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Description of key information

The calculated half-lives were 2 to 9 days using a tropospheric concentration of OH radicals of 7.7 * 105molecule cm-3 over a 24 h period.

Key value for chemical safety assessment

Half-life in air:
5.5 d
Degradation rate constant with OH radicals:
770 000 cm³ molecule-1 d-1

Additional information

The constituents of MS-Silane contain no chromophores that would absorb visible or UV radiation so direct photolysis is not likely to be significant.  Indirect photolysis resulting from gas-phase reaction with photochemically produced hydroxyl radicals may occur. 

The SRC AOPWIN program was used to obtain values of the rate constant kOH for reaction of silanes with hydroxyl radicals. 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 OHConcair(molec/cm3) x 12 x 3600

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

Where:

kdegair= total rate constant for degradation in air

kOH= rate constant for reaction with hydroxyl radicals

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

DT50= half-life

The results are given in the table below.

CAS

kdegair/ d-1

DT50 days

Chlorinated compounds

75-78-5

6.46E-03

107.2532

4525-44-4

2.90E-02

23.87162

1450-14-2

1.94E-02

 

1560-28-7

1.62E-02

42.90127

4342-61-4

1.29E-02

53.62658

75-54-7

3.23E-03

214.5063

75-77-4

9.69E-03

71.50211

2401-73-2

1.29E-02

53.62658

2943-62-6

1.62E-02

42.90127

4518-94-9

5.90E-02

11.75722

N/A

4.06E-02

17.08613

Silane diols

75-78-5

1.56E-01

4.45746

4525-44-4

1.78E-01

3.892414

1450-14-2

1.94E-02

 

1560-28-7

9.07E-02

7.644151

4342-61-4

1.62E-01

4.279599

75-54-7

1.52E-01

4.552052

75-77-4

8.42E-02

8.230775

2401-73-2

1.62E-01

4.279599

2943-62-6

9.07E-02

7.644151

4518-94-9

2.08E-01

3.332518

10026-04-7

2.98E-01

2.325373

N/A

1.15E-01

6.022767

Rate constants for OH-radical reactions were obtained for another four organosilicon compounds in a smog chamber coupled to a quadruple mass spectrometer. The rate constants for OH-radical reactions were 1.28 ± 0.46 * 10-12cm3molecule-1s-1for tetramethylsilane, 1.19 ± 0.30 * 10-12cm3molecule-1s-1for hexamethyldisiloxane and 3.95 ± 0.95 * 10-12cm3molecule-1s-1for trimethylsilanol and 1.26 ± 0.40 * 10-12cm3molecule-1s-1for octamethylcyclotetrasiloxane. The calculated half-lives were 2 to 9 days using a tropospheric concentration of OH radicals of 7.7 * 105molecule cm-3over a 24 h period (Sommerlade 1993).

An indirect photodegradation study, which is considered reliable, is also available for the hydrolysis product of dichloro(dimethyl)silane,dimethylsilanediol (Tuazon et al., 2000). This study measured the rate of gas-phase reactions of dimethylsilanediol with the OH radical, produced via N2H4 + O3 reaction as a nonphotolytic source of OH radicals, with analysis by Fourier transform infrared (FT-IR) spectroscopy in a 5870 L chamber. The measured value of the OH radical reaction rate constant at 298 ± 2 K is 8.1 ± 1.0 x 10-13cm3molecule-1s-1, which using the equation above is equivalent to 20 days.

Chlorosilanes are very hydrolytically unstable, therefore reaction with water vapour, rather than photolysis, is expected to be the primary degradation process.