2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.016 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.16 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.002 mg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

7.1 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

0.064 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.006 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.004 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

No reliable ecotoxicity data are available for the registered substance. The required endpoints are therefore addressed by read-across of good quality data for the related substances[3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1) and 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane (CAS 10127-34-2).

The hydrolysis half-life of 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane is 4 h at pH 7, 20-25⁰C. The registered substance will hydrolyse in contact with water and atmospheric moisture to 2-(3,4-epoxycyclohexyl)ethylsilanetriol and methanol. REACH guidance (ECHA 2012, 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 ECHA guidance, (EC 2003, ECHA 2012) 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. Therefore, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance. As described in Sections 1.3 and 4.1 of the CSR (4.8 and 5.1.2 of IUCLID), the silanol hydrolysis products may be susceptible to condensation reactions as well as subsequent hydrolysis reactions of the epoxide functional group.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil REACH Annex VII-VIII 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, because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour. The registered substance and the substance used as surrogate for read-across are part of a class of epoxyalkyl alkoxysilanes in which it is possible that the epoxide functional group may act by a specific mechanism of toxicity, particularly known to be relevant in fish and invertebrates. The registered substance hydrolyses rapidly in water and therefore the selection of surrogate substance is based on log Kow of the resulting silanols and the epoxide chemical groups present in them.

In the following paragraphs the read-across approach is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties, presented in the Table below. Additional information is given in a supporting report (PFA 2013ah) attached in Section 13 of the IUCLID 5 dossier.

Table: Physicochemical parameters and ecotoxicity data for the registered and surrogate substances

CAS Number	3388-04-3	2897-60-1	10127-34-2
Chemical Name	2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane	[3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane	2-(3,4-epoxycyclohexyl)ethyltriethoxysilane
Si hydrolysis product	2-(3,4-epoxycyclohexyl)ethylsilanetriol	[3-(2,3-epoxypropoxy)propyl]methylsilanediol	2-(3,4-epoxycyclohexyl)ethylsilanetriol
Molecular weight (parent)	246.38	248.40	288.46
Molecular weight (hydrolysis product)	204.30	192.29	204.30
log Kow(parent)	2.5	2.7	3.9 (predicted, no reliable measurements available)
log Kow(silanol hydrolysis product)	-0.6	-0.7	-0.6
Water sol (parent)	1400 mg/l	1200 mg/l	830-860 mg/l
Water sol (silanol hydrolysis product))	1E+06 mg/l (limited by condensation reactions)	1E+06 mg/l (limited by condensation reactions)	1E+06 mg/l (limited by condensation reactions)
Vapour pressure (parent)	0.46 Pa	0.42 Pa	0.3 Pa
Vapour pressure (hydrolysis product)	7.5E-06 Pa	0.011 Pa	7.5E-06 Pa
Hydrolysis t1/2at pH 7 and 25°C	4 h	11.7 h	31 h
Hydrolysis t1/2at pH 4 and 25°C	0.2 h	0.4 h	0.7 h
Hydrolysis t1/2at pH 9 and 25°C	0.1 h	0.2	0.4 h
Short-term toxicity to fish (LC50)	n/a	180 mg/l (nominal) (study reflects effects of mixture of parent and silanol. Result expressed in terms of silanol = 139 mg/l)	42 mg/l (study reflects effects of parent)
Short-term toxicity to aquatic invertebrates (EC50)	n/a	20 mg/l (nominal) (study reflects effects of mixture of parent and silanol. Result expressed in terms of silanol = 15.5 mg/l)	58 mg/l (study reflects effects of parent)
Algal inhibition (ErC50and NOEC)	n/a	ErC50 >32 mg/l; NOEC 3.2 mg/l (nominal) (study reflects effects of mixture of parent and silanol. ErC50 result expressed in terms of silanol = >25 mg/l)	ErC50 >108 mg/l; NOEC 6 mg/l (study reflects effects of mixture of parent and silanol. ErC50 result expressed in terms of silanol = >76 mg/l)
Long-term toxicity to fish (NOEC)	n/a	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a	16 mg/l (geo mean measured as parent; 26 mg/l nominal) (study reflects effects of mixture of parent and silanol. Result expressed in terms of silanol = 18 mg/l based on nominal loading at NOEC)
Long-term sediment toxicity (NOEC)	n/a	n/a
Short-term terrestrial toxicity (L(E)C50)	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	n/a

• Read-across from [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane to 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane

