possible and adjusting the phase ratio determines the product form from lotions to gels. This technology is the basis for the clear antiperspirant gels seen on the market today. It is also possible to make non-aqueous emulsions using silicones to deliver hydrophilic ingredients or those that are sensitive to hydrolysis.

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