Trimethoxyoctylsilane

Endpoint summary

Additional information

Approach to the Terrestrial Chemical Safety Assessment

 

No long-term terrestrial toxicity data are available with trimethoxy(octyl)silane (CAS 3069-40-7; EC No. 221-338-7).

 

The registered substance will hydrolyse rapidly (half-life 4 hours at pH 7 at 20-25°C) in contact with water and atmospheric moisture to form octylsilanetriol and methanol. Therefore the terrestrial chemical safety assessment for trimethoxy(octyl)silane is based on its hydrolysis products octylsilanetriol and methanol. These form the basis of the chemical safety assessment and are assessed separately.

 

Methanol:

Methanol is well characterised in the public domain literature and is not expected to be hazardous to terrestrial organisms. Methanol is therefore not considered further.

 

Octylsilanetriol: 

In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of the silanol hydrolysis product in a short- or long-term terrestrial toxicity to invertebrates/higher plants study because, as indicated in guidance R.7.11.6 (ECHA 2016), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio is below 1, and therefore the risk is already adequately controlled and further testing is not justifiable.

 

Octylsilanetriol is highly water soluble, has low potential for bioaccumulation and low potential for adsorption (based on log K_ow <3 (1.1) and log K_oc 1.6). The compartment of most relevance is therefore expected to be the aquatic compartment.

 

Long-term toxicity testing to aquatic invertebrates has been carried out with a structural analogue of the registered substance. During this test, it is likely that organisms were exposed to the hydrolysis products of the test substance. The occurrence of more severe toxic effects in the terrestrial compartment than in this aquatic study (conducted at concentrations up to 100 mg/L (nominal)) would be considered unlikely.

 

The substance has no structural alerts for ecotoxicity and there is no reason to expect any specific mechanism of toxicity beyond narcosis.

 

In the context of the Integrated Testing Strategy for the terrestrial compartment (ECHA guidance Chapter R.7c, terrestrial hazard classification scheme, Table R.7.11-2) (ECHA 2017) the aquatic toxicity tests do not indicate that the substance is very toxic (EC/LC₅₀ >1 mg/l for algae, Daphnia and fish). However, despite rapid degradation of the parent substance, the silanol hydrolysis product, octylsilanetriol, is not readily biodegradable and may persist in the environment. The substance is therefore assigned to Hazard Category 3.

 

In this situation, a screening approach is applied: a confirmatory long-term terrestrial test is usually appropriate, in addition to the equilibrium partitioning approach with an extra factor of ten, in order to determine whether further full tests are necessary. 

  

The PNECscreen(EQPM) for octylsilanetriol is derived from the long-term test result with Daphnia and has a value of 0.2 mg/kg dwt.  For the purpose of the screening assessment comparison only, an extra factor of ten is applied (PECx10/PNECscreen(EQPM)). Based on the exposure assessment of trimethoxy(octyl)silane (CAS 3069-40-7), the highest terrestrial PECx10/PNECscreen(EQPM) for octylsilanetriol is 0.474 x 10 = RCR 4.74.

 

A confirmatory long-term terrestrial toxicity test is therefore required in accordance with the recommendations for Hazard Category 3 substances.

No toxicity was observed in the aquatic microorganism tests, therefore a toxicity to soil microorganisms test is not required.

  

However, prior to terrestrial toxicity tests being carried out, the technical feasibility of testing the silanol hydrolysis product, octylsilanetriol, would need to be assessed. To achieve the test concentrations required by terrestrial testing guidance (up to 1000 mg/kg), aqueous stock solutions would need to be prepared at very high concentrations, well above 1000 mg/l. Silanetriols are susceptible to condensation reactions. Condensation of octylsilanetriol is expected to become important at loadings above about 100 mg/l causing the formation of insoluble polymeric particles (sols) and gels over time. It therefore may not be possible to test terrestrial organisms at high enough test concentrations to meaningfully assess the terrestrial toxicity of octylsilanetriol. Prior to any testing, the feasibility of dosing at concentrations up to 1000 mg/kg dw would have to be assessed. Further details on technical feasibility of terrestrial testing with silanetriol substances are given in Annex 7 of the CSR and attached in Section 13 of IUCLID.

 

In addition, if a confirmatory long-term terrestrial test were to be conducted, an assessment factor of 100 would be applied to derive PNECsoil from one long-term test. For this value to be more conservative than the PNECsoil derived by the equilibrium partitioning method, the NOEC derived in the terrestrial test would have to be ≤20 mg/kg dw. There is no basis to expect such toxicity for octylsilanetriol based on the low toxicity observed in aquatic tests and the low potential for adsorption to and bioaccumulation in soil.  

 

In the case of octylsilanetriol, the Registrant considers that a long-term terrestrial study is unlikely to affect the outcomes of the chemical safety assessment. As such the Registrant proposes that further testing is currently not necessary.

 

Details on how the PNEC and the risk characterisation ratio have been derived can be found in IUCLID Section 6.0, and Chapters 7, 9 and 10 of the Chemical Safety Report.

 

 

References

ECHA (2017). Guidance on Information Requirements and Chemical Safety Assessment. Chapter R.7c: Endpoint specific guidance. Version 3.0 June 2017.