Knowing the difference between a silicone fluid and bimodal silicone fluid is important for formulators using them. A simple specification such as viscosity will not suffice in duplicating a product that is a bimodal blend.
It is important to note that all polymers are oligomeric compositions. This means there are a series of different molecular weight polymers in the product. This is not a situation unique to silicone polymers. We see a Gaussian distribution of polymers in ethoxylates and many other common polymers.
The fact that polymerization results in several species that are present in a polymer because of the polymerization process should not be confused with intentionally blended silicone polymers that have two different oligomer distributions, one from each polymer. Since many silicone fluids are sold by viscosity, it is often difficult to determine if a particular product is the result of blending or a single silicone fluid without the proper analytical methodology.
Gel permeation chromatography (GPC)1 is a typical recommended system to complete this analysis. It shows the distribution of polymeric species in a polymer (see Figure 1).
Figure 1. Typical distribution of polymer oligomers
Figure 2. Bimodal blend of 50 cSt and 1,000 cSt to make 350 cSt
The 100,000 cSt fluid would be as thick as hot tar. It would not spread well on the skin and would only be viable in a personal care product in a blend. The 50 cSt by itself product would be very thin and have little or no cushion. In blending the two, the 50 cSt fluid lowers the viscosity of the blend, provides for ease of spread and acts like a "delivery system" for the high viscosity fluid. The desirable properties of the high viscosity fluid are delivered in a cosmetically elegant way. Also, keep in mind that the 50 cSt, the 100,000 cSt and the blend all share the INCI name of dimethicone.
Figure 3. Bimodal blend of 50 cSt and 1,000 cSt to make 350 cSt