3-(trimethoxysilyl)propyl isocyanate

Endpoint summary

Description of key information

The registration substance, 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6), is not stable in water, which affects the approach to the determination of physicochemical properties.

3-(Trimethoxysilyl)propyl isocyanate is a liquid at standard temperature and pressure, with a measured melting point of <-70°C at 1013 hPa and a measured boiling point of 222.1°C at 97.5 kPa. It has a measured relative density of 1.08 at 20°C, a predicted kinematic viscosity of 2.2 mm²/s at 20°C and a measured vapour pressure of 7.1 Pa at 25°C.

The substance is not classified for flammability according to EC Regulation 1272/2008 on the basis of a measured flash point of 102°C at 1013 hPa and a measured boiling point of 222.1°C at 97.5 kPa. It has a measured auto-ignition temperature of 275°C at 1011.8 - 1028.1 hPa and it is not explosive and not oxidising on the basis of chemical structure.

The submission substance, 3-(trimethoxysilyl)propyl isocyanate has two types of hydrolysable groups, trimethoxy (-OCH₃) 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 35052-51-0) 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-(trimethoxysilyl)propylamine (CAS 13822-56-5), as an intermediate hydrolysis product and carbon dioxide (CAS 124-38-9). The hydrolysis half-lives of 3-(trimethoxysilyl)propylamine (CAS 13822-56-5) have been predicted using a validated QSAR estimation method to be 0.2 h at pH 4, 0.3 h at pH 5, 2.6 h at pH 7, and 0.1 h at pH 9 and 20-25°C. The ultimate silanol hydrolysis product under dilute conditions is 3-aminopropylsilanetriol (CAS 58160-99-9). The other product of hydrolysis is methanol (CAS 67-56-1) according to the following equation:

(CH₃O)₃Si(CH₂)₃NCO + 4H₂O ¿ NH₂(CH₂)₃Si(OCH₃)₃ + CO₂ ¿ H₂N(CH₂)₃Si(OH)₃ + 3CH₃OH

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-(trimethoxysilyl)propylamine have been estimated using appropriate QSAR methods. The unionised form of the intermediate hydrolysis substance has a predicted log K_ow of 0.2. In solution, the amine group will protonate; therefore, the estimated log K_ow for the unionised form was corrected for ionisation using the equation CORR = 1/1 +10^A(pH-pKa) [where A = 1 for acids, -1 for bases; pH = pH-value of the environment; pKa = acid/base dissociation constant]. Log K_ow values of -4, -4, -2.8 and -0.8 at pH 2, pH 4, pH 7 and pH 9 respectively were determined. 3-(Trimethoxysilyl)propylamine has a predicted water solubility value of 5.7E+05 mg/L at 20°C.

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

 

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. 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 K_ow of -4 at pH 2, pH 4 and pH 7. At pH 9, the predicted log K_ow 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 dossier.

 

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

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