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Octamethylcyclotetrasiloxane
Chemical compound

Octamethylcyclotetrasiloxane, also called D4, is an organosilicon compound and one of a number of cyclic siloxanes (cyclomethicones). It is a colorless viscous liquid with a high boiling point. It has historically been used on a large scale in personal care products including cosmetics, hair conditioners and emollients (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 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).

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Production and polymerization

Commercially D4 is produced from dimethyldichlorosilane. Hydrolysis 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:4

[(CH3)2SiO]4n → n [(CH3)2SiO]4

D4 and D5 are also precursors to the polymer. The catalyst is again KOH.

Safety and environmental considerations

D4 is of low acute toxicity. 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.5

As the smallest cyclic dimethylsiloxane that does not experience considerable ring strain,6 D4 is one of the most abundant siloxanes in the environment, e.g. in landfill gases.7 D4 and D5 have attracted attention because they are pervasive. Cyclic siloxanes can be detected in some species of aquatic life.8 An independent, peer-reviewed study in the US found "negligible risk from D4 to organisms"9 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."10

In the European Union, D4 was characterized as a substance of very high concern (SVHC) due to its PBT and vPvB properties and was thus included in the candidate list for authorisation.11 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 2020.12 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.13 That decision prompted the State of Oregon to follow suit in 2018.14

See also

References

  1. Moretto, Hans-Heinrich; Schulze, Manfred; Wagner, Gebhard (2005). "Silicones". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_057. ISBN 978-3527306732. 978-3527306732 ↩

  2. Philippe P. Quevauviller; Patrick Roose; Gert Verreet (24 August 2011). Chemical Marine Monitoring: Policy Framework and Analytical Trends. John Wiley & Sons. p. 2010. ISBN 978-1-119-97759-9. 978-1-119-97759-9 ↩

  3. US EPA, OCSPP (29 October 2020). "Risk Evaluation for Octamethylcyclotetra- siloxane (D4)". www.epa.gov. https://www.epa.gov/assessing-and-managing-chemicals-under-tsca/risk-evaluation-octamethylcyclotetra-siloxane-d4 ↩

  4. Moretto, Hans-Heinrich; Schulze, Manfred; Wagner, Gebhard (2005). "Silicones". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_057. ISBN 978-3527306732. 978-3527306732 ↩

  5. Brooke DN, Crookes MJ, Gray D, Robertson S (2009). Environmental Risk Assessment Report: Octamethylcyclotetrasiloxane (PDF) (Report). Environment Agency. https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/290565/scho0309bpqz-e-e.pdf ↩

  6. Brook, Michael A. (2000). Silicon in Organic, Organometallic and Polymer Chemistry. New York: Wiley. p. 262. ISBN 978-0-471-19658-7. 978-0-471-19658-7 ↩

  7. Franz-Bernd Frechen (2009). Odours and VOCs: Measurement, Regulation and Control Techniques. kassel university press GmbH. p. 287. ISBN 978-3-89958-609-1. 978-3-89958-609-1 ↩

  8. 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, doi:10.1016/j.chemosphere.2012.10.041. /wiki/Doi_(identifier) ↩

  9. Josie B. Nusz, Anne Fairbrother, Jennifer Daley, G. Allen Burton (2018). "Use of multiple lines of evidence to provide a realistic toxic substances control act ecological risk evaluation based on monitoring data: D4 case study". Science of the Total Environment. 636: 1382–1395. Bibcode:2018ScTEn.636.1382N. doi:10.1016/j.scitotenv.2018.04.335. PMID 29913599.{{cite journal}}: CS1 maint: multiple names: authors list (link) https://doi.org/10.1016%2Fj.scitotenv.2018.04.335 ↩

  10. "Cyclic volatile methyl siloxanes: Environment tier II assessment" Archived 2020-05-27 at the Wayback Machine Australian Department of Health. Retrieved 2018-12-27. https://www.nicnas.gov.au/chemical-information/imap-assessments/imap-assessments/tier-ii-environment-assessments/cvms ↩

  11. "Candidate List of substances of very high concern for Authorisation – Octamethylcyclotetrasiloxane". ECHA. Retrieved 2018-07-18. https://echa.europa.eu/candidate-list-table/-/dislist/substance/external/100.008.307 ↩

  12. "Commission Regulation (EU) 2018/35 of 10 January 2018 amending Annex XVII to Regulation (EC) No 1907/2006 of the European Parliament and of the Council concerning the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) as regards octamethylcyclotetrasiloxane ('D4') and decamethylcyclopentasiloxane ('D5')". Retrieved 2018-07-18. https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:32018R0035 ↩

  13. "Concise Explanatory Statement: Children's Safe Products Reporting Rule" State of Washington Department of Ecology. Retrieved 2018-12-27. https://fortress.wa.gov/ecy/publications/documents/1704034.pdf ↩

  14. "Notice of Proposed Rulemaking" State of Oregon. Retrieved 2018-12-09. https://www.oregon.gov/oha/PH/HEALTHYENVIRONMENTS/HEALTHYNEIGHBORHOODS/TOXICSUBSTANCES/Documents/333-016_Notice%20of%20Rulemaking%20-%20TFK%20Rule%20Revision.pdf?mkt_tok=eyJpIjoiTjJFek5UWXpZakEwT1RnNSIsInQiOiI2dWszZlFPNm5oK0ZKMG5vVTdUalJCZDhRMDk5Zys5NDFCUW1VTU1cL3pkUXVFZHQ2ZjBhbVQreFNjd0RaakM2NTVOdTFtMTBCWWFXZmhMbUJBQzIzYXR5aTUzaWRTREc0T1dIZUxpYjZpdDRZN01pdXRhZlN5UGhMVDNNbWNKenQifQ%3D%3D ↩