2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

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:

3 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.3 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:

3.33 mg/kg food

Assessment factor:

90

Additional information

2,4,6,8-Tetramethyl-2,4,6,8-tetravinyltetrasiloxane (Vi4-D4, CAS 2554-06-5) hydrolyses slowly to form methylvinylsilanediol. The half-life value of 63 hours at pH 7, 0.8 h at pH 9, and 1.1 h at pH 4 and 20 – 25°C is applicable to hydrolysis of the parent substance but complete reaction to methylvinylsilanediol will take longer. The water solubility of the substance is low (0.0073 – 0.0088 mg/l at 23°C), the measured vapour pressure is 93.5 Pa at 25°C, and the log K_ow is high (6.5).

REACH Guidance (ECHA 2017, R.7b) indicates that the parent substance should be tested if the DT₅₀ is greater than 3 days, or on a case by case basis if between 1 hour and 3 days. REACH guidance (ECHA 2016, R.16) states that “for substances where hydrolytic DT₅₀ is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. TGD and REACH guidance (EC 2003) and R.16 (2016) also suggest that when the hydrolysis half-life is less than 12 hours, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity.

Due to the slow hydrolysis half-life of the registration substance, the environmental Chemical Safety Assessment is therefore based on the properties of the parent substance rather than the hydrolysis products.

 

Due to the low water solubility and high log K_ow, aquatic organisms are likely to be exposed to only very low dissolved concentrations of the substance in the environment.

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

It is likely that, under the semi-static and flow-through exposure conditions of the long-term Daphnia and long and short-term fish studies, respectively, the test organisms will have predominantly been exposed to the parent substance. In the static short-term invertebrate study and algal test it is likely that exposure will have been initially to the parent substance followed by a mixture of parent substance and its hydrolysis products.

The registration substance has an average purity of >70% 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4), with <20% 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane Vi5-D5 (CAS 17704-22-2; Impurity 1) and <10% 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane Vi3-D3 (CAS 3901-77-7; Impurity 2) present as impurities. For each endpoint, the 3 components are considered separately.

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 2,4,6,8-tetramethyl-2,4,6,8-tetravinyltetrasiloxane (Vi4-D4) in the environment is dominated by the high partition coefficient (log K_ow 6.5) and adsorption coefficient properties (log K_oc 4.1) of the substance.

In the following paragraphs the read-across approach for Vi4-D4 and its impurities are assessed taking into account structure and physico-chemical properties. Vi4-D4 and the substances used as surrogates are siloxanes. Substances in this group tend to have slow hydrolysis rates, low water solubility, high log K_ow, high adsorption coefficients and a tendency for high persistence in the sediment compartment.

The physico-chemical and ecotoxicological properties and data of the registered and surrogate substances are summarised in the table below.

Sediment toxicity: Read-across from octamethylcyclotetrasiloxane (D4) to Vi4-D4

Octamethylcyclotetrasiloxane (D4, CAS 556-67-2) is a cyclic siloxane made up of 4 Si with methyl groups, linked by oxygen atoms. Vi4-D4 is a cyclic siloxane made up of 4 Si with methyl and vinyl groups, linked by oxygen atoms. Vi4-D4 and D4 have slow hydrolysis rates (63 h at pH 7 and 20-25°C (predicted) and 69-144 h at pH 7 and 25°C, respectively) and similar physico-chemical properties: high molecular weight (MW 344.7 and 296.6 respectively), low water solubility (0.056 mg/l and 0.0073 – 0.0088 mg/l respectively) high log K_ow (both 6.5) and high log K_oc (both close to 4). The environmental behaviour and toxicity is driven by the low water solubility, high log K_ow and high log K_oc, therefore read-across of sediment toxicity data is considered to be appropriate.

Sediment toxicity data with Corophium volutator and Hyalella azteca are available with Vi4-D4. Data with Lumbriculus variegatus are read across from D4. The lowest sediment toxicity value is with L. variegatus where a NOEC of 13 mg/kg dwt in natural sediment is reported.

