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Trimethoxysilane is a liquid at standard temperature and pressure, with a melting point of -113.6°C and a boiling point of 84.4°C. It has a predicted density of 0.95 g/cm3 at 20°C and a measured vapour pressure of 11370 Pa at 20°C.

The substance is classified for flammability according to Regulation (EC) No. 1272/2008 on the basis of a flash point of 7°C and a boiling point of 84.4°C. In addition, the substance is also classified as a substance in contact with water that emits highly flammable gas (i.e. hydrogen), the substance is not classified for pyrophoricity. It has a measured self-ignition temperature of 302°C at 1013 hPa and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, trimethoxysilane reacts very rapidly (half-life of =0.2 minutes at pH 4 and pH 9, =0.3 min at pH 7 and 2°C) to produce silanetriol as an intermediate hydrolysis product and methanol according to the following equation:

(CH3O)3SiH + 3H2O ¿ HSi(OH)+ 3CH3OH

The Si-H bond is also unstable, resulting in further hydrolysis to form monosilicic acid Si(OH)4 as the ultimate hydrolysis product and hydrogen as by-product.

HSi(OH)3 + H2O ¿ Si(OH)4 + H2.

The rate of this reaction has not been determined but is expected to be fairly rapid. Both silanetriol and monosilicic acid exist only in dilute aqueous solutions and readily condense at concentrations above approximately 150 mg/L to give a dynamic equilibrium between monomer, oligomers and insoluble polymeric species.


Therefore, requirements for testing of water-based physicochemical properties for the substance are waived on the basis of instability in water.

However, water based properties of the parent submission substance have been predicted. The substance has a predicted log Kow of 0.2 and water solubility value of 1.7E+5 mg/L at 20°C.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12,790 Pa at 20°C) (OECD 2004).

The water solubility of both silanol hydrolysis products, silicic acid and silanetriol will be limited by the strong tendency of Si(OH)4 to undergo condensation reactions of the type Si-OH ¿ Si-O-Si; this can lead to formation of a highly cross-linked network (gel). They are expected to have negligible vapour pressures and they are not surface active. log Kow is not relevant for inorganic compounds such as silanetriol and silicic acid.

The by-product of the Si-H reaction, hydrogen, is exempt from registration under REACH Annex V. It is therefore not considered further.

Additional information