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Physical & Chemical properties

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The substance, tri(isopropyl)silyl acrylate, is not stable in water, which affects the approach to the determination of physicochemical properties.

Tri(isopropyl)silyl acrylate is a liquid at ambient temperature and pressure. It has a measured melting point of <-70°C and the substance was observed to decompose at 180°C without boiling. A relative density of 0.9104 at 20°C was obtained for the substance and it has a predicted vapour pressure of 10 Pa at 25°C. The substance has a measured kinematic viscosity value of 2.96 mm2/s at 25°C.

The substance is not classified for flammability in accordance with Regulation (EC) No. 1272/2008 on the basis of a measured flash point of 96.7°C and predicted boiling point of 200°C. It has a measured auto-ignition temperature of 250°C at 958.5 mBar. It is not explosive and not oxidising on the basis of structural examination.

In contact with water, tri(isopropyl)silyl acrylate reacts very rapidly with a half-life of <<1 h at pH 7 and 25°C to produce tris(1-methylethyl)silanol and acrylic acid according to the following equation:

CH2=CHC(=O)O[Si][CH(CH3)2]3+ H2O → [CH(CH3)2]3SiOH + CH2=CHCOOH

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, tris(1-methylethyl)silanol and acrylic acid are assessed instead.

Acrylic acid is miscible with water, has low log Kow (0.46 at 25°C, and -2.3 for ionised form at pH 7) and vapour pressure of 380 Pa at 20°C (ECB 2002).

The silanol hydrolysis product, tris(1-methylethyl)silanol, may undergo condensation reactions in solution to give the siloxane dimer and a dynamic equilibrium is established. The overall rate 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 monosilanols may be modelled as an equilibrium between monomer and dimer. The reaction is reversible unless the dimer concentration exceeds its solubility; in this case, the dimer forms a separate phase, driving the equilibrium towards the dimer. At loadings above about 10 mg/l the concentration of the dimer is predicted to exceed its solubility, resulting in formation of a separate phase. In addition, the dimer is expected to have a high volatility from water and this may cause losses from water under some conditions. Further information is given in a supporting report (PFA 2016am) attached in Section 13.

The water solubility of the silanol hydrolysis product (tris(1-methylethyl)silanol) has been calculated using a validated QSAR method to be 60 mg/L. However, the saturation concentration may in reality be limited by the formation of less soluble dimer rather than the solubility of the monomer itself. It has a predicted log Kow (3.9 at 20°C) and predicted vapour pressure of 4 Pa at 25°C. It is not surface-active based on structural examination. The dissociation constant of a structurally analogous silanol (trimethylsilanol) has been reported to be around pKa = 11. The non-silanol hydrolysis product, acrylic acid, has a reported pKa of 4.25 at 25°C (CRC 2010).


CRC (2010) CRC Handbook of Chemistry and Physics: A Ready-Reference Book of Chemical and Physical Data, 91st edition, CRC Press, Boca Raton, 2010

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

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