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Physical & Chemical properties

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The substance, 3-[tris(acetoxy)silyl]propyl methacrylate 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 1.4.1 of the Chemical Safety Report.

In contact with water, 3-[tris(acetoxy)silyl]propyl methacrylate hydrolyses very rapidly (half-life <1 minute at 25°C and pH 4, 7 and 9) to produce 3-(trihydroxysilyl)propyl methacrylate and acetic acid according to the following equation:

H2C=C(CH3)C(=O)OCH2CH2CH2Si(OC(=O)CH3)3+ 3H2O → H2C=C(CH3)C(=O)OCH2CH2CH2Si(OH)3+ 3CH3C(=O)OH

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, 3-(trihydroxysilyl)propyl methacrylate and acetic acid are assessed instead.

The silanol hydrolysis product, 3-(trihydroxysilyl)propyl methacrylate, 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 3-(trihydroxysilyl)propyl methacrylate, 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, 3-(trihydroxysilyl)propyl methacrylate, 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 -0.9. It is not surface active. The first dissociation constant of 3-(trihydroxysilyl)propyl methacrylate is predicted to be around pKa = 10. The silanol hydrolysis product is much less volatile than the parent substance (predicted vapour pressure = 1.2E-05 Pa at 25°C).

3-[Tris(acetoxy)silyl]propyl methacrylate is a liquid at standard temperature and pressure, with a measured melting point of <-75°C, and above 128°C, decomposition of the submission substance can occur with boiling observed from about 215°C. It has a predicted density of 1.1 g/cm³ at 20°C and a predicted vapour pressure of 0.009 Pa at 25°C based on predicted boiling point of 320°C.

The substance is not classified for flammability on the basis of a measured flash point of >110°C. It has a measured auto-ignition temperature of 406°C, and is not explosive and not oxidising on the basis of structural examination.


PFA (2016am). Silanols and aquatic systems. Reference 404.105.003

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