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:

PNEC STP

PNEC value:

1 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

13 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

1.3 mg/kg sediment dw

Assessment factor:

500

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

3.77 mg/kg soil dw

Assessment factor:

100

Extrapolation method:

assessment factor

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) is >1 year at pH 7 and 25˚C. The water solubility of the substance is low (ca. 5 µg/l) and the log K_owis high (8.87). It is therefore likely that, under the flow-through exposure conditions of the aquatic toxicity tests reported in the following sections, that the test organisms will have been predominately exposed to very low concentrations of the registered substance.

READ-ACROSS JUSTIFICATION

In order to reduce animal 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 are part of a class of compounds called siloxanes. Substances in this group tend to have low water solubility, high adsorption and partition coefficientsand slow degradation in the sediment compartment. For substances with a log K_owof 8 and above no long-term toxicity effects are seen with aquatic organisms due to the low water solubility limiting the effects seen. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

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.

Additional information is given in a supporting report (PFA, 2015i) attached in Section 13 of the IUCLID 5 dossier.

Consideration of low functionality side chains and reactive groups

Vinyl group: From the available dataset in the whole of the silanes and siloxanes database, vinyl fragments are understood to not have any specific mode of action with respect to ecotoxicity. For example, trimethoxyvinylsilane (CAS 2768-02-7) is an alkoxysilane with vinyl groups and a log K_ow1.1. Short-term toxicity data for the substance report L(E)C₅₀values in the range 169-210 mg/l (Dow Corning 1979 and Hüls 1993), which are consistent with non-polar narcosis as seen with other alkoxysilanes.

Phenyl group: The toxicity of phenyls is also related to non-polar narcosis. The toxicity of substances with phenyl groups increases with increasing log K_ow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log K_owof the hydrolysis product is -0.2. Short-term toxicity data with trichlorophenylsilane determine no effects up to 100 mg/l (Springborn Smithers 2009a, b and c).

Si-H reaction: Some of the substances can undergo a Si-H reaction. For example, trimethoxysilane (CAS 2487-90-3) is a substance that undergoes strong Si-H bond reaction and with a log K_owvalue 0.2 for the parent and -2.9 for hydrolysis product. Short-term toxicity data with trimethoxysilane determine no effects up to 100 mg/l (Springborn Smithers 2004a, b and c). This indicates that the Si-H further reaction does not have any specific mode of action with respect to ecotoxicity. This conclusion is relevant for siloxanes as well as alkoxysilanes such as trimethoxysilane.

Read-across from decamethylcyclopentasiloxane (D5) to dodecamethylcyclohexasiloxane (D6):

The registered substance, dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) and the surrogate substance decamethylcyclopentasiloxane (D5, CAS 541-02-6) 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 (both insoluble, at 0.0051 and 0.017 mg/l respectively), high log K_ow(8.87 and 8.02 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 studies in other trophic levels conducted with both substances. Environmental toxicity data for siloxanes are consistent with a non-polar narcosis mechanism (Redman 2012,Peter Fisk Associates 2013bc). 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).

Table 1. Summary of ecotoxicological and physicochemical properties for the registered substance and the surrogate substance(aquatic and terrestrial 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)	8.02	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 t1/2at 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 (ErC50and NOEC)	ErC50: >12 μg/l; NOEC: ≥12 μg/l	EC50: >2.0 μg/l; NOEC: ≥2.0 μg/
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 dwtL. variegatus 130 mg/kg dwt, H. azteca	<22 mg/kg dwt,C. riparius ≥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	n/a

 

Conclusion on classification

D6 is not classified in the EU for environmental hazards according to Regulation (EC) No 1272/2008, as amended. The substance was found to be non-toxic at its functional water solubility limit in long-term studies with fish (using a surrogate test substance),Daphniaand algae.