Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.25 mg/L
Assessment factor:
500
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
2.5 mg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.025 mg/L
Assessment factor:
5 000

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
3.2 mg/L
Assessment factor:
10
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
2.4 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
0.24 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
0.15 mg/kg soil dw
Extrapolation method:
equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential for bioaccumulation

Additional information

1,1,3,3-Tetramethyldisiloxane hydrolyses very rapidly (t ½ = 11.3 min) to produce the intermediate hydrolysis product, dimethylsilanol. Dimethylsilanol then hydrolyses more slowly to produce the ultimate hydrolysis products, dimethylsilanediol (CAS 1066-42-8) and hydrogen.

Hydrogen gas is a ubiquitous element and is considered to be non-hazardous.The PNECs for the aquatic freshwater and marine water compartments have therefore been determined on the basis of data for dimethylsilanol, while those for the sediment and soil compartments have been derived using data for dimethylsilanediol.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

In aqueous media, tetramethyldisiloxane hydrolyses very rapidly (half-life11.3 min at pH 7 and 25°C, measured) to dimethylsilanol first then to dimethylsilanediol after an estimated 2.5 days (see Section 4.1.1.1) producing a non silanol hydrolysis product, hydrogen. Dimethylsilanol and dimethylsilanediol are a monosilanol and silanediol respectively with different physicochemical properties. Because of the long exposure to the intermediate and ultimate hydrolysis product to aquatic organisms, the assessment is based on the two products and surrogates are proposed for both substances. Hydrogen is considered to be non-hazardous and so the hazard of this substance has not been assessed. Further considerations on hydrogen are given below.

Dimethylsilanol and dimethylsilanediol and the substances used as surrogates for read-across are part of a class of non-functional compounds which act via a non-polar narcosis mechanism of toxicity.Substances in this group include alkly, aryl, alkoxy, hydroxyl or chloro groups having secondary features that do not affect the toxicity of the substances.The registered substance hydrolyses very rapidly in water and therefore the selection of surrogate substance is based on the structure and log Kow of the silanol hydrolysis products. Additional information is given in a supporting report (PFA 2013i)attached in Section 13 of the IUCLID 5 dossier.

In the following paragraphs the read-across approach for tetramethyldisiloxane is assessed for each surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

Read-across from trimethylsilanol to dimethylsilanol (intermediate hydrolysis product):

Hydroxytrimethylsilane has been used as a surrogate to read-across to dimethysilanol because they are structurally analogous monosilanes with three and two methyl side chains respectively. They have very similar properties: low log Kow (-0.4 and 0.6 respectively), high water solubility (1E+06 and 19 000 mg/l respectively) and low molecular weight (MW 90.2 and 76.17 respectively) and are regarded as not biodegradable within 28-days.

The data from hydroxytrimethylsilane indicate that the substance is of low short-term toxicity to aquatic organisms, with the lowest short-term results being a 48 h EC50 124 mg/l for invertebrates.

Read-across from dimethylsilanediol (ultimate degradation product):

Data from dimethylsilanediol have been read-across to determine the hazard of the ultimate hydrolysis product of the registered substance, which is dimethylsilanediol itself; therefore the read-across is considered to be directly relevant.

 

QSAR

Estimated values derived for neutral organics from the established software ECOSAR ® v.1.11 have been used to support the data for dimethylsilanediol. The predictions indicate that dimethylsilanediol is of low short term toxicity to aquatic organisms.

 

Read-across from trimethoxysilane to tetramethyldisiloxane:

Tetramethyldisiloxanecontain the Si-H function, which can undergo a reaction to give Si-OH with release of hydrogen,to produce dimethylsilanediol. To support the notion that the Si-H bond reaction does not contribute to toxicity of the registered substance, read-across from trimethoxysilane has been used. From the available dataset, the Si-H group is understood to not have any specific mode of action with respect to ecotoxicity. Short-term toxicity data withtrimethoxysilane (CAS 2487-90-3) determine no effects up to 100 mg/l.This indicates that Si-H reactions with electrophilic compounds, such as proteins, are not expected to affect toxicity.

 

Considerations on the non-silanol hydrolysis products:

The other hydrolysis product hydrogen gas, is a ubiquitous element present in the atmosphere at 0.55 ppmV. Any anthropogenic contribution of hydrogen gas to the atmosphere from hydrolysis of dimethylsilanol is negligible therefore substance is not considered in the risk assessment.


Table 7.1.1 Summary of ecotoxicological and physico-chemical properties for the registered substance, its hydrolysis products and the surrogate substances.

CAS Number

3277-26-7

1066-42-8

1066-40-6

2487-90-3

Chemical Name

1,1,3,3-tetramethyldisiloxane

Dimethylsilanediol (DMSD)

Trimethylsilanol

Trimethoxysilane

Si hydrolysis product

Dimethylsilanediol

Dimethylsilanediol

Trimethylsilanol

Silicic acid

Molecular weight (parent)

134.33

92.17

90.2

122.2

Molecular weight (hydrolysis product,)

92.17 (DMSD)

76.17 (dimerthylsilanol)

92.17

90.2

96.1

log Kow(parent)

n/a

-0.38

1.2

0.2

log Kow(silanol hydrolysis product)

-0.38 (DMSD)

0.6 (dimethylsilanol)

-0.38

1.2

-2.9

Water sol (parent)

n/a

1E+06 mg/l

995 mg/l

54,000 mg/l

Water sol (silanol hydrolysis product))

1E+06 mg/l (DMSD)

1.9E+04 mg/l (dimethylsilanol)

1E+06 mg/l

995 mg/l

760,000 mg/l

Vapour pressure (parent)

18000 Pa

7 Pa

1900 Pa

113.7 hPa

Vapour pressure (hydrolysis product)

7 Pa (DMSD)

770 Pa (dimerthylsilanol)

7 Pa

1900 Pa

<1E-06 Pa

Hydrolysis t1/2at pH 7 and 25°C

 11.3 min

n/a

n/a

≤17 s at 2°C

Short-term toxicity to fish (LC50)

n/a

>126 mg/L

270 mg/l

>100 mg/l

Short-term toxicity to aquatic invertebrates (EC50)

n/a

>117 mg/l

124 mg/l

>100 mg/l

Algal inhibition (ErC50and NOEC)

n/a

>118 mg/l and ≥118 mg/l

>750 and 70 mg/l

>100 and <6.3 mg/l

Conclusion on classification

1,1,3,3-Tetramethyldisiloxane hydrolyses very rapidly to dimethylsilanol, then further reactions take place and the ultimate degradation product is dimethylsilanediol. Neither of the silanol hydrolysis products are readily biodegradable.

1,1,3,3–Tetramethyldisiloxane is not classified in the EU according to the Regulation (EC) No 1272/2008 because data for the substance and appropriate read-across data for its hydrolysis products indicate that it is not toxic up to a concentration of 100 mg/l, i. e. lowest E(L)C50 is >100 mg/l. Additionally, the hydrolysis products have a log Kow of <3.