The addition of structure to organic oils results in some unique aesthetics in cosmetic formulations. The organic oils that can be structures include triglycerides and esters. The structuring agent can simply increase viscosity and improve flow or provide rigid thixotropic gels. The most interesting cosmetic aesthetics occurs when structuring agents (also called gelling agents) are added to low viscosity esters and triglycerides that are dry on the skin.
By using such esters and structuring them into a gel a soft butter like material results that thins under pressure and provide a dry feel in formulation. One interesting application of this technology is in providing serums with a buttery feeling or structuring oils used in emulsions. A series of patented gels were made and analysed using C8-C10 triglyceride and C26 alkyl dimethicone.1
One of the most important attributes of a cosmetic product is aesthetics. Aesthetics drive consumer perception of the product. The ability to alter skin aesthetics by modifying the feel of an oil on the skin, during application and after rub-in is a crucial property in a product’s performance. These properties are driven by rheology. A product that goes on the skin as a luxurious gel and spreads well into a dry feeling uniform film is highly desired.
In order to understand the performance of products that contain rheological modifiers, one must first understand the terms used to describe oils. The terms oils, fats, butters and waxes have been misused over the years. Butters, oils and fats all can be triglycerides, the difference between them is the temperature at which they become solid, called the titer point. Fats have a titer point of over 40.5˚C, oils have a titer point of below 40.5˚C. Butters have a titer below 40.5˚C but above 20˚C. Consequently, oils are liquid at room temperature, butters are soft solids and fats are harder solids than butters.
Because oils, fats, butters and waxes are complex mixtures of homologues of similar chemical structures, it is difficult to obtain a true melting point. The term titer is therefore used to define the resolidification point of the melted oil, fat, butter or wax. The procedure is to heat the product to be tested until it is completely liquid, then to cool it slowly by stirring. This is done until the temperature stays constant for 30 seconds, or begins to rise. The titer point is the highest temperature indicated by this rise. As the lower molecular weight fractions melt, they act as solvents to dissolve the higher molecular weight products. This results in a very wide melting ‘range’ for these compounds. For this reason, titer point is generally determined on fats, oils, waxes and butters. Complex mixtures have a tendency to be better butters in how they perform on the skin. In fact one class of butters prepared by partial hydrogenation of unsaturated oils, results in complex mixtures of triglycerides that are saturated and unsaturated and contain both cis and trans triglycerides.3 Simple triglycerides like glyceryl tristearate are hard waxes and do not exhibit the rheological properties of butters. Additionally, we have observed the properties of butters prepared by partial hydrogenation cannot be achieved by blending glyceryl stearate and glyceryl oleate to the same iodine value. Put another way, the oleate (unsaturated akyl) and the stearate (saturated alkyl) cannot be on different glycerin molecules, but the glycerin must contain both solid and liquid to be a butter. Titer point is described later in this article.
A hydrogenation-free approach that results in butter-like rheology is the addition of specific structuring agents. Selection of the proper structuring agents can obtain a buttery gel that thins under pressure to spread on the skin and deliver the benefit of a dry ester. Structuring agents will also add structure to oil phases that are emulsified, improving the emulsion stability and feel. Emulsions thickened with the proper structuring agents will be more mobile, and fluid, rather than stiff. Some of these gels will thin when force is applied (thixotropic) and return to viscosity when the force is removed. These gels make it possible to create thick products that can be shaken, or stirred under high shear, for easier bottling, or for spraying.3 There are a number of gelling agents that are effective in the water phase, but oil phase gelling agents are less common.
The technology described in this article is the topic of a US Patent.1 The patent relates to providing a structured gel composition comprising specific solid alkyl dimethicone polymers combined with specific liquid esters including triglycerides glyceryl esters, trimethylolpropane esters, and pentaerythritol esters.
Alkyl dimethicone structure
Figure 1 shows the structure of alkyl dimethicone compounds that are claimed in US Patent 7,875,263, used to gel esters.
Silicone to alkyl ratio (‘a’ to ‘b’ ratio)
These alkyl dimethicone polymers not only have the ability to form solid and liquid products, they also have the ability to change the physical properties of the solid. By tuning, or changing, the silicone to alkyl ratio (changing the ‘a’ to ‘b’ ratio in the structure), the physical properties of the solid can range from very brittle opaque solid to translucent wax. Increasing the percentage of silicone will result in a softer wax with minimal effect upon melting point. This ability to control the tensile strength applies not only to the polymer in bulk but also when it is added into a products formulation. The addition of a silicone with the correct silicone to alkyl ratio provides a formulation chemist the ability to ‘stiffen’ or ‘soften’ a solid.
