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Description of key information

Key value for chemical safety assessment

Degradation rate constant with OH radicals:
0 m³ molecule-1 s-1

Additional information

No measured data are available for 3,3-Bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane (ViM4Q).

ViM4Q contains 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 AOPWIN program (v1.92, EPA 2010) has been used to obtain values of the rate constant kOH for reaction of ViM4Q with hydroxyl radicals. This prediction method has not been validated to assess applicability to siloxanes; 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)

Where:

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 105OH molecules/ cm3

DT50= half-life

The concentration of hydroxyl radicals in air of 5 x 105OH 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 4.1.2.

Table4.1.2: Results of photodegradation in air calculations

Parameter

Result, ViM4Q

kOH (cm3 / molecule.sec)

1.1 x 10-10

kdegair (d-1)

4.6

DT50(days)

0.2

Measured data for reaction with hydroxyl radicals in air are available for some organosilanes. A summary of these measured data is in the table below.

AOPWIN predictions are also presented for comparison with the measured data.


Table4.1.3Measured data and AOPWIN predictions for reaction with hydroxyl radicals in air.

Substance

Rate constantfor reaction with hydroxyl radicals (kOH(cm3/ molecule. sec))

Half-life (days)

Tetramethylsilane

1.28 x 10-12 (Sommerlade, 1993)

0.6 x 10-12 (AOPWIN)

1.0 x 10-12 (Atkinson, 1991)

8.5 x 10-13 (Tuazon, 2000)

13

27

16

19

Hexamethyldisiloxane

1.19 x 10-12 (Sommerlade, 1993)

0.9 x 10-12 (AOPWIN)

1.4 x 10-12 (Atkinson, 1991)

14

18

12

Octamethylcyclotetrasiloxane

1.26 x 10-12 (Sommerlade, 1993)

1.2 x 10-12 (AOPWIN)

1.0 x 10-12 (Atkinson, 1991)

13

13

16

Hexamethylcyclotrisiloxane

0.9 x 10-12 (AOPWIN)

0.5 x 10-12 (Atkinson, 1991)

18

31

Decamethylcyclopentasiloxane

1.5 x 10-12 (AOPWIN)

1.6 x 10-12 (Atkinson, 1991)

11

10

Dimethylsilanediol

7.2 x 10-12 (AOPWIN)

8.1 x 10-13 (Tuazon, 2000)

2

20

Trimethylsilanol

3.95 x 10-12 (Sommerlade, 1993)

3.9 x 10-12 (AOPWIN)

7.2 x 10-13 (Tuazon, 2000)

4

4

22

The measured values from Sommerlade (1993) and Atkinson (1991) are in sufficient agreement, and correlate well with the predicted values. Indeed, the data from these two studies were used in the training set for the AOPWIN program.

The measured values from Tuazon (2000) indicate slightly lower rates of reaction for the silanols compared to the AOPWIN predictions and the measured value from Sommerlade (1993).

References:

EPA, 2010. US Environmental Protection Agency.AOPWIN program v1.92a (September, 2010)

ECHA (2016). European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version:3.0February 2016.

Sommerlade et al. (1993). Product Analysis and Kinetics of the Gas-Phase Reactions of Selected Organosilicon Compounds with OH Radicals Using a Smog Chamber-Mass Spectrometer System. Environ. Sci. Technol. 1993, 27, 2435-2440.

Tuazon E C, Aschmann S M and Atkinson R (2000) Atmospheric Degradation of Volatile Methyl-Silicon Compounds Environmental Science and Technology, Vol. 34, No. 10, 1970-1975

Atkinson R. 1991. Kinetics of the Gas-Phase Reactions of a Series of Organosilicon Compounds with OH and NO3 Radicals and O3 at 297 +/- 2 K. Environ. Sci. Technol. 25(5):863-866.