Triethoxypropylsilane

Endpoint summary

Additional information

Note: An acute fish study, a daphnia test and a biodegradation screening study are currently ongoing for the registered substance (i.e. triethoxy(propyl)silane (CAS 2550-02-9)) in order to support the read across from (3-chloropropyl) triethoxysilane (CAS 5089-70-3) . This dossier will be updated including an analogue justification according to RAAF once the final study reports are available.

The registered substance is part of a group of organosilicon substances, the toxicity of which is determined by a non-polar mechanism of toxicity, and as such log Kow drives toxicity. Substances in this group include alkoxy- and chloro-silanes having secondary features that do not affect the toxicity of the substances. Additional information is given in a supporting report (PFA, 2013y) attached in Section 13 of the IUCLID 5 dossier.

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.

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

Read-across from (3-chloropropyl) triethoxysilane (CAS 5089-70-3) to triethoxy(propyl)silane (CAS 2550-02-9):

Measured data were not available for triethoxy(propyl)silane. However, measured data were available for the close structurally analogous substance, (3-chloropropyl) triethoxysilane (CAS 5089-70-3) which was considered suitable for read across.

(3-chloropropyl) triethoxysilane (CAS 5089-70-3) is a trialkoxysilane that hydrolyses (half-life of 1.45 days) to form chloropropylsilanetriol and ethanol. The effects of ethanol are assessed below. Chloropropylsilanetriol and propylsilanetriol are trisilanols. The silanols have similar physical properties with low log Kowvalues(-1.1 and -1.4, respectively) and low molecular weight and due to the slower hydrolysis half-life of the registered substance (DT50of 1.4 days) indicates that it is appropriate to conduct the hazard assessment for the environment based on the properties of the close structurally analogous substance, (3-chloropropyl) triethoxysilane (CAS 5089-70-3) (DT50 of 1.45 days).

Short- and long-term data available with (3-chloropropyl) triethoxysilane (CAS 5089-70-3) indicate that this substance is of low toxicity to aquatic organisms.

Reliable short-term toxicity tests results were available for each of the trophic levels. An 96 hour LC50 of 80 mg/l was reported for freshwater fish (Brachydanio rerio) (EU method C.1 acute toxicity to fish) and EC50of 21.2 mg/l and an EC50of 140 mg/l for invertebrates (Daphnia magna) (OECD Guideline 202 (Daphnia sp. Acute Immobilisation Test) and a 72-hour EC50value of >819 mg/l was determined for the effects of the test substance on the growth rate and biomass of Scenedesmus subspicatus. The study was conducted according to EU Method C.3 (Algal Inhibition test).

A 21-day NOEC of ≥100 mg/l has been determined for the effects of the test substance on mortality and reproduction of Daphnia magna. The result has been reinterpreted by the reviewer in terms of the mean measured concentration of the test substance to give a NOEC of ≥ 66 mg/l. The apparent inconsistency in the results of the short- and long-term tests with Daphnia magna might be explained by the flat dose response in the short-term test. As a consequence it is possible that the 48-h EC50 value could be closer to 100 mg/l than is indicated by the value determined by linear regression. The results of the second short-term test with D. magna (48-h EC50: 140 mg/l and NOEC: 100 mg/l) are consistent with this thesis.

Read-across from trimethoxy(propyl)silane (CAS 1067-25-0) to triethoxy(propyl)silane (CAS 2550-02-9):

Additional good quality data for the analogue substance, trimethoxypropylsilane, have been read across. Both substances share the same silanol hydrolysis product, propylsilanetriol; the other hydrolysis products being ethanol and methanol. As trimethoxypropylsilane hydrolyses rapidly (half-life approximately 4.6 hours at pH 7, 20-25°C) to propylsilanetriol, under ecotoxicity testing conditions the organisms will therefore be exposed predominantly to the hydrolysis product. Data were available for trimethoxy(propyl)silane for aquatic toxicity for each of the three trophic levels. The reported effect concentration was an EC50of 913 mg/l for the effect on growth rate on the algae Scenedesmus subspicatus (new name: Desmodesmus subspicatus) (EU Method C.3 (Algal Inhibition test). For fish an 96 hour LC50of 746 mg/l (EU Method C.1 (Acute Toxicity for Fish) was reported and for Daphnia magna an 48 hour EC50of 816 mg/l (EU Method C.2 (Acute Toxicity for Daphnia).

But due to the slower hydrolysis half-life of the registered substance (DT50of 1.4 days) indicates that it is appropriate to conduct the hazard assessment for the environment based on the properties of the close structurally analogous substance, (3-chloropropyl) triethoxysilane (CAS 5089-70-3) (DT50 of 1.45 days) and not for hydrolysis product.

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 (OECD2004a - SIDS for methanol, CAS 67-56-1, OECD 2004b - SIDS for ethanol, CAS 64-17-5).

 

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

CAS Number	2550-02-9	5089-70-3	1067-25-0
Chemical Name	triethoxypropylsilane	chloropropyltriethoxysilane	trimethoxypropylsilane
Si hydrolysis product	propylsilanetriol	chloropropylsilanetriol	propylsilanetriol
Molecular weight (parent)	206.36	240.8	164.28
Molecular weight (hydrolysis product)	122.2	n/a	122.2
log Kow(parent)	3.1	3.13	1.7
log Kow(silanol hydrolysis product)	-1.4	-1.1	-1.4
Water sol (parent)	280 mg/l	113 mg/l (after 2 hours)	9200 mg/l
Water sol (silanol hydrolysis product))	1000000 mg/l	1000000 mg/l	1000000 mg/l
Vapour pressure (parent)	13 Pa at 25°C	30.9 Pa at 25°C	9240 mg/l
Vapour pressure (hydrolysis product)	0.0046 Pa at 25°C	n/a	0.0046 Pa at 25°C
Hydrolysis t1/2at pH 7 and 20°C-25°C	1.4 days 0.7 days (at 25°C)	35 hours	2.6 hours
Hydrolysis t1/2at pH 4 and 25°C	3.6 minutes	< 24 min at 20°C	0.2 hours
Short-term toxicity to fish (LC50)	n/a	80 mg/l	> 746 mg/l
Short-term toxicity to aquatic invertebrates (EC50)	n/a	21.2 mg/l and 140 mg/l	> 816 mg/l
Algal inhibition (ErC50and NOEC)	n/a	EC50> 819 mg/l NOEC of 237 mg/l	> 913 mg/l
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	NOEC >= 100 mg/l	n/a