Most silicones that are used in personal care formulations are based on polydimethylsiloxane (PDMS). The INCI name for methyl-terminated PDMS is the aforementioned Dimethicone; when the terminal groups on PDMS are silanols (Si-OH groups), the INCI name is Dimethiconol. The structures for these basic silicone polymers are shown in Figures 1 and 2. Dimethicone is the oldest and most widely used silicone in personal care formulations. It provides many important benefits in skin care, hair care, and color cosmetic formulations. Dimethicones are good emollients that provide a unique combination of physical properties that set them apart from other cosmetic oils. Their low surface energy and highly flexible siloxane polymer backbone allows for effective spreading and a pleasant skin feel.
The physical and esthetic properties of Dimethicones can be controlled by varying the chain length and molecular weight of the polymer. As chain length increases, the viscosity of the Dimethicone also increases. Low viscosity Dimethicones spread quickly and easily while providing a light, silky skin feel. High viscosity Dimethicones form more persistent hydrophobic films with good water barrier properties.
Dimethicone fluids with viscosities below about 5 cSt are somewhat volatile, but Dimethicones with lower viscosities such as 1 cSt are volatile enough to be noticeable to consumers. A related material called Disiloxane, which is essentially a methyl siloxane dimer, has volatility comparable to ethanol. Trisiloxane, another
Formulating Skin Care Products with Silicones 61 volatile low molecular weight silicone, is the shortest possible PDMS with only one dimethyl siloxane unit.
Dimethiconols are more polar than Dimethicones because of their silanol end groups and have somewhat limited reactivity. Silanol groups are weak acids and can condense with each other to form longer chains over time.5 This affects the shelf life of Dimethiconol itself, but has little practical significance for the formulation chemist. Because of their polarity, short chain (low viscosity) Dimethiconols can impact the solubility of other ingredients with which they are mixed, but as the chain length increases, the effect of the silanol group’s polarity rapidly diminishes.
Many of the Dimethiconols used in personal care formulations are very long-chain PDMS (silicone gums) and the effect of the silanol end groups is minimal. The term silicone gum is used for very long-chain PDMS because physically they resemble a soft putty or dough-like consistency. Despite their appearance, silicone gums are liquids and will flow, albeit very slowly. High molecular weight Dimethiconols (and Dimethicones) are often sold as blends with low viscosity silicones. These blends (solutions) are relatively low viscosity liquids that are easier to handle when compared to the silicone gums themselves and easier to emulsify. If silicone gums are blended with volatile silicone or hydrocarbon solvents, unique skin interaction effects may be achieved in formulations, because the volatile solvent serves as a carrier or delivery system for the silicone gum. Normally the silicone gum would be too viscous to easily apply as a film on surfaces. Creating a low-viscosity solution of silicone gum in a volatile carrier allows for relatively easy application to skin and leaves a film of silicone gum when the volatile carrier evaporates.
Figure 1. Dimethicone
Figure 2. Dimethiconol
As mentioned earlier, very low molecular weight Dimethicones are volatile, but their use is limited by higher cost compared to long-chain Dimethicones. Another class of volatile silicones that was introduced to the market in the late 1970s has become the dominant technology for providing transient silicone benefits in formulations.
These are low molecular cyclic PDMS oligomers that have been assigned the generic INCI name Cyclomethicone. The INCI name Cyclomethicone covers a family of compounds that contain four, five, or six dimethyl siloxane units. These cyclic siloxanes are often referred to by an industry shorthand notation as D4, D5, and D6 according to the number of dimethyl siloxane units. The INCI names for these compounds are Cyclotetrasiloxane, Cyclopentasiloxane, and Cyclohexasiloxane, respectively.
Cyclopentasiloxane (Figure 3) is the most widely used of the Cyclomethicones. They exhibit esthetic properties similar to low viscosity Dimethicones but are volatile and odorless. They also have a very low heat of vaporization, and so produce no cooling effect on the skin. The first formulation to incorporate large percentages (>70%) of Cyclomethicone was an antiperspirant called Dry Idea that was introduced by Gillette circa 1980. This was an anhydrous formulation (i.e. containing no water) in which the aluminum salts (the active antiperspirant ingredient) were suspended in Cyclomethicone (in those days, D4).6 The name of the product reflected the use of the novel vehicle that provided a pleasant “dry” feel on the skin during application and then evaporated to leave the active ingredient on the skin.
