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Physical & Chemical properties

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The substance,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilaneis 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 1.4.1 of the CSR.

2-(3,4-Epoxycyclohexyl)ethyltrimethoxysilane is a liquid at standard temperature and pressure, with a measured melting point of <-80°C, and a predicted boiling point of 260°C. It has a measured relative density of 1.07 at 20°C and a predicted vapour pressure of 0.46 Pa at 25°C.

The substance is not classified as a flammable liquid according to Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 136°C and a predicted boiling point of 260°C. It has a measured auto-ignition temperature of 250°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water,2-(3,4-epoxycyclohexyl)ethyltrimethoxysilanehydrolyses rapidly (half-life 0.2 h at pH 4, 4 h at pH 7, 0.1 h at pH 9 and 20- 25°C) to produce 2-(3,4 -epoxycyclohexyl)ethylsilanetriol and methanol according to the following equation:

(OCH3)3CH2CH2Si(C6H9O) + 3H2O → Si(OH)3CH2CH2(C6H9O) + 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 product, 2-(3,4 -epoxycyclohexyl)ethylsilanetriol and methanol are assessed instead.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12790 Pa at 20°C) (OECD, 2004a: SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18 - 20 October 2004, Methanol, CAS 67-56-1).

The silanol hydrolysis product, 2-(3,4-epoxycyclohexyl)ethylsilanetriol, 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 1000 mg/l of 2-(3,4-epoxycyclohexyl)ethylsilanetriol, 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, 2-(3,4-epoxycyclohexyl)ethylsilanetriol, 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 -0.6. It is not surface active. The silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 7.5E-06 Pa at 25°C).

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