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The substance, trichlorosilane, 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.

In contact with water, trichlorosilane is expected to react very rapidly (t1/2 ≤17 seconds at pH 4, pH 7 and pH 9 and 1.5°C, based on read-across from a related chlorosilane substance) to form silanetriol as intermediate hydrolysis product and hydrochloric acid according to the following equation:

HSiCl3+ 3H2O → HSi(OH)3+ 3HCl

The Si-H bond is also unstable, resulting in further reaction to monosilicic acid Si(OH)4with hydrogen as a by-product. The rate of this reaction has not been determined but is expected to be rapid. The overall equation is given below:

HSiCl3+ 4H2O → Si(OH)4+ 3HCl + H2

Both silanetriol and monosilicic acid exist only in dilute aqueous solutions and readily condense at concentrations above approximately 100 -150 mg/L to give a dynamic equilibrium between monomer, oligomers and insoluble amorphous polysilicic acid.

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 initial and final silanol hydrolysis products, silanetriol and monosilicic acid, are assessed instead.

The water solubility of both silanol hydrolysis products, silicic acid and silanetriol will be limited by the strong tendency of Si(OH)4 to undergo condensation reactions of the type Si-OH → Si-O-Si; this can lead to formation of a highly cross-linked network (gel). They are expected to have negligible vapour pressures and they are not surface active. Log Kow is not relevant for inorganic compounds such as silanetriol and silicic acid.

For both substances, solubility is limited by condensation reactions as discussed above. Their volatility is expected to be very low. They are not surface active and do not undergo significant dissociation within the environmentally-relevant pH range; the first dissociation constant is approximately 10.

Trichlorosilane is a liquid at standard temperature and pressure, with a melting point of -126.5°C and a boiling point of 31.5 - 33°C. It has a density greater than that of water (1.33-1.35 g/cm3) at 20°C and moderate viscosity of 0.33 mPa.s (dynamic) at 20°C. It is highly volatile, vapour pressure of 72200 Pa at 22.5°C.

The substance is classified as extremely flammable on the basis of a flash point of <-19.5°C and boiling point of 31.5-33°C. In addition, the substance is also classified as a substance in contact with water that emits highly flammable gas (i.e. hydrogen) in accordance with UN Test Method N.5. In accordance with UN Test method N.3, the substance is not classified for pyrophoricity. It has an auto-ignition temperature of 224°C at 1013 hPa, is not explosive and is not oxidising based on structural examination.

The by-product of the Si-H reaction, hydrogen, is exempt from registration under REACH Annex V. It is therefore not assessed in this CSA.


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