Octamethylcyclotetrasiloxane, also called D₄, is an

organosilicon compound Organosilicon chemistry is the study of organometallic compounds containing carbon–silicon bonds, to which they are called organosilicon compounds. Most organosilicon compounds are similar to the ordinary organic compounds, being colourless, f ...

and one of a number of cyclic

siloxane In organosilicon chemistry, a siloxane is an organic compound containing a functional group of two silicon atoms bound to an oxygen atom: . The parent siloxanes include the oligomeric and polymeric hydrides with the formulae and . Siloxanes ...

s (cyclomethicones). It is a colorless viscous liquid with a high boiling point. It has historically been used on a large scale in

personal care Personal care products are consumer products which are applied on various external parts of the body such as skin, hair, nails, lips, external genital and anal areas, as well as teeth and mucous membrane of the oral cavity, in order to make ...

products including cosmetics, hair conditioners and

emollient A moisturizer, or emollient, is a cosmetic preparation used for protecting, moisturizing, and lubricating the skin. These functions are normally performed by sebum produced by healthy skin. The word "emollient" is derived from the Latin verb ''mo ...

s (moisturising creams), Global production in 1993 was 136,000 tons, however it is now facing significant pressure from regulators. It is a

substance of very high concern A substance of very high concern (SVHC) is a chemical substance (or part of a group of chemical substances) which has been proposed as a candidate for inclusion on the Authorization or Restriction list (seeECHA Lists of REACH. The addition of a ...

In the EU, where it was classified as a PBT and effectively banned in personal care products in 2018. The US EPA began reevaluating its risks in 2020. Replacements include branched-chain alkanes (isoparaffins).

Production and polymerization

Commercially D₄ is produced from

dimethyldichlorosilane Dimethyldichlorosilane is a tetrahedral organosilicon compound with the formula . At room temperature it is a colorless liquid that readily reacts with water to form both linear and cyclic Si-O chains. Dimethyldichlorosilane is made on an indust ...

Hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of the dichloride produces a mixture of cyclic dimethylsiloxanes and polydimethylsiloxane. From this mixture, the cyclic siloxanes including D₄ can be removed by distillation. In the presence of a strong base such as KOH, the polymer/ring mixture is equilibrated, allowing complete conversion to the more volatile cyclic siloxanes: : D₄ and D₅ are also precursors to the polymer. The catalyst is again KOH.

Safety and environmental considerations

D₄ is of low

acute toxicity Acute toxicity describes the adverse effects of a substance that result either from a single exposure or from multiple exposures in a short period of time (usually less than 24 hours). To be described as ''acute'' toxicity, the adverse effect ...

. The LC₅₀ for a single four hour inhalation exposure in rats is 36 mg/L. The oral LD₅₀ in rats is above 4800 mg/kg and the dermal LD₅₀in rats is above 2400 mg/kg. As the smallest cyclic dimethylsiloxane that does not experience considerable

ring strain In organic chemistry, ring strain is a type of instability that exists when bonds in a molecule form angles that are abnormal. Strain is most commonly discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles ar ...

, D₄ is one of the most abundant siloxanes in the environment, e.g. in landfill gases. D₄ and D₅ have attracted attention because they are pervasive. Cyclic siloxanes can be detected in some species of aquatic life.Wang, De-Gao; Norwood, Warren; Alaee, Mehran; Byer, Jonathan D.; Brimble, Samantha "Review of Recent Advances in Research on the Toxicity, Detection, Occurrence and Fate of Cyclic Volatile Methyl Siloxanes in the Environment" Chemosphere 2013, volume 93, pages 711–725, . An independent, peer-reviewed study in the US found "negligible risk from D4 to organisms" while a scientific assessment by the Australian government stated, "the direct risks to aquatic life from exposure to these chemicals at expected surface water concentrations are not likely to be significant." In the

European Union The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are Geography of the European Union, located primarily in Europe. The u ...

, D₄ was characterized as a

(SVHC) due to its PBT and vPvB properties and was thus included in the candidate list for authorisation. D₄ 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. Conversely, a detailed review and analysis of the science by the State of Washington in 2017 led to the removal of D₄ from their CHCC listing. That decision prompted the State of Oregon to follow suit in 2018."Notice of Proposed Rulemaking"
State of Oregon. Retrieved 2018-12-09.

References

External links

* * {{CPID, id=3487, name=Octamethylcydotetrasiloxane (a misspelling) PBT substances Siloxanes

Production and polymerization

Safety and environmental considerations

See also

References

External links