Aquatic, sediment and terrestrial toxicity: Read-across from decamethylcyclopentasiloxane (D5) to Impurity 1, Vi5-D5

Decamethylcyclopentasiloxane (D5, CAS 541-02-6) is a cyclic siloxane made up of 5 Si with methyl groups, linked by oxygen atoms. Impurity 1, Vi5-D5 is a cyclic siloxane made up of 5 silicon atoms with methyl and vinyl groups, linked by oxygen atoms. Vi5-D5 and D5 have slow hydrolysis rates (1600 h at pH 7 and 20-25°C, predicted and 1590 h at pH 7 and 25°C respectively) and similar physico-chemical properties: high molecular weight (MW 431 and 370.8 respectively), low water solubility (9.1E-06 mg/l and 0.017 mg/l respectively) high log K_ow (9.0 and 8.0 respectively) and high log K_oc (6 and 5.2 respectively). D5 and Vi5-D5 are structural analogues with very similar properties. The environmental behaviour and toxicity is driven by the water solubility, high log K_ow and high log K_oc, therefore read-across of aquatic, sediment and terrestrial toxicity data is considered to be appropriate. The aquatic data indicates that D5 is not toxic to aquatic organisms in the long-term up to the limit of solubility. The lowest sediment toxicity study is with C. Riparius where a NOEC of 70 mg/kg dwt in artificial sediment is reported. The lowest terrestrial toxicity study result is with F. candida where a NOEC of 377 mg/kg dry weight is reported.

Aquatic toxicity: Read-across from hexamethylcyclotrisiloxane (D3) to Impurity 2, Vi3-D3

Hexamethylcyclotrisiloxane (D3, CAS 541-05-9) is a cyclic siloxane made up of 3 Si with methyl groups, linked by oxygen atoms. Vi3-D3 is a cyclic siloxane made up of 3 silicon atoms with methyl and vinyl groups, linked by oxygen atoms. Vi3-D3 and D3 have rapid hydrolysis rates (0.6 h at pH 7 and 20-25°C, predicted, and 23 minutes at pH 7 and 25°C, measured, respectively) and similar physico-chemical properties: similar molecular weight (MW 259 and 223 respectively), low water solubility (0.13 mg/l and 2 mg/l respectively) and high log K_ow (6.7 and 4.4 respectively). D3 and Vi3-D3 are structural analogues with very similar properties. The environmental behaviour and toxicity is driven by the water solubility and log K_ow, therefore read-across of aquatic toxicity data is considered to be appropriate.

Aquatic toxicity: Read-across from trimethoxyvinylsilane to Vi4-D4

The registration substance has an average purity of 70% Vi4-D4, with <20% 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane Vi5-D5 (CAS 17704-22-2; Impurity 1) and <10% 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane Vi3-D3 (CAS 3901-77-7; Impurity 2) present as impurities. Data with decamethylcyclopentasiloxane (D5, CAS 541-02-6) and hexamethylcyclotrisiloxane (D3, CAS 541-05-9) are read across to assess the toxicity of the impurities. Vi4-D4, Vi5-D5 and Vi3-D3 contain a vinyl side-chain whereas D5 and D3 do not. To provide evidence that the vinyl side chain does not contribute significantly to the ecotoxicity of the impurities of the registered substance, supporting read-across from trimethoxyvinylsilane (CAS 2768-02-7) has been used. From the available dataset, the vinyl fragment of Vi5-D5 and Vi3-D3 is understood to not have any specific mode of action with respect to ecotoxicity; data with trimethoxyvinylsilane (CAS 2768-02-7) indicate the substance is of low short-term toxicity with L(E)C₅₀ values in the range >89-191 mg/l, which are consistent with non-polar narcosis as seen with other alkoxysilanes.

Trimethoxyvinylsilane is used to read-across to short-term toxicity to fish, invertebrates and algae endpoints. E(L)C₅₀ values of 191, 169 and >89 mg/l, respectively, have been determined.

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 (MoE Japan 2006, Hüls 1993 and Dow Corning 1979), which are consistent with non-polar narcosis as seen with other alkoxysilanes.

 

Table: Summary of physicochemical and ecotoxicological properties for the registered substance, its impurities and their surrogate substances.