Anhydrous antiperspirant formulations based on Cyclomethicone still occupy a significant market share despite the cost associated with the silicone vehicle. A survey of the leading antiperspirants in the United States by the authors revealed
Figure 3. Cyclopentasiloxane
Formulating Skin Care Products with Silicones 63 that all of them contain Cyclomethicone. Another popular use of cyclic dimethyl siloxanes is in color cosmetics. Here they serve as spreading agents for pigments.
Pigments are usually derived from inorganic compounds such as iron oxide and titanium dioxide that are used in the form of small particles. These pigment particles need to be spread evenly on the skin surface, and various cosmetic oils are blended with the pigments to facilitate spreading. Cyclomethicone is ideal for this purpose because it provides good spreading and then evaporates leaving a pigment film that resists smearing.4 When combined with a silicone resin, the pigment/cyclomethicone combination is effective for producing wear-resistant cosmetics. When the surface of the pigment particle is treated to impart a more lipophilic character, pigment spreading is facilitated in a silicone-based formulation because the pigment particles are more easily wetted. In fact, a case could be made that the evolution of surface treatments for pigment was spurred by the use of Cyclomethicone and other silicones as carriers in color cosmetic formulations.
From applications in antiperspirants and color cosmetics, the use of Cyclomethicone has grown steadily due to its versatility alone and in combination with other silicones. In hair conditioning formulations, the use of Cyclomethicone in combination with organic hair conditioning agents such as long-chain quaternary ammonium salts (e.g. Cetrimonium Chloride) was shown to provide very effective hair conditioning.16 Cyclomethicone works with other hair conditioning agents to provide lubrication for improved wet combing, but evaporates as the hair is dried and leaves a minimal feel of residue. Using silicone emulsifiers, novel inverse emulsions based on Cyclomethicone allow the esthetic properties of the silicone to be maximized while improving the delivery of active ingredients. This approach has been used by formulation chemists to prepare clear antiperspirant emulsion gels where an aqueous solution of antiperspirant salts is emulsified into a continuous phase of Cyclomethicone. Similarly, inorganic pigments are delivered from an invert emulsion foundation that has an elegant feel because of silicone in the external phase.
The introduction of hydrophobic surface-treated pigments into the color cosmetic market helped drive the trend towards silicone-based invert emulsion formulations because these pigments are ideal for use in this type of formulation. (Invert emulsions are discussed later in this chapter in the section on Formulation Considerations.)
Any discussion of the Cyclomethicones would be incomplete without some mention of the potential toxicity and environmental concerns related to the two most commonly used Cyclomethicones: Cyclotetrasiloxane (D4) and Cyclopentasiloxane (D5). The rapid growth in use and expanding applications of these compounds since the late 1970s prompted the silicone industry to conduct extensive studies on the toxicity and environmental fate of these silicones. The Cosmetic Ingredient Review (CIR), an organization sponsored by PCPC, first published its assessment of the safety of Cyclomethicone in 1991. More recently, the CIR has published an article that provides a comprehensive review of safety studies conducted on the individual members of the Cyclomethicone family.7 This publication also includes
a review of the usage of the Cyclomethicones in various formulation categories.
