1 Tony O'Lenick PhD Siltech LLC, US SILICONES. Thomas O'Lenick SurfaTech corporation , US.. PEG/PPG dimethicone structure and function PEG/PPG dimethicone compounds should ABSTRACT. be used in instances where they provide benefits that cannot be obtained by organic Despite the increasing usage of silicone polymers in personal care products, the compounds. These properties include: selection of the proper polymer for a particular application remains somewhat elusive. w Lowering of surface tension (into the This directly results in inefficiency of formula development. This problem is exacerbated range of 25 dynes/cm). by the reliance upon INCI names in selection of compounds. While helpful in placing w Provide unique skin feel. polymers in a generic class, it is of limited help in optimising formulation efficiency. w Provide unique emulsification properties Compounds that are water insoluble, water dispersible or water insoluble can all share (especially in invert emulsions).
2 A common INCI name. Likewise polymers that provide wetting, emulsification, or w Provide film formation. conditioning can share a common INCI name. This article is intended to clarify w Provide foaming of non-traditional selection of products for formulation. formulations. These properties are a direct result of the structure, but the structure needs to be In studio for carefully selected. PEG/PPG dimethicone new artwork polymers have increased polarity, by virtue of their polyoxyalkylene groups. It is important to note that these modifications are done by chemical reaction between a reactive silicone and a vinyl containing compound. A very common structure is shown in Figure 1. Figure 1: PEG/PPG dimethicone structure. There are several variables that can be modified to change the performance of the Surface tension DMC polymer. One of the most important When added into water at low is the ratio of a (silicone soluble portion) concentrations, PEG/PPG dimethicone to b (water soluble portion) as well as the (DMC) migrates to the air/water interface.
3 Total number of a and b, both of which are As more DMC is added into the water, the important to functionality. The higher the interface becomes saturated and a critical ratio of a to b the less water soluble the point is achieved. When additional DMC. In studio for product. Also, the molecular weight of polymer is added, it cannot migrate to the new artwork the DMC polymer can drastically change interface, so they start to form micelles. properties. The studies presented in this This critical point where the interface is paper keep the ratio of D/D* constant (that saturated and micelles start to form is is the ratio of a/b is kept constant relative called the critical micelle concentration to each other). (CMC). Critical micelle concentrations are Figure 2: Illustration of Table 1: Surface tension and molecular weight. PEG 8 dimethicone ( solution). PEG 8 dimethicone ( wt% solution).
4 Product Molecular Surface weight tension (dynes/cm). In studio for A 607 new artwork B 808 C 1108 D 1610 E 2111 Figure 3: Graphic surface tension and molecular weight. April 2013 P E R S O N A L C A R E 1. SILICONES. In studio for In studio for new artwork new artwork Figure 4: Contact Figure 5: Wetting time (seconds) versus molecular weight. determined by monitoring surface tension. formulation. As will become clear Table 2: Function of dimethicone Figure 2 shows an illustration of what subsequently, interaction with other raw copolyol versus molecular weight. happens upon addition of a surfactant materials in the formulation needs to be into bulk solution. considered as well. Molecular weight (Da) Function The surface tension starts to decrease The surface tensions are related to 500 Wetting in a linear relationship with the amount the molecular weight of a series of PEG 8. of surfactant added.
5 The moment the dimethicone in which the ratio of silicone 2500 Emulsification interface gets saturated, the surface units to polyoxyalkylene units (a:b) are 10000 Conditioning tension stops decreasing. kept constant but increases. The results 50000 Waterproofing The key to surface tension efficiency is are shown in Table 1 and Figure 3. the ability of a polymer to migrate to the The structure/function relationship is Table 3: Draves wetting. interface and take up the maximum free seen in these same compounds in both volume on that interface. This maximisation wetting times and eye irritation as shown Sample Wetting time of free volume at the interface and its below. (Seconds). affinity for the interface makes the DMC. A 607 extremely effective at lowering surface Wetting properties as a tension. This leads to DMC being used function of molecular weight B 808 at very low concentrations to drastically The term wetting' refers to how a material C 1108 change the properties of a solution.