CAS Number	2554-06-5	556-67-2	2768-02-7	17704-22-2	541-02-6	3901-77-7	541-05-9
Chemical Name	2,4,6,8 -Tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane	Octamethylcyclotetrasiloxane (D4)	Trimethoxy(vinyl)silane	2,4,6,8,10 -Pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Impurity 1 (Vi5-D5))	Decamethylcyclopentasiloxane (D5)	2,4,6 -Trimethyl-2,4,6-trivinylcyclotrisiloxane (Impurity 2 (Vi3-D3))	Hexamethylcyclotrisiloxane (D3)
Ultimate Si hydrolysis product	Methylvinylsilanediol	Dimethylsilanediol	Vinylsilanetriol	Methylvinylsilanediol	Dimethylsilanediol	Methylvinylsilanediol	Dimethylsilanediol
Molecular weight (parent)	344.66	296.62	148.2	431	370.8	259	223
Molecular weight (hydrolysis product)	104.18	92.17	106.2	104.18	92.17	104.18	92.17
log Kow(parent)	6.47	6.49	1.1	9.0	8.07	6.7	4.4
Water sol (parent)	0.0073 – 0.0088 mg/l at 23°C	0.056 mg/l	l9400 mg/l	9.1E-06 mg/l	0.017 mg/l	0.13 mg/l	2 mg/l
Vapour pressure (parent)	93.5 Pa	132 Pa	1190 Pa	0.6 Pa	33 Pa	67 Pa	671 Pa
Hydrolysis t1/2 at pH 7 and 25°C	approximately 63 hours	69-144 hours	0.2 h (at 20⁰C)	1600 hours	1590 hours	0.6 hours	23 minutes
Hydrolysis t1/2 at pH 4 and 25°C	1.1 hours	1.8 hours	0.04 h	3.3 hours (pH5)	9.3 hours	0.1 hours	2 minutes
Hydrolysis t1/2 at pH 9 and 25°C	0.8 hours	0.7-0.8 hours	0.004 h	11 hours	24.8-31.6 hours	0.02 hours	0.4 minutes
Short-term toxicity to fish (LC50)	LL50 >1000 mg/l loading rate (WAFs), equivalent to >7.3 mg/l*	10 µg/l	191 mg/l	n/a	>16 µg/l (>LOS)	n/a	>1.6 mg/l (>LOS)
Short-term toxicity to aquatic invertebrates (EC50)	EL50 272 mg/l loading rate (WAFs) *	15 μg/l	169 mg/l	n/a	>2.9 µg/l (>LOS)	n/a	>1.6 mg/l (>LOS)
Algal inhibition (ErC50 and NOEC)	EL50 (70.5 h): >988 mg/l NOELR: ≥988 mg/l*	>22 µg/l	ErC50 >89 mg/l; NOEC ≥89 mg/l	n/a	96-h EC50 >12 µg/l and 96-h NOEC ≥12 µg/l. (>LOS)	n/a	>1.6 mg/l (>LOS)
Long-term toxicity to fish (NOEC)	≥0.036 µg/l	≥4.4 μg/l	n/a	n/a	≥14 µg/l (>LOS)	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	0.00378 mg/l	NOEC 7.9 µg/l	n/a	n/a	≥15 µg/l (>LOS)	n/a	n/a
Sediment toxicity (NOEC)	≥100 mg/kg dry weight, Hyalella azteca	13 mg/kg dry weight,Lumbriculus variegatus; 44 mg/kg dry weight,Chironomus riparius	n/a	n/a	Chironomus riparius 70 mg/kg dry weight	n/a	n/a
Terrestrial toxicity (NOEC or EC50)	n/a	n/a	n/a	n/a	F. Candida 377 mg/kg dry weight	n/a	n/a

 * The studies have been conducted at loading rates above the solubility limit of the substance. The studies indicate that the substance is not toxic at the limit of solubility.

Conclusion on classification

The substance has reliable short-term E(L)L₅₀ values of >1000 mg/l in fish, 272 mg/l in invertebrates and >988 mg/l in algae. It has a reliable NOELR of ≥988 mg/l in algae and reliable long-term NOECs of ≥0.036 µg/l in fish and ≥0.00378 mg/l in Daphnia.

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.