Alkyl effects (number of carbon atoms in ‘R’)
Along with the ability to lower surface tension and increase solubility of the silicone in oil, alkyl dimethicone polymers can also change the physical properties of materials they are added to. By changing the alkyl group attached to the silicone backbone, a wide variety of products can be obtained. Attaching alkyl groups containing 16 or fewer carbons atoms in their alkyl chain will result in a liquid product at ambient temperature. Alkyl groups with 18 or more carbon atoms in their alkyl chain are solid at ambient temperatures. Adding alkyl dimethicone, with a long carbon chain in the pendant group (≥18 carbons), to a liquid product will cause the product’s melt point to increase and under high enough concentrations the product will gel. The opposite is true for a product that has a high melt point. Adding alkyl dimethicone, with a small carbon chain in the pendant group (≤18 carbons), can drastically lower the melt point of the product and add liquidity.
Silicone content effects (% silicone by weight)
Increasing the amount of silicone in the alkyl dimethicone results in a polymer that is less soluble in the organic phase. The effect is to make a more opaque blend when added to the organic phase. Additionally, the higher the concentration of polymer in the organic phase, the more playtime and cushion experienced with the rub out of the blend.
Analytical techniques
Critical gel concentration
The methodology used to determine the critical gel concentration (CGC), to prepare the solution of the alkyl dimethicone in the specified solvent at the specified concentration. Allow to cool to ambient temperature in plastic jars shown in Figure 2. Turn jars over and observe flow.
In the case of the analysis in Figure 2, the critical gel concentration (CGC) of the alkyl dimethicone in 50 cst dimethicone is between 10% and 20%. The 20% did not flow after 24 hours, while the 10% dropped in several minutes. The next step would be to make up samples between 10% and 20% and re-evaluate.
Melting range
The blends that we are making have a melting range, not a melting point. This is because the structured gels are rather complex mixtures. The more narrow the range, the more uniform the structured gel becomes.
The melt points reported are using Fisher Johns Melting Apparatus as shown in Figure 3.
Blends with C8-C10 triglyceride
Alkyl dimethicone A, B and C were blended into C8-C10 triglyceride at levels of 5% by weight, 10% by weight and 20% by weight for analysis, heated until clear (70˚C), allowed to cool overnight and were evaluated (Fig. 6).
Melting range analysis
There was minimal change in melting point by changing the alkyl dimethicone used to make the blend, despite the fact there was a significant difference in the melting point of the specific alkyl dimethicone compounds evaluated. It appears the interaction of the alkyl group and the specific ester chosen were the salient factors in determining melting range of the mixture, rather than the melting point of the alkyl dimethicone.
Appearance
5% Additive – The use of 5% alkyl dimethicone, independently of if it was A, B or C resulted in a solid which passed the critical gel concentration, but which cracked then stressed. The crack lines are evident in Figure 7.
As the concentration of the alkyl dimethicone is increased the surface cracking disappears (Fig. 8).
10% Additive – The use of 10% alkyl dimethicone resulted in elimination of the stress cracking observed in the 5% materials.
20% Additive – The 20% alkyl dimethicone added materials likewise showed no stress cracking observed in the 5% materials.
The 20% additive blends were all harder than the 10% and took greater amount of pressure to liquefy. The A series was the most waxy in feel. The B series was judged to be the best in terms of feel and lack of greasy feeling. Finally, the C series was found to be most greasy in feel. It is also quite interesting that as the concentration of alkyl group increases in the alkyl dimethicone, the clarity of the blend improves. This is because the alkyl dimethicone is more soluble in the ester.
Microscopic evaluation
The resulting blends were evaluated at 100x and 400x with an optical microscope (Fig. 9).
Conclusion
The inclusion of alkyl dimethicone compounds into C8-C10 triglycerides at concentrations of 10%-20% results in a series of products that are gels that liquefy under pressure to provide buttery gels.
The higher alkyl content in the silicone polymer, the more waxy the feeling. The higher molecular weight the product, the more greasy the product.
The effect that is achievable is slightly different for each alkyl group in the alkyl dimethicone and each ester evaluated.
The optical microscopy work shows clearly better uniformity in cases where the concentration of alkyl dimethicone is 10% and above. The uniformity increases with the concentration and explains the difference in hardness.
References
1 US Patent 7,875,263. O’Lenick K. Polymeric structured gels. 25 January 2011 [assigned to Siltech LLC].
2 O’Lenick AJ, Steinberg DC, Klein K, Lavay Carter Oils of Nature. Illinois: Allured Publishing, 2008.
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