Cyclopentasiloxane

Cyclopentasiloxane is a silicone that leaves. Five silicon atoms and five oxygens close into a ring, each silicon carrying two methyl groups, and the resulting molecule is small enough to evaporate from skin at ordinary temperatures. That one property made it the standard carrier for a generation of long-wear cosmetics.

It is also the reason many of those formulations are being rebuilt. Its volatility carries it into the air and its stability keeps it in the environment, and European regulators have restricted it on the second ground.^[1]

Hydrolysing dimethyldichlorosilane does not give one product. It gives a mixture of open chains and closed rings, and the rings are named by how many silicon atoms they contain: D3, D4, D5, D6 and upward. Winton Patnode and Donald Wilcock at General Electric separated the series from three to ten units and described their properties in 1946.^[2]

D5 is the five-membered ring, decamethylcyclopentasiloxane, CAS 541-02-6.^[3] It boils at around 210 degrees Celsius, but its vapour pressure is high enough that a thin film evaporates steadily at room temperature. Older ingredient lists often say cyclomethicone, which is not this substance: it is a separately registered INCI name, with its own CAS number, for a mixture of these rings. A product labelled cyclomethicone may contain cyclopentasiloxane among other ring sizes, but the two names do not identify the same material.

Volatile silicones entered colour cosmetics as the solvent half of a two-part idea: suspend the durable material in something that will disappear, apply the mixture, and let the solvent go. Revlon's patent on cosmetic compositions with improved transfer resistance, with a priority date of 15 December 1992, claims a volatile solvent at one to seventy per cent of the product, chosen from volatile silicones, C8 to C20 isoparaffins, or mixtures of the two.^[4]

Procter and Gamble's transfer-resistant compositions, with a priority date of 7 November 1995, use the same architecture with a silicone MQ resin as the film former.^[5] The long-wear foundation and the transfer-proof lipstick of the middle 1990s are both built on this pattern. Both patents list volatile silicones among the permitted carriers, and cyclopentasiloxane was the usual choice where a silicone carrier was wanted; neither ranks the carriers by market use, and the Procter and Gamble specification in fact prefers a hydrocarbon, naming isododecane as most preferred.^[5]

Cyclopentasiloxane is a solvent and a spreading aid rather than a conditioning agent in its own right; what conditioning effect it has is the residue of that spreading, not a property it brings on its own. It carries pigments, resins and silicone gums into a thin even layer, lowers the apparent viscosity of the mixture while it is being applied, and then leaves.^[6] What remains is the film the formulator actually wanted, at a solids content that could not have been spread directly.

The sensory effect is distinctive. A product with a high load of volatile silicone feels weightless and slightly cool as it dries, because evaporation draws heat from the skin. Foundations, antiperspirants, hair serums and primers all made use of that.

The case against D5 is environmental rather than dermatological. Commission Regulation (EU) 2018/35 of 10 January 2018 provided that D4 and D5 'shall not be placed on the market in wash-off cosmetic products in a concentration equal to or greater than 0,1 % by weight of either substance, after 31 January 2020'.^[7]

Commission Regulation (EU) 2024/1328 of 16 May 2024 extended entry 70 of Annex XVII to the REACH Regulation to cover D6 as well, and carried the same threshold of one tenth of one per cent into leave-on cosmetic products. The restriction applies from 6 June 2026, with leave-on cosmetic products allowed until 6 June 2027.^[8] The threshold bites on these substances on their own, as constituents of other substances and in mixtures, so a raw material carrying D5 as a residue is caught as well as one formulated with it; the 2024 amendment also reaches uses outside cosmetics, among them dry cleaning, waxes and washing and cleaning products, each on its own timetable.^[8] The reasoning in both instruments concerns persistence and bioaccumulation in the environment, not effects on skin. Reformulation towards volatile hydrocarbons and short linear siloxanes has followed.