Dodecamethylcyclohexasiloxane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

13.5 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

1.35 mg/kg sediment dw

Assessment factor:

100

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

66.7 mg/kg food

Assessment factor:

300

Additional information

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 K_ow 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 safety assessment.

 

READ-ACROSS JUSTIFICATION

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 K_ow 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 K_ow (8.87 and 8.07 respectively) and high log K_oc (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)

CAS Number	541-02-6	540-97-6
Chemical Name	Decamethylcyclopentasiloxane (D5)	Dodecamethylcyclohexasiloxane (D6)
Si hydrolysis product	Dimethylsilanediol	Dimethylsilanediol
Molecular weight (parent)	370.78	444.93
Molecular weight (hydrolysis product)	92.17	92.17
log Kow (parent)	8.07	8.87
log Kow (silanol hydrolysis product)	-0.41	-0.38
log Koc (parent)	5.2	5.9
Water sol (parent)	0.017 mg/l	0.0051 mg/l
Water sol (silanol hydrolysis product))	1.0E+06 mg/l	1.0E+06 mg/l
Vapour pressure (parent)	33.2 Pa	5 Pa
Vapour pressure (hydrolysis product)	7 Pa	7 Pa
Hydrolysis t1/2 at pH 7 and 25°C	1590 h	>1 year
Short-term toxicity to fish (LC50)	>16 µg/l	n/a
Short-term toxicity to aquatic invertebrates (EC50)	>2.9 µg/l	n/a
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)	≥14 μg/l	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	≥15 µg/l	≥4.6 µg/l
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	n/a
Long-term terrestrial toxicity (NOEC)	≥4074 mg/kg dwt, E. andrei; 377 mg/kg dwt, F. candida	>1000 mg/kg dw, Eisenia fetida; >100 mg/kg dw, soil microflora

 

References

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, PFA.404.114.001.

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.

Conclusion on classification

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 analogous substance).

 

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