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Physical & Chemical properties

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The substance, trimethoxy(vinyl)silane, is not stable in water, which affects the approach to the determination of physicochemical properties. The significance of this for read-across is discussed in Section 1.5 of the CSR.

Trimethoxy(vinyl)silane is a liquid at standard temperature and pressure, with a measured melting point of -97°C at 1013 hPa, and a measured boiling point of 123°C at 1013 hPa. It has a predicted density of 0.94 g/cm3 at 20°C and a predicted kinematic viscosity of 0.7 mm2/s at 20°C. The submission substance has measured vapour pressure values of 640 Pa at 10°C, 1190 Pa at 20°C and 6000 Pa at 50°C.

The substance is classified for flammability according to Regulation (EC) No. 1272/2008/EC on the basis of a measured flash point of 25.5 - 26°C and a measured boiling point of 123°C. It has a measured auto-ignition temperature of 224°C at 1013 hPa, it is not explosive and not oxidising on the basis of structural examination.

In contact with water, trimethoxy(vinyl)silane reacts very rapidly (half-life of 0.1 hours at pH 7, 0.04 hours at pH 4, 0.004 hours at pH 9 and 20-25°C) to produce vinylsilanetriol and methanol according to the following equation:

CH2=CHSi(OCH3)3 + 3H2O ¿ CH2=CHSi(OH)+ 3CH3OH

 

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 silanol hydrolysis products, vinylsilanetriol and methanol are assessed instead. However, the water solubility and log Kow of trimethoxy(vinyl)silane have been predicted to be 3.0E+04 mg/L and 1.1 at 20°C respectively.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12790 Pa at 20°C) (OECD 2004).

The silanol hydrolysis product, vinylsilanetriol, may undergo condensation reactions in solution to give siloxane dimers, linear and cyclic oligomers and highly cross-linked polymeric particles (a colloidal suspension of small solid particles known as a sol) that may over time form an insoluble gel and a dynamic equilibrium is established. The overall rate and extent of condensation is dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution.

The condensation reactions of silanetriols may be modelled as an equilibrium between monomer, dimer, trimer and tetramer, with the linear tetramer cyclising to the thermodynamically stable cyclic tetramer. At higher loadings, cross-linking reactions between the cyclic tetramers may occur. The reactions are reversible unless the cyclic tetramer concentration exceeds its solubility; in this case, the cyclic tetramer forms a separate phase, driving the equilibrium towards the tetramer. At loadings below 500 mg/L of vinylsilanetriol, the soluble monomer is expected to predominate in solution (>99%), with small amounts of dimer and oligomers. Condensation reactions are expected to become important at loadings above about 1000 mg/L causing the formation of insoluble polymeric particles and gels over time. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The saturation concentration in water of the silanol hydrolysis product vinylsilanetriol is therefore limited by condensation reactions to approximately 1000 mg/L. However, it is very hydrophilic (calculated solubility is 1E+06 mg/L at 20°C using a QSAR method) with a predicted low log Kow of -2.0. It is not surface active and is not expected to undergo significant dissociation within the environmentally-relevant range. The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa of 10. It is much less volatile than the parent substance (vapour pressure = 0.017 Pa at 25°C, predicted).

References:

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

PFA (2016am). Peter Fisk Associates, Silanols and aquatic systems, 404.105.003

Additional information