Dan Thompson: Beauty Busted


Dimethicone vs. Siloxane In Your Personal Care Products

Separating Complicated Information From Headlines

A little while ago, I wrote about how news stories don't always paint the full picture in regards to how cosmetics ingredients function in the human body. I was reminded again this week why it is so important to understand that a simple headline cannot actually explain the complexity of how cosmetic formulations work.
There is such confusion about certain ingredients, because so much press coverage and media attention have been put on personal care products of late, and yet these stories often omit incredibly important facts that could indeed ease consumer concerns.
I want to clarify the dimethicone vs. siloxane issue a little.
While technically true that dimethicone can be a siloxane (and I want to stress the "CAN"), it requires some very specific compounding for this to be true. It depends on the actual construction of the molecule.
The concern is the allergic reaction rate on skin, the occlusion level on the skin, and, of course, the interruption of skin's natural functions—all of which are heightened when a pure siloxane compound is used on the skin.
Dimethicone, in its purest synthetic form, does not cause occlusion on the skin—it actually is semi-permeable, which means it allows the exchange of oxygen, nitrogen, and water (respiration) on the skin, has no known toxicity, and, in fact, has an incredibly low allergic reaction rate (less than 1 in 100, 000). Even the EWG, which regular readers know I do not use as a resource for information, gives dimethicone a low toxicity rating.
Dimethicone is used in many medical applications, but most usually in topical medications because it allows the skin to function as normal and will not inhibit the healing process.
Here is the actual science:
Siloxanes are a form of  a chemical bond of silicone and oxygen in a very specific manner—Si-O-Si. This link must be in this order, uninterrupted by any other element in order to be classified as a siloxane, and the link can be either linear or cyclic and that is why you will see some compounds called cyclopentasiloxane or just simple pentasiloxane. All this refers to is the molecular structure of the molecule (cyclic chains are harder to break as they are closed in a circle and linear ones are straight lines, and thus easier to manipulate). Regardless of the structure, the Si-O-Si link is always in tact and not interrupted.
Dimethicone is also a form of a chemical bond between silicone and oxygen, but is broken with Carbon and Hydrogen, thus changing its nature completely. Dimethicone is written as [SiO(CH3)2], note there is no Si-O-Si link of any kind. Thus in its pure state, dimethicone is not actually a siloxane bond.
What does this mean?
It means that dimethicone does not have the same properties as a siloxane, other than that of being emollient. Dimethicone neither occludes the skin nor does it have high reaction rate on the skin like a siloxane will.
Now (and this is where it gets complicated), dimethicone, when manipulated at the molecular level, CAN be a siloxane. For example, CH3[Si(CH3)2O]2nSi(CH3)3 were n = [SiO(CH3)2] is also dimethicone, but an altered form where the molecule forms an Si-O-Si link. This is called a cross polymer. Technically, it is called polymethylsiloxane and must be listed as such on an INCI list. I think this is where a lot of confusion happens with dimethicone.
However, in this case, dimethicone is part of a larger molecule that contains other elements and changes the behaviour of the dimethicone completely.  
It's like saying because water is part of lemonade then lemonade must have the same properties and will behave the same as pure water. This simply is not true.  
So what does all this mean?
While a siloxane can be dimethicone, dimethicone is not always a siloxane.
The easiest way for a consumer to tell the difference is by the suffix -ane or -one. This does not, however, indicate if the raw material being synthesized is petrochemical or not (which is an entirely different conversation). There are no laws regarding sources on labels.  
What I can tell you is that most dimethicone (and all silicones ending in the suffix -one) are now produced by synthesizing silica, which is either extracted from sandstone or common beach sand. This is a relatively new process due to the fact that the petrochemical version of synthesis is no longer desirable from a marketing perspective
So to summarize, it is my opinion that siloxane should not be referred to AKA dimethicone, even though many consumers believe it to be.
I know a lot of publications (i.e. so-called consumer advocacy books) say to look for dimethicone on the ingredient list in order to avoid siloxane compounds, but this is simply because if the actual list of ingredients were to be published, the reader would give up.
Here is a list of dimethicone derivatives that form the Si-O-Si link that would be found on a cosmetic INCI list:
  • acrylates/bis-hydroxypropyl dimethicone crosspolymer

