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Physical & Chemical properties

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Physico-chemical properties

(Chloromethyl)triethoxysilane is a liquid at standard temperature and pressure. It is predicted to have a melting point of <-20°C, a predicted boiling point of 200°C and a predicted density of 1.04 g/cm3at 25°C. The static viscosity of the substance is predicted to be 1.2 mm2/s at 20°C and the vapour pressure of the substance is predicted to be 6.5 Pa at 25°C. The substance is classified as a flammable liquid on the basis of the boiling point and a measured flash point of 49°C. It is not pyrophoric or flammable in contact with water. It has a measured self-ignition temperature of 225°C, and is not explosive and not oxidising on the basis of chemical structure. It is not surface active.

In contact with water (chloromethyl)triethoxysilane reacts rapidly (half-life 2.5 hours at pH 7 and 20 - 25°C) to produce chloromethylsilanetriol and ethanol according to the following equation:

                       (CH3CH2O)3SiCH2Cl  +  3H2O   →  ClCH2Si(OH)3  + 3CH3CH2OH

Therefore, requirements for testing of the water-based physicochemical properties for the substance and measurement of the surface tension are waived on the basis of instability in water. The properties of the hydrolysis products are assessed in instead.

The silanol hydrolysis product, chloromethylsilanetriol, is predicted to be very soluble in water 20oC (1E+06 mg/l) and has a log Kowof -2.1 (predicted). It is less volatile than the parent substance with a vapour pressure of 2.1E-03 at 25°C (predicted). The first dissociation constant (hydroxyl groups) of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa10.

Methanol is miscible with water, has low log Kowof -0.3 and high vapour pressure of 7910 Pa at 20°C) (OECD, 2004).

Silanetriols may undergo condensation reactions to give siloxane dimers, oligomers and polymers, according to the scheme:

 

RSi(OH)3    RSi(OH)2OSi(OH)2R    RSi(OH)2O[Si(R)(OH)O]nSi(OH)2R

 

(where R is an alkyl or aryl side-chain)

 

A highly cross-linked gel may form. The degree of condensation that will occur may vary with:

  • Concentration of the silanol; the greater the initial concentration, the greater the degree of condensation. Significant condensation is not expected at concentrations less than approximately 100 mg/l, but is dependent on specific conditions.
  • pH; the condensation reaction may be either acid or base catalysed.
  • Temperature.
  • Other species present.  
  • The nature of the R group
  • The number of Si-OH groups; silanetriols condense more rapidly than silanediols.