2-(3,4-Epoxycyclohexyl)ethyltrimethoxysilane (CAS 3388-04-3) and [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane (CAS 2897-60-1) both hydrolyse rapidly in contact with water (4 h at pH 7, 20-25⁰C, and 11.7 h at pH 7, 25⁰C respectively). The organosilicon hydrolysis products are 2-(3,4-epoxycyclohexyl)ethylsilanetriol and [3-(2,3-epoxypropoxy)propyl]methylsilanediol respectively. Both parent substances and both initial hydrolysis products mentioned above have one epoxide functional group present in the structure. The differing alkyl group conformations with a cyclohexyl structure in the registration substance and its silanol hydrolysis product, and presence of a second methyl substituent in the surrogate substance and its silanol hydrolysis product, are considered to be not significant contributors of toxicity in themselves. They share similar physicochemical properties: high water solubility, low log Kow (-0.6 and -0.7 respectively) and low molecular weight (204.3 and 192.3 respectively). Short-term toxicity to fish, invertebrates and algae endpoints are read across from [3-(2,3-epoxypropoxy)propyl]diethoxy(methyl)silane to 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane. E(L)C50 values of 180, 20 and 32 mg/L, respectively, have been determined (139, 15.5 and 25 mg/l when expressed in terms of silanol hydrolysis product of the tested substance).

• Read-across from [2-(3,4-epoxycyclohexyl)ethyl]triethoxysilane to 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane

2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (CAS 3388-04-3) and [2-(3,4-epoxycyclohexyl)ethyl]triethoxysilane (CAS 10217-34-2) both hydrolyse in contact with water but at significantly different rates (4 h at pH 7, 20-25⁰C, and 31 h at pH 7, 25⁰C respectively). At elevated pH, the hydrolysis rates of both substances are very rapid (0.1 h at pH 9, 20-25⁰C and 0.4 h at pH 9, 20-25⁰C respectively). The organosilicon hydrolysis product is 2-(3,4-epoxycyclohexyl)ethylsilanetriol in both cases. The structural side chain is identical and contains one epoxide functional group present in the structure. The parent structures are dissimilar in terms of physicochemical properties: whilst both share high water solubility, the moderate log Kow (2.5 for the registered substance and 3.9, estimated for the surrogate substance respectively). The long-term toxicity to invertebrates endpoint is read across from 2-(3,4-epoxycyclohexyl)ethyltriethoxysilane to 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane. The semistatic exposure regime at pH ~7.7-8.1 with media renewal periods of 48-72 h means the results observed are considered to be partially attributable to the silanol hydrolysis product. A NOEC value of 26 mg/L (nominal), has been determined (16 mg/l when expressed in terms of silanol hydrolysis product of the tested substance). However at Annex VIII this endpoint is not required in the REACH data set.

• Considerations on the non-silanol hydrolysis products:

Methanol and ethanol are well characterised in the public domain literature and are not hazardous at the concentrations relevant to the studies; the short-term EC50 and LC50 values for these substances are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol, CAS 67-56-1, OECD 2004b - SIDS for ethanol, CAS 64-17-5). Therefore, at the loading rates experienced in these tests it is unlikely that the presence of either would significantly affect the results of the tests.

There are no reliable microorganism toxicity data available for 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (CAS 3388-04-3), therefore good quality data for the structurally-related substance, [2-(3,4-epoxycyclohexyl)ethyl]triethoxysilane (CAS 010217-34-2), have been read across. The justification for read across is discussed in the IUCLID Section 6.1.7 Endpoint Summary.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀values of 180 mg/l in fish, 20 mg/l in invertebrates and >32 mg/l in algae based on read-across from a structural analogue (139 mg/l, 15.5 mg/l and >25 mg/l respectively based on the hydrolysis product of the tested substance, [3-(2,3-epoxypropoxy)propyl]methylsilanediol). A reliable NOEC 2.5 mg/l in algae is also read across.

The substance hydrolyses rapidly in water and is not readily biodegradable.

The hydrolysis product 2-(3,4-epoxycyclohexyl)ethylsilanetriol is susceptible to further hydrolysis reactions and the ultimate hydrolysis product [2-(3,4-dihydroxycyclohexyl)ethyl]silanetriol is considered unlikely to exhibit significant ecotoxic effects based on QSAR estimated E(L)C₅₀s >>100 mg/l. The rate of reaction under environmental conditions is uncertain but the half-life is not expected to be more than a few days. Therefore it may be considered whether [3-(2,3-epoxypropoxy)propyl]methylsilanediol meets the criteria of a ‘rapidly degradable’ substance in the context of classification and labelling for the environment. In the absence of definitive evidence, a precautionary approach has been applied.

These data are consistent with the following classification under Regulation (EC) No. 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Category Chronic 3.