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Key value for chemical safety assessment

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The SRC AOPWIN program (v1.92) has been used to obtain a value of the rate constant kOH for reaction of propylsilanetriol with hydroxyl radicals. This prediction method has not been validated to assess applicability to siloxanes/silanes/silanols; therefore, there is uncertainty associated with the calculated values obtained.

The overall half-life in air under default conditions of hydroxyl radical concentration was calculated using the following expressions:

kdegair (d-1) = kOH (cm3/molecule.sec) x OH Concair (molecules/cm3) x 24 x 3600

DT50 (d) = ln 2/ kdegair (d-1)


kdegair = total rate constant for degradation in air

kOH = rate constant for reaction with hydroxyl radicals

OH Concair = concentration of hydroxyl radicals in air = 5 x 105 OH molecules/ cm3

DT50 = half-life

The concentration of hydroxyl radicals in air of 5 x 105 OH molecules/ cm3, and the 24 hour photoperiod, are the values specified in ECHA Guidance on Information requirements and chemical safety assessment, Part R.16 Environmental exposure estimation (ECHA, 2016).

The results are given in Table 4.1.1.

Table 4.1.1 Results of photodegradation in air calculations


Result, propylsilanetriol

kOH (cm3/ molecule.sec)

12.9 x 10-12

kdegair (d-1)


DT50 (days)



Propylsilanetriol may undergo indirect phototransformation; calculated half-life for reaction with hydroxyl radicals in air is 1.2 days.