Triethoxysilane

Endpoint summary

Additional information

Data for the registered substance triethoxysilane (CAS 998-30-1) are only available for invertebrates. Acute toxicity to invertebrates has been investigated for triethoxysilane (CAS 998-30-1) according to OECD guideline 202.Daphnia magna were exposed for 48 hours under static conditions (LPT, 2003) and a 48-hour EC₅₀value of >100 mg/l (nominal) was determined.

The registered substance will hydrolyse rapidly (0.7 h at pH 7 and 20°C - 25°C) in contact with water and atmospheric moisture to silicic acid and ethanol. REACH guidance (ECHA 2010A, R.16) states that “for substances where hydrolytic DT₅₀is 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 2010A) 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 silicate hydrolysis product, silanetriol, silicic acid, and ethanol, in accordance with REACH guidance. However, for completeness it is important to look also at any data from tests of this and related substances.

As described in Section 1.3, the silicic acid may precipitate as insoluble silica (SiO₂) at high enough concentrations. 

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physico-chemical 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 physico-chemical properties and the structure is altered.

Triethoxysilane ismembers of a group of substances that hydrolyse rapidly to form silicic acid, a bioavailable form of silicate.

Silicic acid is a naturally occurring substance which is not harmful to aquatic organisms. It is the major bioavailable form of silica for aquatic organisms and plays an important role in the biogeochemical cycle of Si. Most living organisms contain at least trace quantities of silicon. For some species Si is essential element taken up actively, for example, diatoms, radiolarians, flagellates, sponges and gastropods all have silicate skeletal structures. In addition, silicic acid has been shown to be beneficial in protection against mildew formation in wheat and to not be phytotoxic in non-standard studies (Côte-Beaulieu et al. 2009). Therefore, it is not expected to be harmful to the environment nor the organisms that will be exposed to it.

 

In the following paragraphs the read-across approach for triethoxysilane is assessed for each surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

Additional information is given in a supporting report (PFA, 2013x) attached in Section 13 of the REACH IUCLID 5 dossier.

• Read-across from trimethoxysilane to triethoxysilane

No measured data are available for fish and algae for triethoxysilane (CAS 998-30-1). However, studies are available for the related substance, trimethoxysilane (CAS 2487-90-3). It is considered appropriate to read across for fish and algae results for trimethoxysilane (CAS 2487-90-3) to triethoxysilane (CAS 998-30-1) as both substances hydrolyse in the presence of water to yield silanetriol and further produce silicic acidas silanetriol will break down further into inorganic, naturally occurring substances such as silica and silica acid.The hydrolysis half-lives of triethoxysilane (CAS 998-30-1) and trimethoxysilane (CAS 2487-90-3) indicate that under conditions relevant ecotoxicity assessment, both will hydrolyse first to silanetriol and further to silicid acid. Short term toxicity data for trimethoxysilane (CAS 2487-90-3) with fish and algae were a LC₅₀of >100 mg/l (OECD Guideline 203 Fish, Acute Toxicity Test, Springborne Smithers, 2004) and an EC₅₀of >100 mg/l (OECD Guideline 201, Alga, Growth Inhibition Test, Springborne Smithers, 2004)

• Read-across from dichloro(methyl)silane to triethoxysilane

No measured data are available for toxicity to microorganisms for triethoxysilane (CAS 998-30-1). Studies are available for the structurally analogous substance dichloro(methyl)silane (CAS 75-54-7) and it is considered appropriate to read-across the microorganism toxicity results for dichloro(methyl)silane (CAS 75-54-7) to triethoxysilane (CAS 998-30-1) because both substances hydrolyse rapidly to similar hydrolysis products, dimethylsilanetriol and silanetriol, respectively. An EC₅₀of 160 mg/l and a NOEC of 32 mg/l for toxicity to microorganisms were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test, Desmares-Koopmans 2009).

·        Considerations on the non-silanol hydrolysis products:

 

Triethoxysilane hydrolyses to form both silanetriol and ethanol. As silanetriol will break down further into inorganic, naturally occurring substances such as silica and silica acid and shows no toxicity it is more appropriate to base the hazard assessment on the other hydrolysis product ethanol. TGD and ECHA guidance, (EC 2003, ECHA 2010A) 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 hydrolysis product, in accordance with REACH guidance. The lowest reliable long term toxicity data reported in the SIDS for ethanol are a (10 d) NOEC 9.6 mg/l and a (7 d) NOEC 280 mg/l with invertebrates and higher vascular plants respectively.

Table7.1.1 Key physicochemical parameters and ectoxicological data for the registered and surrogate substances.

CAS Number	998-30-1	2487-90-3
Chemical Name	Triethoxysilane	Trimethoxysilane
Si hydrolysis product	Silanetriol further to Silicic acid	Silanetriol further to Silicic acid
Molecular weight (parent)	164,28	122,2
Molecular weight (silanol hydrolysis products)	n/a	n/a
log Kow (parent)	1.7	0.2
log Kow (silanol hydrolysis products)	Both inorganic (-2.9 (QSAR) forSilanetriol)	Both inorganic (-2.9 (QSAR) forSilanetriol)
Water sol (parent)	5600 mg/l	54000 mg/l
Water sol (silanol hydrolysis products))	Both soluble (7.9E+05 mg/l at 20°C (QSAR) for Silanetriol)	Both soluble (7.9E+05 mg/l at 20°C (QSAR) for Silanetriol)
Vapour pressure (parent)	921 Pa at 20°C	11370 Pa at 20°C
Vapour pressure (hydrolysis products)	The vapour pressure prediction is not applicable to inorganic silanols. The volatility of the hydrolysis product is expected to be very low based on its structure (small molecule silanols are predicted to have low vapour pressures) and its high water solubility.	The vapour pressure prediction is not applicable to inorganic silanols. The volatility of the hydrolysis product is expected to be very low based on its structure (small molecule silanols are predicted to have low vapour pressures) and its high water solubility.
Hydrolysis t1/2at pH 7 and 25°C	0.7 h	0.1 h
Hydrolysis t1/2at pH 4 and 25°C	0.1 h	0.2 mins
Hydrolysis t1/2at pH 9 and 25°C	0.02 h	n/a
Short-term toxicity to fish (LC50)	n/a	>100 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	>100 mg/l	>100 mg/l
Algal inhibition (ErC50and NOEC)	n/a	>100 mg/l
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	n/a