Silicones, or polysiloxanes, are inert synthetic polymers consisting of repeated units of silicon (Si) atoms, oxygen atoms (O), and R organic substituent (R2SiO), occurring in linear or cyclic forms. Based on their polymer length, particle size, and molecular weights, siloxanes can be categorised into different groups: fluids, elastomers and resins. Since the 1940s, siloxane materials are used in many applications from household, medical, and electronic to cosmetic and beauty products.
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Siloxane toxicity
Generally, siloxanes are well tolerated by human organisms. However, the degree of polymerisation, format of the polymer (linear or cyclic), and molecular weight can affect the toxicity of this group of chemicals. Furthermore, impurities like volatile organic compound (VOC) or allyl alcohol ethoxylates remaining from the polymerisation manufacturing process would be another safety concern for these types of ingredients.
Among the large family of siloxanes, cyclomethicone is a generic name for a cyclic siloxane with the chemical name Octamethylcyclotetrasiloxane, with a general formulation of (CH3)2n On Sin, where n=3-7. Examples include D5 (n=5, Decamethylcyclopentasiloxane, INCI:Cyclopentasiloxane), and D4 (n=4, Octamethylcyclotetrasiloxane, INCI: Cyclotetrasiloxane). D5 is a volatile ingredient with low toxicity (Acute LD50>20g/kg bw, slight irritant to the eyes and skin, with low skin absorption of 0.06%).
These ingredients have wide applications in cosmetic products such as emollients, humectant, solvent, viscosity control and hair conditioning.
In 2010, the Scientific Committee on Consumer Safety (SCCS) published an opinion (SCCS/1241/10) on the materials’ safety: “…cyclomethicone (D4, D5) does not pose a risk for human health when used in cosmetic products”.
While assessing D5’s safety, the SCCS concluded that the overall aggregate exposure to D4 was less than 5 µg/kg bw/day, which provides a margin of safety of > 3500.
In the re-assessment published in 2016 (SCCS/1549/15), the SCCS indicated the use of D5 in cosmetic products is safe, except in cases where inhalation exposure is a risk, like hair styling aerosols and sun care spray products. Hence, it is recommended that impurities of D4 in D5 should be kept as low as possible (D5 purity > 99%).
REACH regulation on D4/D5
The Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulation assessed the potential environmental effect of D4/D5. Based on the evaluation, D4 meets the Annex XIII criteria for identification as a persistent, bioaccumulative and toxic (PBT) and very persistent, very bioaccumulative (vPvB) substance, and D5 meets the criteria for a vPvB substance.
Harmonised hazard classification of D4
Index No. | International Chemical Identification | Classification | |
Hazard Class & Category Codes | Hazard statement codes | ||
014-018-00-1 | Octamethylcyclotetrasiloxane (D4) |
Repr. 2 Aquatic Chronic 4 |
H361f: Suspected of damaging fertility H413: May cause long lasting harmful effects to aquatic life |
In January 2018, the EU commission has published an update amending Annex XVII to Regulation (EC) No 1907/2006 of REACH. After 31st January 2020, the concentration of D4/D5 in cosmetic products that are rinsed off after application (“wash-off”) placed on the market should be less than 0.1% by weight of either substance.
Furthermore, the European commission has requested The European Chemical Agency (ECHA) to prepare a new dossier regarding restriction of D4/D5 in leave-on consumer product. The scope of this restriction will include products containing D4/D5 concentration of more than 0.1%. It is expected that ECHA will publish the final dossier in April 2018.
Read additional regulatory articles
Resources for further reading:
- COMMISSION REGULATION (EU) 2018/35 of 10 Jan 2018
- Mojsiewicz-Pieńkowska K Et al, Front Pharmacol. 2016; 7: 132. Direct Human Contact with Siloxanes (Silicones) – Safety or Risk Part 1. Characteristics of Siloxanes (Silicones)
- ANNEX XV: RESTRICTION REPORT PROPOSAL FOR A RESTRICTION of D4/D5
- Scientific Committee on Consumer Safety (SCCS/1549/15 Final version of 29 July 2016) opinion on decamethylcyclopentasiloxane (cyclopentasiloxane, D5) in cosmetic products [PDF]
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it seems Ethyl Macadamiate, which is derived from Macadamia oil with varying viscosities & low reflective index, is the best D4/D5 alternative. I greatly appreciate your help.