Tri(isopropoxy)vinylsilane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of tri(isopropoxy)(vinyl)silane is 7 h at pH 7, 20-25⁰C. The registered substance will hydrolyse in contact with water and atmospheric moisture to vinylsilanetriol and isopropanol.

REACH guidance (ECHA 2016b, 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 substance itself”. TGD and REACH guidance (EC 2003, ECHA 2016c, R.7b) 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 chemical safety assessment (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product, vinylsilanetriol, in accordance with REACH guidance. Condensation reactions of the silanetriol are possible, as described in Section 4.

 

READ-ACROSS JUSTIFICATION

In order to reduce animal testing, read-across is proposed to fulfil up to REACH Annex X requirements for the registration 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 registration substance, its silanol hydrolysis productand the substance used as a surrogate for read-across (see detailed discussion below) are part of a class of non-functional compounds which act via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the Si atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. The silanol hydrolysis products may be susceptible to condensation reactions, see Section 1.3 of the CSR. The registered substance hydrolyses rapidly in water and therefore the selection of surrogate substance is based on log K_owof the resulting silanols and the chemical groups present in them.

Additional information is given in a supporting report (PFA 2016y), attached in Section 13.

In the following paragraphs the read-across approach for tri(isopropoxy)(vinyl)silane is assessed for each surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties. The table below presents the properties of the registered and surrogate substances (and their respective hydrolysis products) alongside for comparison.

• Read-across from trimethoxy(vinyl)silane (CAS 2768-02-7) to tri(isopropoxy)(vinyl)silane (CAS 18023-33-1)

Tri(isopropoxy)(vinyl)silane (CAS 18023-33-1) and trimethoxy(vinyl)silane (CAS 2768-02-7) both hydrolyse rapidly in contact with water (6.6 h at pH 7, 20-25°C, and 0.2 h at pH 7, 20°C respectively). The organosilicon hydrolysis product is vinylsilanetriol in both cases, with isopropanol and methanol respectively being released as a by-product.

Trimethoxy(vinyl)silane (CAS 2768-02-7)  is used to read across to short-term aquatic toxicity to invertebrates and algal growth endpoints. E(L)C50 values of 120 mg/l and >64 mg/l (expressed in terms of concentration of the hydrolysis product, vinylsilanetriol) have been determined.

·        Considerations on the non-silanol hydrolysis products:

Methanol and isopropoanol are both 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 are in excess of 1000 mg/l (OECD 2004a - SIDS for methanol; OECD 1997 - SIDS for isopropanol) and do not require quantitative consideration.

 

Table: The key physicochemical parameters and ecotoxicological data for the registered and surrogate substances.

CAS Number	2768-02-7	18023-33-1
Chemical Name	trimethoxyvinylsilane	tri(isopropoxy)(vinyl)silane
Si hydrolysis product	Vinylsilanetriol	Vinylsilanetriol
Molecular weight (parent)	148.2	232.4
Molecular weight (hydrolysis product)	106.2	See left
log Kow(parent)	1.1
log Kow(silanol hydrolysis product)	-2	See left
Water sol (parent)	1.9E+04 mg/l
Water sol (silanol hydrolysis product))	1.0E+06 mg/l	See left
Vapour pressure (parent)	1190 Pa
Vapour pressure (hydrolysis product)	0.02 Pa	See left
Hydrolysis t1/2at pH 7 and 25°C	0.2 h (at 20⁰C)	6.6 h (QSAR)
Hydrolysis t1/2at pH 4 and 25°C	0.04 h	0.3 h (QSAR)
Hydrolysis t1/2at pH 9 and 25°C	0.004 h	0.1 h (QSAR)
Short-term toxicity to fish (LC50)	191 mg/l	Not available
Short-term toxicity to aquatic invertebrates (EC50)	169 mg/l	Not available
Algal inhibition (ErC50and NOEC)	72-h ErC50: >89 mg/l and NOErC >=89 mg/l (7-d) EyC50: 210 mg/l and EyC10 32 mg/l	Not available
Long-term toxicity to fish (NOEC)	Not available	Not available
Long-term toxicity to aquatic invertebrates (NOEC)	Not available	Not available
Long-term sediment toxicity (NOEC)	Not available	Not available
Short-term terrestrial toxicity (L(E)C50)	Not available	Not available
Long-term terrestrial toxicity (NOEC)	Not available	Not available
Toxicity to microorganisms (EC50, EC10, NOEC)	oxygen consumption (P. putida) 5hr EC10 1.1 ml/l	Not available

 

Conclusion on classification

The substance has reliable short-term E(L)C50 values of 263 mg/l in invertebrates and >140 mg/l in algae based on read-across from the hydrolysis product of an analogous alkoxysilane (expressed as concentration of the registration substance to release the same concentration of vinylsilanetriol). It has reliable ErC10 >=140 mg/l in algae, EyC10(7d) = 50 mg/l (recalculated on the same basis).

The substance hydrolyses at a moderate rate in water. It is expected to be readily biodegradable (based on read across from a structural analogue, attributed to degradability of the alkoxy groups; vinylsilanetriol is not expected to be significantly 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.