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Physico-chemical properties

Triethoxysilane is a liquid at standard temperature and pressure, with a melting point of < -20°C and a measured boiling point of 132.5°C. It has a measured relative density of 0.875 at 20°C, a predicted viscosity of 0.76 mPasat 25°Cand a measured extrapolated vapour pressure of 921 Pa at 20°C.

The substance is classified as a highly flammable liquid on the basis of a measured flash point of 19°C and the boiling point of 132.5°C. It has a measured auto-ignition temperature of 237 - 240°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, triethoxysilane is predicted to rapidly hydrolyse (half-life of approximately 1 hour at 20 - 25°C and pH 7) to produce silanetriol and ethanol according to the following equation:

HSi(OCH2CH3)3+ 3H2O → HSi(OH)3+ 3CH3CH2OH

The Si-H bond may also be unstable, resulting in further hydrolysis toSi(OH)4.Silanols may undergo condensation reactions to form siloxane dimers, oligomers and polymers. In the case of triethoxysilane, where no organic side-chains are present, rapid condensation to inorganic siloxanes and finally silica is expected. The precise rate of this condensation and the species present are dependent on conditions.

Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water. The properties of the initial silanol hydrolysis products, silanetriol and ethanol, are assessed instead.Ethanol is miscible with water, has low log Kow(-0.3) and high vapour pressure (7910 Pa at 25°C).Silanetriol has a predicted solubility in water of 7.9E+05 mg/l at 20°C and has a low log Kow(-2.9, predicted). It is not surface active and the volatility is expected to be very low. It is not expected to undergo significant dissociation within an environmentally-relevant range.