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“If a personal care product is a gourmet meal, silicones should be the salt and spice and not the meat and potatoes.”
—Anthony J. O’Lenick Jr.
Bad news always makes headlines while good news is buried on Page 43, if mentioned at all. Lately, this is the case with silicones when some questions arose concerning their safety, particularly in regard to the environment. And the saber rattling in regulatory agencies, particularly those in Canada, seemed to threaten the industry. But when an extensively researched report clearly validating the safety of silicones in personal care was issued,1 consumer media was strangely silent.
Silicone-based molecules have unique properties and many uses in personal care; however, confusion over the chemistry has caused undue concern. While silicon (no “e” at the end) is an atom, larger polymeric structures formed from it are called silicones; and while an element such as carbon bonds to itself (–C-C-C-C-C-), silicon needs an oxygen in between (–Si-O-Si-O-Si-O-Si-). Concern centers on cyclic silicones, rings formed having four or five silicon atoms, also known as D4 or D5. The D comes from the standard shorthand for silicone units: M (mono), D (di), T (tri), Q (quat). It is easy to see how a row of D units swallowing its tail can form a ring, and how appending the number of D units results in D4, D5, D6—and the problems with cyclic silicones will be discussed later in this column.
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Silicon accounts for 28% of the mass of the earth’s crust, so it is hardly an exotic material. The sand on a beach, concrete and computer chips are all created from silicones. Yet we now have to consider if chemically altered silicones are biodegradable or toxic to the environment. Also, as far as silicones being “natural,” even though the process starts with sand, so much energy and synthetic chemistry is employed to develop the silicones used in commerce that one is loathe to consider the process “green.”
Considering these questions as well as the aforementioned regulatory concerns rocking the foundations of the beauty industry, many companies have moved away from using some silicones as a precautionary response. Only one type of silicone—cyclic methylsiloxane—remains in the crosshairs, but it constituted 64% of the poundage of silicones used in 2009.2 To understand the problem, we must understand why one silicone is safe and appropriate for use in personal care and why there are issues with another kind of silicone.
D4 and The Environment
Human toxicity is not an issue for any silicones utilized in personal care. Genuine concerns arise with the chemical properties of cyclic silicones that determine their fate in the environment. Stevens3 provided a valuable survey, summarized here, of his examination of cyclopentasiloxane, a D4. Though D4 is no longer widely used, the issues demonstrated project onto other cyclic silicones.
Some silicones such as cyclics are volatile, meaning they evaporate readily into the atmosphere. Dimethicones are not. D4 is especially of concern due to its volatility and high molecular weight, 296.
In examining silicones and their fate, one quickly encounters Henry’s Law. In simple terms, it is a relation between the vapor over a liquid to the gases dissolved in the liquid. This is clearly demonstrated by a bottle of carbonated beverage. When the bottle is sealed, the bubbles remain in the liquid; when opened, the gas escapes and the beverage goes flat. That is Henry’s Law in action. For silicones, the law is a key property determining if a molecule will partition in soil, water or air.
A high Henry’s Law Constant for D4 is the consequence of high vapor pressure and low water solubility, thus D4 directly enters the atmosphere. In “down-the-drain” products such as hair conditioners, most of the D4 will go from waste water into the air. Degradation takes place over a time span of approximately a week to a month,4 enough time for long range atmospheric transport. Dimethicones, by contrast, primarily migrate to soil or sludge where they degrade readily into silicone dioxide, water and carbon dioxide with no adverse environmental consequences.
D5 has replaced D4 but the debate continues with regulators, especially in Canada where concerns have been raised that D5 may also be bioaccumulative.* However, a simple drop-in replacement is not available for D5, and it is employed in various personal care products. Antiperspirants, for instance, rely more heavily on D5 than any other product, and its volatility and feel are essential to the performance that users expect. Hair and skin applications hinge more on feel than volatility, so alternatives have already been proposed. (Discussions on these alternatives and suppliers that provide them are available at www.CosmeticsandToiletries.com; ingredient listings are available by clicking the “Directory” tab on www.GCImagazine.com. In addition, a number of ingredient suppliers fully support silicone lines containing cyclomethicone while also offering numerous cyclic-free choices).
Few commercial beauty products prominently claim to be silicone-free. Makeup, particularly face primers, seem to be an exception—with Korres Face Primer, Exuberance’s Helichrysum Silicone-Free Makeup Primer and Tarte’s ReCreate Silicone-Free Primer as notable examples making silicone-free claims.
A positive sign for the current status of silicones is that they do not appear on the Campaign for Safe Cosmetics list of “Chemicals of Concern.” The biggest difference between the silicone issue and phthalates, another ingredient type under heavy regulatory and NGO scrutiny, is that silicones are an environmental concern rather than a human health one. It is also clear that, despite the claims of non-industry special interest groups, the beauty industry and regulators actively investigate both human and environmental issues far more intensively than consumers or activists realize. The official industry response regarding silicones is coordinated by Silicones Environmental, Health & Safety Council of North America5 and the European Silicones Centre.6
It is unlikely that silicones will ever become a lightning rod for conspiracy theories like diethyl phthalate and parabens have. The issues will probably be solved through rational analysis of the science by national regulatory agencies, and there, the science is very positive for confirming the human safety of cyclic silicones. There will be some minor push back from a few consumer companies but the cyclic silicones are far too valuable to be broadly eliminated simply as a precautionary step.
For additional reading on silicones and other ingredient types in the crosshairs, read “Controversial Ingredients: One Brand’s Perspective,” published by GCI in December 2011.
- ec.europa.eu/health/scientific_committees/consumer_safety/docs/sccs_o_029.pdf (Accessed Feb 20, 2012)
- pubs.acs.org/cen/coverstory/89/8918cover.html?utm_source=twitterfeed&utm_medium=twitter (Accessed Feb 20, 2012)
- C Stevens, Environmental fate and effects of dimethicone and cyclotetrasiloxane from personal care applications,Int J Cosm Sci, 20, 297–305 (1998)
- J Lambert, Silicone Safety in the Cosmetic Industry, Cosm Toil, Nov 2011
- www.sehsc.com (Accessed Feb 20, 2012)
- www.silicones.eu (Accessed Feb 20, 2012)