Silicone Oil (Polydimethylsiloxane) & Emulsion

J. Allcock & Sons we have been supplying silicone products (silicone oil, silicone emulsion, silicone grease, and more) to many industries since the 1970′s. Therefore we have decided to explain some of┬áthe chemistry behind the two most popular; silicone oil and silicone emulsion. We hope you find it useful.

Silicone Oil

First of all, silicone oil has many different names; silicone fluid, polydimethylsiloxane, dimethicone, dimethylpolysiloxane, dimethyl siloxane, PDMS. J. Allcock & Sons have branded our silicone oil Allcosil 200 (which only adds further to the confusion!)

Technically, we believe that polydimethylsiloxane best describes the oil. Poly (Latin for many), dimethyl (two methyl (CH3) group) siloxane (Silicone and Oxygen).

The chemical structure of polydimethylsiloxane is:

n= number dimethylsiloxane units.

Buyers of polydimethylsiloxane will be aware that it is available in a number of different viscosities. The most common being; 100cSt(centistoke), 350cSt and 1000cSt. The number of dimethylsiloxane units depicts the size of the polymer chain which determines the viscosity. The smaller the polymer chain the lower the viscosity (e.g. 100 cSt) , the bigger the polymer chain the higher the viscosity (e.g.1000cSt).

Our one product in the Allcosil 200 range that is slightly different is our lowest viscosity Allcosil 200; 0.65cSt. Allcosil 200/0.65 has only two units, this means that it is not a polymer, but a dimer. The dimer is hexamethyldisiloxane.

The chemical structure of hexamethyldisiloxane is:


It is clear that when you compare it to the polydimethylsiloxane structure that the dimer is very similar to polydimethysiloxane.

The polymer chain length also has an effect on other properties of the oil.

Viscosity, cSt Flashpoint, ┬░C COC Freezing Point,┬░C Specific Gravity, @ 25┬░C Surface Tension, mN/m Refractive Index, @ 25┬░C
0.65 -4 -67 0.760 15.9 1,375
1 40 -85 0.816 17.4 1,382
2 48 -90 0.830 18.1 1,387
3 62 -100 0.900 18.9 1,392
5 136 -100 0.910 19.7 1,397
10 162 -65 0.930 20.1 1,399
20 230 -60 0.950 20.6 1,400
50 280 -55 0.959 20.7 1,402
100 >300 -55 0.965 20.9 1,403
200 >300 -50 0.970 21.0 1,403
300 >300 -50 0.970 21.1 1,403
350 >300 -50 0.970 21.1 1,403
500 >300 -50 0.970 21.1 1,403
1000 >300 -50 0.970 21.2 1,403
5000 >300 -50 0.975 21.4 1,403
10000 >300 -50 0.975 21.5 1,403
12500 >300 -50 0.975 21.5 1,403
30000 >300 -50 0.975 21.5 1,403
60000 >320 -50 0.975 21.5 1,403
100000 >300 -50 0.976 21.5 1,404
300000 >300 -45 0.976 21.5 1,404
1000000 >300 -40 0.976 21.5 1,404

(1000000 cSt, longest polymer chain.)

It is clear from the table showing properties, that after 50cSt, the polymer chain length becomes less important in effecting the properties.

According to the FDA Regulation 21 CFR, certain viscosities are food grade. For more information, please contact us.

Silicone Oils have many uses due to their lubrication, dielectric and water repellent properties.

Silicone Emulsion

Silicone Emulsion contains 3 ingredients; polydimethylsiloxane, emulsifier & water.

The key ingredient is the emulsifier which encapsulates the polydimethylsiloxane oil and holds it in suspension.

A typical emulsifier used in making silicone emulsions is ethoxylated glycol ethers. These emulsifiers have a hydrophobic (dislikes water) and hydrophilic (likes water) part to their structure. The hydrophobic faces the polydimethylsiloxane and the hydrophillic faces the water. This creates a barrier between the water and the polydimethylsiloxane.

Here is a really good picture i found on google. A surfactant (Surface active agent) is just a type of emulsifier.

Unfortunately emulsifiers, due to their chemical make-up, are vulnerable to bacteria. Bacteria can digest certain emulsifiers causing the polydimethylsiloxane to float to the top or disrupt the pH enough for non-digestible emulsifiers to split from the oil. This is called splitting. Splitting in an emulsion can be seen and smelt. The smell comes from the bacterial growth, it usually smells like sour milk.

Emulsions are a key mould realease/lubricant and antifoams for many industries.Our biggest selling emulsion is Allcosil 435 FG, this is sold to the food packaging industry. It is food grade and kosher certified.


If you have any questions, please feel free to leave a comment.

7 thoughts on “Silicone Oil (Polydimethylsiloxane) & Emulsion

  1. Hi Luke,

    Want to say thanks for a wonderful blog post, what a fascinating and informative read!

    Best of luck in all of your future ventures,


  2. I agree with you: silicone is such a fascinating material! I work in a factory that needs a lot of silicone sheets and mats to develop his activity, so I have to work with silicone every day.

  3. Terrific work! That is the type of info that are supposed to be shared across the internet. Disgrace on the search engines for not positioning this submit upper! Come on over and talk over with my web site . Thanks =)

  4. Hi Luke,
    Very good explanation about this topic and very informative too.
    Well, I have one question about the stability of the emulsifiers.

    Right now, I am doing research about silicone oil and emulsions.
    And I have problem that my silicone emulsion has a bad odor after a week.
    You said that it is caused by bacteria.
    Can you give me some advices about the good emulsifiers for making silicone emulsion?

    I am using nonylphenol (C10-C15) as the emulsifiers.
    I thought the bacteria can not digest nonylphenol but maybe it happened.

    Oh, one more question.
    Can you give me some advices about handling silicone emulsion, such as temperature storage, sunlight exposure, etc?
    Because I thought the sunlight can make the silicone emulsion unstable.


    • Hi Bill,

      Thanks for the question and i’m sorry for the slow reply. I’ve just come back from a holiday :)

      I’m not too sure if the bacteria can digest nonylphenol (C10-C15) but sometimes bacteria can disrupt the pH of an emulsion enough for the emulsifier and oil silicone to split anyway. Emulsions are an extremely good environment for bacteria, even with a non-digestible emulsifier.

      You could try: Poly(oxy-1,2-ethanediyl),alpha-(2-propylheptyl)-omegahydroxy (CAS No. 160875-66-1). We think this is a better/stronger emulsifier.

      I will also suggest adding some fungicide / bactericides. We sell a product called Allcosil Stabiliser. This has Sodium Benzoate and Potassium Sorbate – both food grade preservatives. Allcosil Stabiliser prolongs the onset of splitting by “killing” the bacteria.

      The best conditions for handling silicone emulsion would be : keep cool and away from direct of sunlight (very much the same conditions you would keep Milk in). However, good/strong silicone emulsions will be fine in RT conditions. Exposure to sunlight is not critical but obviously it will heat up the emulsion and give a better environment for the bacteria, so better to keep away if possible. We keep our emulsions in our warehouse, which has never been a problem. Do not allow the emulsion to freeze as this will also cause the emulsion to split.

      Hope this answers your questions and if you have anymore, feel free to ask :)