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Physical & Chemical properties

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Physico-chemical properties

Trimethoxy(propyl)silane is a liquid at standard temperature and pressure, with a predicted melting point of < -20°C and a measured boiling point of 137°C. It has a measured density of at 0.94 g/cm3 at 20°C, a measured viscosity of 0.7 mPa sat 20°C and a predicted vapour pressure of 290 Pa at 25°C.

The substance is classified as a flammable liquid on the basis of a measured flash point of 35°C and the boiling point of 137°C. It has a measured auto-ignition temperature of 245°C, and is not explosive and not oxidising on the basis of chemical structure.

In contact with water, trimethoxy(propyl)silane is predicted to rapidly hydrolyse (half-life of approximately 2.6 hour at 20-25°C and pH 7) to produce propylsilanetriol and methanol according to the following equation:

(CH3O)3Si(C3H7) + 3 H2O  →  C3H7Si(OH)3  + 3 CH3OH

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, propylsilanetriol are assessed instead. Propylsilanetriol has a predicted solubility in water of 1.0E+06 mg/l at 20°C and has a low log Kow (-1.4, predicted). It is not surface active and the volatility is expected to be very low. It is not expected to undergo significant dissociation within an environmentally relevant range. The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa=10.

Methanol is miscible with water, has low log Kow (-0.82 to -0.64) and high vapour pressure (12790 Pa at 25°C).

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 thanapproximately100 mg/l, but is dependent on specific conditions
  • pH; the condensation reaction may be either acid or base catalysed
  • Temperature
  • Other species present
  • Timescale
  • The nature of the R group
  • The number of Si-OH groups; silanetriols condense more rapidly than silanediols