6 Coats a surface. For example, if a water D 1610 The molecular weight is a key concept in drop is placed onto a lotus leaf, the water E 2111 determining CMC. As the molecular weight will minimise the interaction with the leaf increases, the CMC decreases. This is due and have a high contact angle. If a good in part to the fact that the surface can wetting agent is added either to the surface Table 4: PEG 8 dimethicone 1% foam test. accommodate fewer large molecules' of the leaf or in the water solution that Sample Immediate 3 minutes than small molecules. The CMC of a DMC same drop will spread out coating the is a key property to be determined before surface of the leaf, in turn making the leaf A 607 90 62. using a specific DMC in formulation, this wet', and this is where the term wetting B 808 90 63. is based upon the observation that if the comes from. Figure 4 illustrates the C 1108 95 67.
7 DMC cannot get to the interface, it will Young's equation and how the contact D 1610 90 62. not provide desired properties to the angle is defined. E 2111 90 61. Table 5: Foam height. In studio for PEG 18/PPG 18 dimethicone 1% in water new artwork Sample Immediate 3 Minutes F 7600 97 50. Table 6: Surface tension Sample Molecular Surface tension weight (dynes/cm). E 211 Figure 6: Sample F PEG 18/PPG 18 dimethicone structure. F 7600 2 P E R S O N A L C A R E April 2013. SILICONES. Table 7: Property comparison. Property Sample E Sample G. Star Product Surface tension ( aqueous). Foam height 90 185. (1% aqueous) In studio for Cloud point ( C) 58 89 new artwork Wetting time >200 >200. (seconds). Figure 7: PEG 8 dimethicone structure. The contact angle is defined as the tabular form the effect of molecular weight and patented (US Patent 7,951,893 to angle the water makes with the surface.)
8 On wetting time and Figure 5 shows the O'Lenick et al, issued May 31, 2011). In this illustration, a low contact angle same data in graphic form. This class of compounds, referred to as ( <90 ) means the surface is hydrophilic. Star Silicones, have unique properties Large contact angles ( >90 ) lead to a Foam and are a model useful in explaining the surface that is hydrophobic. Ability to generate foam is an important properties observed above. The structure of Another measure of wetting is the time functionality in any surfactant, including PEG 8 Dimethicone is shown in Figure 7. required for a standard skein of fibre, or silicone surfactants. The ability to generate The structure of the Star compound of hair to drop in an aqueous solution. foam results from the balance of silicone (Sample G) is evaluated in shown in Figure This type of wetting time is called Draves portion to water soluble portion, which 8 and a comparison of the properties of Wetting (ASTM D2281).
9 The lower includes as an important variable the Sample E and G are shown in Table 7. molecular weight DMC polymer has faster amount of PEG/PPG. The cylinder shake It is noteworthy that the surface tension wetting times than their high molecular foam test data is shown in Table 4. of Sample G is more reminiscent of a weight counterparts. The lower molecular The data in Table 4 indicates that fatty compound than of a silicone weight polymers allow for more efficient changes to the PEG-8 dimethicone with the (30s dynes/cm not 20s dynes/cm). It is packing and dynamics. The materials with D/D* ratio evaluated has really no effect also quite significant that the foam level lower molecular weight were extremely upon foam, either initial or after 3 minutes. effective at the higher concentration of A change was made to Sample E replacing Table 8: Draize primary w. Their wetting speeds were almost the PEG 8 group with PEG 18/ PPG 18.
10 Ocular irritation scale. instantaneous meaning it is controlled by This material was designated Sample F. Moderately irritating the diffusion of the DMC polymer to the and is shown in Figure 6. air/water interface. The interesting finding Table 5 shows the result of the foam Mildly irritating is that the wetting speed is lost slowly as testing on Sample F. Table 6 shows the Minimally irritating the molecular weight is increased. The result of the surface tension testing on Practically non-irritating slope increases once one gets to a Sample F. Non-irritating molecular weight over 2000. This implies The inclusion of more EO and PO. that a rapid wetting conditioner can be lowered foam, but interestingly also raised Table 9: Eye irritation of PEG 8. prepared by proper selection of molecular the surface tension. Simply put, the highly dimethicone by molecular weight. weight. As the molecular weight of a alkoxylatyed DMC polymer had a surface polymer is increases, its ability to function tension like a fatty alkoxylate, not like a Sample 1-day 7-day in different capacities changes.