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Silicones in the Crosshairs

By: Steve Herman
Posted: April 4, 2012, from the April 2012 issue of GCI Magazine.

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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).