Because of their volatility, inhalation toxicity was tested using several animal species. Data from D4 inhalation studies using rats showed that there was a potential for reproductive toxicity, although it was later concluded that these results are not relevant to humans. Nevertheless, this concern led the industry to shift from D4 to D5 in many applications beginning in the late 1990s. The overall conclusion of the most recent CIR panel was that all of the Cyclomethicones used today (D4, D5, D6, and D7) “are safe as cosmetic ingredients in the practices of use and concentration as described in this safety assessment.”7
Although concerns about potential human toxicity for D5 have been addressed by extensive testing with no evidence of a harmful effect, the high volume of its use and hence exposure led to concerns about its potential environmental effects. The environmental fate of these cyclic siloxanes in the atmosphere was established many years ago. Studies showed that they rapidly break down when exposed to oxygen and UV radiation from sunlight, producing silica and carbon dioxide.8 A more recent field study on D5 confirmed that it is effectively removed from the atmosphere.9 The environmental fate of D4 and particularly D5 after they have been released into oceans and lakes (via sewage discharges) has been difficult to establish. Since they are not biodegradable and hydrophobic (potentially fat-soluble) the concern is for a potential bioaccumulation in the food chain. Because D5 is not broken down biologically, it may persist in the bodies of organisms and thereby accumulate in growing concentrations as these organisms are eaten by animals higher in the food chain. One model that is widely used to predict lipophilicity and can be used in the estimation of bioaccumulation potential is the partition coefficient between water and octanol. High solubility in octanol correlates with high bioaccumulation potential for many compounds. Data from this model suggested that D5 has a high potential to accumulate in animals imparting unknown and potentially toxic effects. To answer questions about bioaccumulation, the silicone industry sponsored environmental sampling studies in multiple aquatic systems around the world. Results from some of the studies have been published,10,11 but others have not and the results so far are inconclusive. Regulatory bodies in the European Union, United Kingdom, and Canada have been considering rules that could restrict the usage of D5 for several years. The Canadian government was the first to issue a conclusion concerning D5 based on data presented by the silicone industry. The conclusion was that “D5 is not entering the environment in a quantity or under conditions that constitute a danger to the environment.”12
The silicones that have been discussed so far are referred to as methyl silicones because they have no other type of organic group attached to the silicon atoms. A large class of modified methyl silicones exists that contain other types of organic groups. These organic groups are introduced into the silicone to provide a specific functionality. These silicones are often referred to as “organo-modified” silicones.
The different organic groups are introduced for various reasons such as to change the
Formulating Skin Care Products with Silicones 65 solubility of the silicone or increase the affinity of the silicone for particular types of surfaces. Organo-modified dimethyl silicones developed for use in the textile industry were found to be effective hair conditioning agents and began to appear in hair conditioning products in the mid 1970s. One of the first of these silicones used for hair care was Amodimethicone, a dimethyl siloxane onto which amine groups are grafted (Figure 4). The polar amine groups which acquire a positive charge in water help to deposit the silicone onto both textiles and hair. The amine groups anchor the silicone to the surface of the hair fibers while the dimethyl siloxane backbone provides lubrication to facilitate combing and detangling. The INCI name Amodimethicone now refers to a diverse group of silicones that contain ethylenediamine groups that are very basic (i.e. ionize in water to generate hydroxide and ammonium ions and therefore generate a positive charge on the polymer over a wide pH range). Amodimethicone can also contain silanol groups that can produce further polymerization after the silicone is deposited, leading to a relatively durable coating. Silanols on Amodimethicone are more reactive because the ethylenediamine groups catalyze the silanol condensation reaction. Variants of Amodimethicone were developed over the years that contain other types of amine groups and quaternary nitrogen groups with the intention of improving hair conditioning performance.
(Table 1 provides examples of the most commonly used amine-modified silicones.)
Silicone surfactants that were originally developed for use in the manufacture of polyurethane foams have also found applications in personal care formulations.
These silicones, like the Amodimethicones, are dimethyl siloxane polymers that have been modified by grafting polar groups onto the siloxane. For this family of silicones that are commonly referred to as silicone polyethers, the polar groups are
Figure 4. Amodimethicone (where R is –OH or –CH3)
polyethylene glycol (PEG), polypropylene glycol (PPG), polybutylene glycol, or some combination of these. Since the dimethyl siloxane backbone is very hydrophobic and the polyethers are hydrophilic, silicone polyethers are essentially a type of nonionic surfactant. The properties of silicone polyethers are dependent on the type and ratio of silicone to glycol in the polymer. If the silicone polyether contains enough PEG, it will be water-soluble and exhibit a variety of properties one would expect from an organic nonionic surfactant. These properties include foaming, wetting, and emulsification of oils. As with other nonionic surfactants, silicone polyethers can be assigned an HLB value using the formula: wt. % polyether/5. Silicone polyethers with an HLB value of 10 or greater is water-dispersible.
The growth in popularity of Cyclomethicone-based formulations led to the development of high molecular weight silicone polyethers for stabilizing “invert”
emulsion preparations. The term invert emulsion refers to the characteristic that these emulsions consist of the water phase dispersed in the oil phase, which is the inverse of conventional emulsions where the oil phase is dispersed into the water phase. This emulsifier technology enabled the production of novel antiperspirant and color cosmetic formulations, as discussed earlier. These special-purpose emulsifiers generally have an HLB of less than 5 and are not water-dispersible. They must be mixed with the oil phase prior to the preparation of an emulsion. (Information about the preparation of water-in-oil emulsions can be found in this chapter’s section on Formulation Considerations.)
