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EC number: 208-762-8
CAS number: 540-97-6
The hydrolysis half-life of dodecamethylcyclohexasiloxane (D6, CAS
540-97-6) is >1 year at pH 7 and 25˚C. The water solubility of the
substance is low (5.13 µg/l) and the log Kow is
high (8.87). It is therefore likely that, under the exposure conditions
of the aquatic toxicity tests reported in the following sections, the
test organisms will have been predominately exposed to very low
concentrations of the registered substance.
In the sediment and soil compartments adsorption to particulate and
organic matter will be high.
ECHA Guidance Chapter R.7b (ECHA, 2017) states that “OECD recommends
testing parent compound for Disappearance Time 50 (DT50) >3
days, breakdown products for DT50 <1h and case-by-case basis
for anything in between.” Due to the slow hydrolysis half-life of D6,
the parent substance is therefore the relevant entity for chemical
In order to reduce testing, read-across is proposed to fulfil up to
REACH Annex X requirements for the registered substance from substances
that have similar structure and physicochemical properties.
Ecotoxicological studies are conducted in aquatic medium or in moist
environments; therefore, the hydrolysis rate of the substance is
particularly important since after hydrolysis occurs the resulting
product has different physicochemical properties and structure.
In aqueous media, D6 hydrolyses very slowly in water (half-life >1 year
at pH 7 and 25°C).
D6 and the substance used as surrogate for read-across, decamethylcyclopentasiloxane
(D5, CAS 541-02-6), are members of the Reconsile Siloxanes Category.
Substances in this category tend to have low water solubility, high
adsorption and partition coefficients and slow degradation rates in the
sediment compartment. For substances with a log Kow of 8 and
above no long-term toxicity effects are seen with aquatic organisms due
to the low water solubility limiting the bioavailability and uptake of
the substance. In the environment the substances will adsorb to
particulate matter and will partition to soil and sediment compartments.
Additional information is given in a supporting report (Siloxane
Category Report, PFA 2017) attached in Section 13 of the IUCLID dossier.
The Siloxane Category report (PFA, 2017) addresses the Assessment
Elements of the RAAF and, in addition to describing the siloxanes
category, forms the RAAF justification report.
In addition, in the following paragraphs the read-across approach
for D6 is assessed for the surrogate substance taking into
account structure, hydrolysis rate and physico-chemical properties.
Read-across from decamethylcyclopentasiloxane (D5, CAS 541-02-6) to
dodecamethylcyclohexasiloxane (D6, CAS 540-97-6):
The registered substance, dodecamethylcyclohexasiloxane (D6, CAS
540-97-6) and the surrogate substance decamethylcyclopentasiloxane (D5,
CAS 541-02-6) are both members of the siloxanes category and both are
cyclic siloxanes. D6 is a cyclic siloxane with six dimethylated silicon
atoms linked by six oxygen atoms. D5 is a directly analogous structure
with five silicon and five oxygen atoms. The substances have similar
physicochemical properties: high molecular weight (445 and 370
respectively), low water solubility (0.0051 and 0.017 mg/l
respectively), high log Kow (8.87 and 8.07
respectively) and high log Koc (5.9 and 5.2
respectively). Both substances have negligible biodegradability and
similar slow hydrolysis rates.
No effects at the limit of solubility have been reported in short-term
and long-term aquatic studies conducted with both substances.
Environmental toxicity data for siloxanes are consistent with a
non-polar narcosis mechanism (Redman 2012). Given the similar
properties, structural similarities, and expected mode of action it is
considered valid to read-across data from decamethylcyclopentasiloxane
(D5) to dodecamethylcyclohexasiloxane (D6).
Long-term toxicity of D5 to fish (Oncorhynchus mykiss) data are
read across to the aquatic compartment.
Long-term toxicity of D5 to the sediment dwelling amphipod Hyalella
azteca data are read across to the sediment compartment.
Terrestrial toxicity testing according to OECD TG 222 and 216 has
also been conducted with the registration substance. No effects were
reported in the tests at the highest concentrations tested.
Table 1. Summary of ecotoxicological and physicochemical
properties for the registered substance and the surrogate substance
(aquatic and sediment toxicity)
Si hydrolysis product
Molecular weight (parent)
Molecular weight (hydrolysis product)
log Kow (parent)
log Kow (silanol hydrolysis product)
log Koc (parent)
Water sol (parent)
Water sol (silanol hydrolysis product))
Vapour pressure (parent)
Vapour pressure (hydrolysis product)
Hydrolysis t1/2 at pH 7 and 25°C
Short-term toxicity to fish (LC50)
Short-term toxicity to aquatic invertebrates (EC50)
Algal inhibition (ErC50 and NOEC)
ErC50: >12 μg/l; NOEC: ≥12 μg/l
EC50: >2.0 μg/l; NOEC: ≥2.0 μg/l
Long-term toxicity to fish (NOEC)
Long-term toxicity to aquatic invertebrates (NOEC)
Sediment toxicity (NOEC)
70 mg/kg dwt, C. riparius;≥1272 mg/kg dwt, L. variegatus;130 mg/kg dwt, H. azteca
<22 mg/kg dwt, C. riparius (disregarded study);
≥420 mg/kg dwt, L. variegatus;≥620 mg/kg dwt, C. riparius
Short-term terrestrial toxicity (L(EC)50)
(IC50 ) 209 mg/kg dwt, H. vulgare;>4054 mg/kg dwt, T. pratense
Long-term terrestrial toxicity (NOEC)
≥4074 mg/kg dwt, E. andrei;377 mg/kg dwt, F. candida
>1000 mg/kg dw, Eisenia fetida;
ECHA (2017). European Chemicals Agency. Guidance on Information
Requirements and Chemical Safety Assessment, Chapter R.7b: Endpoint
specific guidance. Version 4.0 June 2017.
PFA, 2017, Siloxane Category Report for Environmental Endpoints,
Redman, A. D., Mihaich, E., Woodburn, K., Paquin, P., Powell, D.,
McGrath, J. A. & Di Toro, D. M. (2012). Tissue-Based Risk Assessment of
Cyclic Volatile Methyl Siloxanes. Environmental Toxicology and
Chemistry 31, 1911–1919.
The substance has a reliable short-term EC50 value of
>2.0 μg/L in algae. It has a reliable NOEC value of ≥2.0 μg/l in algae
and reliable long-term NOEC values of ≥4.6 µg/L in Daphnia
and ≥14.4 μg/l in fish (the long-term fish data are read-across from an
The available short- and long-term aquatic toxicity data indicate
that there are no effects on aquatic organisms at the limit of
solubility of the substance in water. The substance hydrolyses slowly in
water and is not readily biodegradable.
These data are consistent with the following classification under
Regulation (EC) No 1272/2008 (as amended) (CLP):
Acute toxicity: Not classified.
Chronic toxicity: Not classified
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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