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The substance, triethoxy(3-isocyanatopropyl)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 relevant sections.

Triethoxy(3-isocyanatopropyl)silane is a liquid at standard temperature and pressure, with a measured melting point of <-100°C at 1013 hPa and a measured boiling point of 242°C at 1013 hPa. It has a measured relative density of 1.00067 at 20°C, measured kinematic viscosity of 1.6 mm2/s at 20°C and approximately 1.2 mm2/s at 40°C. The substance has measured vapour pressures of 2 Pa at 20°C, 3.2 Pa at 25°C and 24 Pa at 50°C.

The substance is not classified for flammability according to Regulation (EC) No 1272/2008 on the basis of a measured flash point of 106.5°C at 1013 hPa and a measured boiling point of 242°C at 1013 hPa. It has a measured auto-ignition temperature of 255°C and is not explosive and not oxidising on the basis of chemical structure.

The submission substance, triethoxy(3-isocyanatopropyl)silane, has two hydrolysable groups, triethoxy (-OCH3CH2) and isocyanate (-N=C=O). In contact with water, the isocyanate group is expected to hydrolyse very rapidly, for example the hydrolysis half-lives of 2,2,4- (or 2,4,4)-trimethylhexane-1,6-diisocyanate (CAS 32052-51-0) were measured in accordance with OECD 111 and in compliance with GLP (Lange 2013). Very rapid hydrolysis following pseudo-first order kinetics was observed; hydrolysis half-lives of 3.81 min at pH 4, 4.88 min at pH 7 and 1.93 min at pH 9 and 20°C were determined. Similarly, n-butyl isocyanate (CAS 111-36-4) was reported to undergo complete hydrolysis in water within a few minutes at 20°C (OECD 2005).

For the submission substance, this means very rapid hydrolysis to form 3-aminopropyltriethoxysilane (CAS 919-30-2) as an intermediate hydrolysis product and carbon dioxide. The hydrolysis half-lives of 3-aminopropyltriethoxysilane have been measured in accordance with OECD 111 to be 0.8 h at pH 5, 8.5 h at pH 7, and 0.15 h at pH 9 and 24.7°C. The ultimate silanol hydrolysis product under dilute conditions is 3-aminopropylsilanetriol. The other product of hydrolysis is ethanol according to the following equation:

(CH2CH3O)3Si(CH2)3NCO + 4H2O → NH2(CH2)3Si(OCH2CH3)3 + CO2 → H2N(CH2)3Si(OH)3 + 3CH2CH3OH

 

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

The properties of the intermediate hydrolysis product 3-aminopropyltriethoxysilane have been estimated using appropriate QSAR methods. The unionised form of the intermediate hydrolysis product has a predicted log Kow of 1.7. In solution, the amine group will protonate; therefore,the estimated log Kow for the unionised form was corrected for ionisation using the equation CORR = 1/1 +10A(pH-pKa) [where A = 1 for acids, -1 for bases; pH = pH-value of the environment; pKa = acid/base dissociation constant]. Log Kow values of -4 at pH 2, -4 at pH 4, -1.3 at pH 7 and 0.7 at pH 9 were determined for the intermediate hydrolysis product. 3-Aminopropyl(triethoxy)silane has a predicted water solubility value of 1.7E+04 mg/L at 20°C.

Ethanol is miscible with water, has low log Kow (-0.3) and high vapour pressure (7910 Pa at 25°C).

The ultimate silanol hydrolysis product, 3-aminopropylsilanetriol, 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 between the monomer, dimer and tetramer of 3-aminopropylsilanetriol. 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-aminopropylsilanetriol, 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-aminopropylsilanetriol, is therefore limited by condensation reactions to approximately 1000 mg/L. However, it is very hydrophilic (calculated solubility is 1.0E+06 mg/L using a QSAR method) and the ionised form of the silanol hydrolysis product have predicted log Kow of -4 at pH 2, pH 4 and pH 7. At pH 9, the predicted log Kow is -3.5.

3-Aminopropylsilanetriol is not surface active and is much less volatile than the parent substance with a predicted vapour pressure of 2.5E-04 Pa at 25°C. The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa of 10. The amine group has a pKa of approximately 9.5 based on available data.

The by-product of the isocyanate hydrolysis, carbon dioxide, is exempt from registration under REACH Article 2(7) because (a) it is a substance with sufficiently known information and (b) it is considered to cause minimum risk due to its intrinsic properties. It is therefore not assessed in this CSA.

 

References:

Lange (2013). Lange, J. (2013). Vestanat TMDI - hydrolysis as a function of pH. Dr. U. Noack-Laboratorien, Sarstedt (Germany). Test report. Testing laboratory: Dr. U. Noack-Laboratorien, Sarstedt (Germany). Report no.: CPH14411. Owner company: Evonik lndustries AG. Report date: 2013-01-21.

OECD (2005). SIDS Initial Assessment Report for SIAM 21, Washington, 18-21 October 2005, n-Butyl isocyanate, CAS 111-36-4.

PFA (2016am). Peter Fisk Associates, Analogue Report - Silanols and aquatic systems. Reference:404.105.003

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