  • behenyl dimethicone/bis-vinyldimethicone crosspolymer

  • bis-phenylisopropyl phenylisopropyl dimethicone/vinyl dimethicone crosspolymer

  • bis-vinyldimethicone/bis-isobutyl PPG-20 crosspolymer

  • bis-vinyldimethicone crosspolymer

  • bis-vinyldimethicone/ PEG-10 dimethicone crosspolymer

  • bis-vinyldimethicone/PPG-20 crosspolymer

  • butyldimethicone methacrylate/methyl methacrylate crosspolymer

  • C30-45 alkyl cetearyl dimethicone crosspolymer

  • C4-24 alkyl dimethicone/divinyldimethicone crosspolymer

  • C30-45 alkyl dimethicone/polycyclohexene oxide crosspolymer

  • cetearyl dimethicone crosspolymer

  • cetearyl dimethicone/vinyl dimethicone crosspolymer

  • cetyl dimethicone/bis-vinyldimethicone crosspolymer

  • cetyl hexacosyl dimethicone/bis-vinyl dimethicone crosspolymer

  • crotonic acid/vinyl C8-12 isoalkyl esters/VA/bis-vinyldimethicone crosspolymer

  • dimethicone/bis-isobutyl PPG-20 crosspolymer

  • dimethicone/bis-vinyldimethicone/silsesquioxane crosspolymer

  • dimethicone crosspolymer

  • dimethicone crosspolymer-3

  • dimethicone/divinyldimethicone/silsesquioxane crosspolymer

  • dimethicone/lauryl dimethicone/bis-vinyldimethicone crosspolymer

  • dimethicone/PEG-10 crosspolymer

  • dimethicone/PEG-10/15 crosspolymer

  • dimethicone/PEG-15 crosspolymer

  • dimethicone/phenyl vinyl dimethicone crosspolymer

  • dimethicone/polyglycerin-3 crosspolymer

  • dimethicone/PPG-20 crosspolymer

  • dimethicone/titanate crosspolymer

  • dimethicone/vinyl dimethicone crosspolymer

  • dimethicone/vinyltrimethylsiloxysilicate crosspolymer

  • diphenyl dimethicone crosspolymer

  • diphenyl dimethicone/vinyl diphenyl dimethicone/silsesquioxane crosspolymer

  • divinyldimethicone/dimethicone crosspolymer

  • hydroxypropyl dimethicone/polysorbate 20 crosspolymer

  • isopropyl titanium triisostearate/triethoxysilylethyl polydimethylsiloxyethyl dimethicone crosspolymer

  • lauryl dimethicone PEG-15 crosspolymer

  • lauryl dimethicone/polyglycerin-3 crosspolymer

  • lauryl polydimethylsiloxyethyl dimethicone/bis-vinyldimethicone crosspolymer

  • PEG-10 dimethicone crosspolymer

  • PEG-12 dimethicone crosspolymer

  • PEG-8 dimethicone/polysorbate 20 crosspolymer

  • PEG-12 dimethicone/bis-isobutyl PPG-20 crosspolymer

  • PEG-12 dimethicone/PPG-20 crosspolymer

  • PEG-10 dimethicone/vinyl dimethicone crosspolymer

  • PEG-10/lauryl dimethicone crosspolymer

  • PEG-15/lauryl dimethicone crosspolymer

  • PEG-15/lauryl polydimethylsiloxyethyl dimethicone crosspolymer

  • perfluorononyl dimethicone/methicone/amodimethicone crosspolymer

  • polydimethylsiloxyethyl dimethicone/bis-vinyldimethicone crosspolymer

  • polyglyceryl-3/lauryl polydimethylsiloxyethyl dimethicone crosspolymer

  • silicone quaternium-16/glycidoxy dimethicone crosspolymer

  • styrene/acrylates/dimethicone acrylate crosspolymer

  • trifluoropropyl dimethicone/PEG-10 crosspolymer

  • trifluoropropyl dimethicone/trifluoropropyl 2 divinyldimethicone crosspolymer

  • trifluoropropyl dimethicone/vinyl trifluoropropyl crosspolymer

  • vinyl dimethicone/methicone silsesquioxane crosspolymer

  • vinyldimethyl/trimethylsiloxysilicate/ dimethicone crosspolymer

  • vinyldimethyl/trimethylsiloxysilicate stearyl dimethicone crosspolymer 

  • dimethicone/silsesquioxane crosspolymer

  • trimethylsiloxysilicate/dimethicone crosspolymer

  • vinyl dimethicone/lauryl/behenyl dimethicone crosspolymer

  • vinyl dimethicone/lauryl dimethicone

Daunting list, isn't it? So it becomes easier for news stories and so-called experts to simply say "dimethicone" is the same as "siloxane," which it actually is not.
You will find simple dimethicone (and even Cyclomethicone or Simethicone) are not to be found on this list at all. An accurate AKA list for siloxane compounds (derived from dimethicone) is actually rather lengthy and complicated.
It quickly becomes apparent why this topic (and really any of product integrity) is much more complex than the latest newspaper article about personal care products.