Silicone polyethers have been assigned a wide variety of INCI names because the INCI nomenclature system requires that the chain length of each type of polyether be included in the name. This nomenclature requirement was put in place circa March 2001. Prior to that time, silicone polyethers were assigned a generic INCI name, Dimethicone Copolyol. Despite the fact that the INCI name Dimethicone Copolyol was eliminated from the INCI dictionary more than ten years ago, some manufacturers still use it for ingredient labels. In addition to reflecting the lengths of the polyether chains in the INCI name, the nature of the polyether (block versus random) must also be reflected in the INCI name, therefore the INCI name contains information about how the silicone polyether is made. For example, the INCI name PEG-12 Dimethicone refers to a family of dimethyl siloxane polymers onto which polyethylene glycol chains that contain an average of 12 PEG units have been grafted (Figure 5).
Figure 5. PEG-12 Dimethicone
Formulating Skin Care Products with Silicones 67 The INCI name does not provide any information about the relative amounts of PEG versus dimethyl siloxane. PEG-12 Dimethicone may be water-soluble, but only if it contains a sufficient number of PEG-12 chains to overcome the insolubility of the dimethyl silicone backbone. The polyether chains can be grafted along the dimethyl siloxane polymer chain, or attached to the ends of the siloxane polymer.
An example of the latter case is Bis-PEG/PPG-14/14 Dimethicone. The bis in the name indicates that the polyethers are attached to each end of the dimethyl siloxane polymer and in this case the polyethers are random copolymers with about 14 PEG units and 14 PPG units. The water solubility of this silicone polyether will depend upon the ratio of the polyether to the dimethyl siloxane, which for this copolymer is determined by the length of the dimethyl siloxane chain. Examples of some of the most common silicone polyethers are given in Table 1.In addition to methyl groups, silicones are produced that contain other types of hydrocarbon groups.
Many of these have found their way into topical formulations.
Silicones with phenyl groups have been used for many years and the most widely used of these is Phenyl Trimethicone. This compound is not based on dimethyl siloxane and is usually a mixture of oligomeric (short chain) siloxanes that have both phenyl and trimethyl siloxy groups (Figure 6). Phenyl Trimethicone is soluble in a much wider variety of oils and waxes compared to Dimethicone and this makes it very easy to formulate with. In addition solubility modification of the silicone, phenyl groups raise the refractive index and this can produce higher gloss films.
Other phenyl-containing silicones such as Diphenyl Dimethicone have been introduced more recently.
The silicones discussed so far in this section are all liquids at room temperature.
There are several important classes of silicones used in topical formulations that are solids. These include alkyl-modified silicone waxes, silicone resins, and silicone
Figure 6. Phenyl Trimethicone
elastomers. The alkyl-modified silicone waxes are based on dimethyl siloxanes that have various amounts of methyl groups replaced with long-chain (C16 or higher) alkyl groups. The melting point of the silicone wax depends upon the degree of alkyl substitution and the chain length of the hydrocarbon. For example, two commercial waxes that conform to the INCI names Cetyl Dimethicone and Stearyl Dimethicone have roughly the same amount of alkyl substitution but different melting points.
The structure for Cetyl Dimethicone is shown in Figure 7. Cetyl Dimethicone melts at slightly below room temperature while Stearyl Dimethicone melts at about 32o C which is, interestingly, the temperature of the surface of the skin. In addition to raising the melting point, replacing methyl groups with long-chain hydrocarbons changes the solubility of the silicone and also makes the films more impermeable to small molecules like water. Some alkyl-modified silicones such as C30-45 Alkyl Methicone, which has half of the methyl groups replaced with a mixture of long- chain hydrocarbons, are nearly as occlusive as petrolatum. Figure 8 shows the relative occlusivity of several alkyl-modified silicones as measured in-vitro using the Payne Cup method. The in vitro data was shown to correlate with transepidermal water loss (TEWL) results13.
Figure 8. Water Permeability for Alkyl-Modified Silicones versus other selected materials
Figure 7. Cetyl Dimethicone