3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane

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:

no potential to cause toxic effects if accumulated (in higher organisms) via the food chain

Additional information

3,3-Bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane (ViM4Q, CAS 60111-54-8) is very poorly soluble in water (<10 ng/l) and has a predicted log K_ow value of 9.0. The substance hydrolyses slowly in water (half-life >200 - 2000 h at pH 7, 1.9 – 6.6 h at pH 4, and 2.0 – 13.9 h at pH 9 and 20-25°C (QSAR).

Aquatic organisms are therefore likely to be exposed to extremely low dissolved concentrations of the substance in the environment.

In the sediment and soil compartments adsorption to particulate and organic matter will be high.

 

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to fulfil up to REACH Annex IX 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.

The behaviour of ViM4Q in the environment is going to be dominated by the high partition coefficient (log K_ow 9) and adsorption coefficient properties (log K_oc 6) of the substance.

ViM4Q and the substances used as surrogate for read-across are part of the Reconsile Siloxanes Category. Substances in this group 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 effects are seen with aquatic organisms due to the substance 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.

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 the following paragraphs the read-across approach for ViM4Q is assessed for the surrogate substances taking into account structure, hydrolysis rate and physico-chemical properties.

Read-across from D6 to ViM4Q (Sediment toxicity)

ViM4Q and the surrogate substance dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) are siloxanes with 5 silicon atoms and 6 silicon atoms respectively. The registration substance, ViM4Q, is a quaternary-branched structure of five Si atoms, with four terminal Si atoms linked to a central Si atom by Si-O-Si bonds. Each of the four terminal Si atoms is substituted by one vinyl and two methyl groups. The surrogate substance is a cyclic siloxane consisting of six Si atoms, each having two methyl branches and alternated by oxygen. ViM4Q and D6 have similar physicochemical properties: high molecular weight (432.89 and 444.92, respectively), low solubility (both insoluble at 7.0E-05 mg/l and 5.1E-03 mg/l, respectively), high log K_ow (9 and 8.87, respectively) and high log K_oc (6.0 and 5.9 respectively). Both substances have negligible biodegradability and hydrolysis rates. A comparison of the key physicochemical properties is presented in the table below.

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.

Given the similar properties and structural similarities, it is considered valid to read across sediment toxicity data from D6 to ViM4Q.

Read-across from D5 to ViM4Q (Sediment toxicity)

Decamethylcyclopentasiloxane (CAS 541-02-6, D5) is a cyclic siloxane with five Si having methyl groups and linked by oxygen atoms. The registered substance is a composite siloxane having a central siloxane with four oxygens each linking to another silicon with two methyl groups and a vinyl side chain. The registered substance has a total of 5 silicon atoms. Both the registered substance and D5 have similar physico-chemical properties: high molecular weight (432.89 and 370.78 respectively), high log K_ow (9 and 8.07 respectively) and high log K_oc (both 6). Both substances have negligible biodegradability and 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. The registered substance and the surrogate substance share similar physico-chemical properties but are not close structural analogues (branched and cyclic siloxanes). However, given the similar properties and structural similarities, it is considered valid to read-across data from D5 to ViM4Q.

Table: Summary of ecotoxicological and physico-chemical properties for the registered substance and the surrogate substances.

CAS Number	60111-54-8	541-02-6	540-97-6
Chemical Name	3,3-Bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane (ViM4Q)	Decamethylcyclopentasiloxane (D5)	Dodecamethylcyclehexasiloxane (D6)
Main ultimate Si hydrolysis product	Dimethylvinylsilanol^1	Dimethylsilanediol	Dimethylsilanediol
Molecular weight (parent)	432.89	370.78	444.92
Molecular weight (silanol hydrolysis product)	102.2	92.17	92.17
Log Kow (parent)	9.0	8.07	8.87
Log Kow (silanol hydrolysis product)	1.5	-0.38	-0.38
Water solubility (parent)	<10 ng/l	0.017 mg/l	0.0051 mg/l
Water solubility (silanol hydrolysis product)	n/a	1.0E+06 mg/l	1.0E+06 mg/l
Vapour pressure (parent)	<10 Pa	33.2 Pa	4.7 Pa
Vapour pressure (hydrolysis product)	n/a	7 Pa	7 Pa
Hydrolysis t1/2 at pH 7 and 25°C	>200 h	1590 h	>1 year
Hydrolysis t1/2 at pH 4 and 25°C	1.9 h	9.3 h
Hydrolysis t1/2 at pH 9 and 25°C	2.0 h	24.8 - 31.6 h
Short-term toxicity to fish (LC50)	>LoS (>14.2 ng/l)	n/a	n/a
Short-term toxicity to aquatic invertebrates (EC50)	>LoS (>19 ng/l)	n/a	n/a
Algal inhibition (ErC50 and NOEC)	>LoS (>15 ng/l)	ErC50: >2.0 μg/l; NOEC: ≥2.0 μg/l	EC50: >2.0 μg/l; NOEC: ≥2.0 μg/
Long-term toxicity to fish (NOEC)	n/a	≥4.4 μg/l	≥4 μg/l(supporting, as bioaccumulation study)
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	≥4.6 µg/l	≥4.6 µg/l
Long-term toxicity to sediment organisms (NOEC)	n/a	70 mg/kg dwt, C. riparius; ≥1272 mg/kg dwt L. 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/EC50)	n/a	(IC50) 209 mg/kg dwt, H. vulgare; >4054 mg/kg dwt,  T. pratense	n/a
Long-term terrestrial toxicity (NOEC)	≥1000 mg/kg dwt, Eisenia fetida; EC25 and EC50 >100 mg/kg dwt, soil microorganisms	≥4074 mg/kg dwt,  E. Andrei; 377 mg/kg dwt,  F. candida	n/a

Notes

LoS = limit of solubility

1 – Silicic acid is also generated by the hydrolysis of the registration substance

 

 

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_ow 1.1. Short-term toxicity data for the substance report L(E)C₅₀ values in the range >89-191 mg/l (Dow Corning 1979, Hüls 1993 and MOEC 2006), which are consistent with non-polar narcosis as seen with other alkoxysilanes.

Conclusion on classification

The submission substance is not classified for the environment in the EU according to the regulation (EC) No 1272/2008 because no short-term toxicity effects have been reported at the limit of solubility. In addition, the substance has a log K_ow 9 and very low water solubility (<10 ng/l) and no structural alerts, therefore based on non-polar narcosis mechanism of toxicity, no toxicity is expected to be expressed at the limit of solubility.