Trimethoxysilane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of trimethoxysilane (CAS 2487-90-3) is =0.2 minutes at pH 4 and pH 9, =0.3 minutes at pH 7 and 2°C; the substance will therefore undergo very rapid hydrolysis in contact with water. This half-life relates to hydrolysis of the Si-OMe bonds to give silanetriol and methanol. The Si-H bond of silanetriol reacts in water, forming monosilicic acid (Si(OH)4) as the ultimate hydrolysis product. The precise rate of this reaction is uncertain but the half-life for Si-H reactivity of trimethoxysilane is estimated as <12 hours (possibly much less than 12 hours) at pH 7 and 25°C.

Methanol will not significantly influence the hydrolysis and condensation reactions of the silanetriol species formed by initial hydrolysis of trimethoxysilane.

Monosilicic acid (Si(OH)₄) condenses at concentrations above approximately 100-150 mg/L as SiO₂ to give insoluble amorphous polysilicic acid.

The log K_ow of monosilicic acid (Si(OH)₄) is -4 and the water solubility is approximately 1.0E+6 mg/L (OECD 2004). Above 100-150 mg/L, monosilicic acid condenses to give insoluble amorphous polysilicic acid (see Section 4.8 of IUCLID for further discussion).

Methanol is miscible in water and has log K_ow -0.82 to -0.64 (OECD 2004a).

 

REACH guidance (ECHA 2016, 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”. ECHA Guidance Chapter R.7b (ECHA 2017) states that where degradation rates fall between >1 hour and <72 hours, testing of parent and/or degradation product(s) should be considered on a case-by-case basis.

The environmental hazard assessment is therefore based on the properties of the silanol hydrolysis product, in accordance with REACH guidance.

 

As described below, condensation reactions of the silicic acid are possible.

 

The registered substance is part of a class of alkoxysilane compounds which hydrolyse rapidly or moderately rapidly to produce monosilicic acid (Si(OH)₄) and another non-Si hydrolysis product. Si(OH)₄ has not been isolated and only exists in dilute aqueous solution. It readily and rapidly (within minutes) condenses to give amorphous polysilicic acid at concentrations above 100 - 150 mg SiO₂ / L. Depending on the pH and concentration, solutions will contain varying proportions of monosilicic acid, cyclic and linear oligomers and polysilicic acid of three-dimensional structure. Further details are given in supporting reports (PFA 2015ao and PFA 2013x) attached in Section 13.

 

Measured short-term toxicity to fish, Daphnia and algae data are available with the registered substance. LC₅₀ or EC₅₀ values for the three organisms are all >100 mg/L indicating that the substance is not acutely toxic to aquatic organisms.

 

References

ECHA (2016). REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Assessment Version: 3.0. February 2016.

ECHA (2017). European Chemicals Agency. Guidance on Information Requirements and Chemical Safety Assessment, Chapter R.7b: Endpoint specific guidance. Version 4.0 June 2017.

OECD SIDS (2004). SIDS Initial Assessment Report for SIAM 18, Paris, France, 20-23 April, 2004, Soluble Silicates, CAS 1344-09-8 Silicic acid, sodium salt; CAS 6834-92-0 Silicic acid (H2SiO3), disodium salt; CAS 10213-79-3 Silicic acid (H2SiO3), disodium salt, pentahydrate; CAS 13517-24-3 Silicic acid (H2SiO3), disodium salt, nonahydrate; CAS 1312-76-1 Silicic acid, potassium salt.

OECD (2004a). SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1

PFA (2013x). Peter Fisk Associates. Analogue report - Ecotoxicity of (poly)silicic acid generating compounds, PFA.300.003.001.

PFA (2015ao). Peter Fisk Associates. The aquatic chemistry of inorganic silicic acid generators, PFA.404.001.001.

Conclusion on classification

The substance very rapidly hydrolyses to methanol and inorganic silicate moieties. Hydrolysis product methanol has a harmonised classification in Annex VI of Regulation No 1272/2008 and does not require classification for the environment. Hydrolysis product monosilicic acid is a naturally-occuring substance which is not harmful to aquatic organisms at relevant concentrations. All available studies with aquatic organisms report no effects at 100 mg/L in short-term toxicity studies (reference PFA 2013x).

The substance has reliable short-term E(L)C₅₀ values of >100 mg/L in fish, >100 mg/L in invertebrates and >100 mg/L in algae. It has reliable NOEC <6.3 mg/L in algae based on biomass and growth effects.

The substance hydrolyses very rapidly in water.

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.