N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.12 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

1.2 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.012 mg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

7.4 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

1.19 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.12 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:

0.17 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The registered substance, N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine (CAS 7691-02-3), will hydrolyse very rapidly (half-life <<1 min at pH 4, 7 and 9 and 25°C, based on read-across) in contact with water and atmospheric moisture to dimethylvinylsilanol and ammonia.

 

REACH guidance (ECHA 2016, R.16) states that “for substances where hydrolytic DT₅₀ is less than 12 hours, environmental effects are likely to be attributed to the hydrolysis product rather than to the parent itself”. ECHA Guidance Chapter R.7b (ECHA 2017) states that OECD recommends testing breakdown products for DT₅₀ <1 hour. In addition, it states that where degradation is rapid (e.g. half-life < 1 hour), the available test data will frequently define the hazard of the degradation products since it will be these that have been tested. These data may be used to classify the parent substance in the normal way.

 

The substance may be released to the environment through wastewater treatment plant (WWTP) effluent only. The residency time in the waste-water treatment plant is approximately 15 hours with an average temperature of 15°C (assumed to be at neutral pH). Significant degradation by hydrolysis would be expected before the substance is released to the receiving waters.

 

The environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is therefore based on the properties of the hydrolysis products dimethylvinylsilanol and ammonia since both contribute to the toxicity, in accordance with REACH guidance.

 

As described in Sections 4.8 of IUCLID and 1.4 of the CSR, the silanol hydrolysis products may be susceptible to condensation reactions.

The environmental chemical safety assessment for aquatic, sediment and soil compartments, including PNEC derivation, is based on the hydrolysis products dimethylvinylsilanol and ammonia. The non-silicon hydrolysis product, ammonia, is known to be toxic and is likely to drive toxicity in aquatic tests. The toxicity of dimethylvinylsilanol and ammonia are therefore separately assessed and PNECs are derived separately for dimethylvinylsilanol and ammonia.

There are no reliable aquatic toxicity data available for N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine (CAS 7691-02-3).

Following the Assessment Entity approach, separate PNECs can be derived for (1) the silanol and (2) ammonia as follows:

 

1)      Silanol: PNEC derived from short-term data for trimethylsilanol (CAS 1066 40-6), which is a structurally-analogous read-across substance for the silanol hydrolysis product of N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine i.e. dimethylvinylsilanol.

 

2)      Ammonia: PNEC derived from the chronic data for ammonium compounds.

  

Acute data for trimethylsilanol (CAS 1066-40-6) 

Fish: LC50 (96 h): 271 mg/l;

Daphnia: EC50 (48 h): 124 mg/l;

Algae: EC50 (96 h): >684 mg/l.

 

The standard approach to PNEC calculation when acute data are available for three trophic levels is to apply an assessment factor of 1000 to the lowest lethal or effect concentration (E(L)C50).

 

Dimethylvinylsilanol is part of a category of organosilicon substances containing only alkyl, halogen, alkoxy or silanol groups attached to the Si atom. The category has a low hazard profile and data are consistent with a simple polar narcosis mechanism.

 

Published data are available for ammonia and are used to derive PNECs for ammonia. Please see the Ammonia endpoint summary for further discussion on data and PNEC derivation.

READ-ACROSS JUSTIFICATION 

In order to reduce 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 because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour.

 

The registered substance and the substance used as surrogate for read-across, trimethylsilanol (CAS 1066-40-6) are part of a group of organosilicons with low functionality side chains attached to the silicon. The toxicity of the substances in this group is determined by a non-polar narcosis mechanism of toxicity, and as such log K_ow drives toxicity.

 

Therefore the selection of surrogate substance is based on the log K_ow of the silanol hydrolysis product, as well as the groups present on the side chains.

 

Additional information is given in a supporting report (PFA 2016y) attached in Section 13.

 

The analogue approach for fulfilling the data requirement by read-across is discussed according to the Read-across Assessment Framework (RAAF).

 

Please see the report attached in Section 13 for the analogue approach to address ecotoxicity of N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine (CAS 7691-02-3) by read-across from trimethylsilanol (CAS 1066-40-6).

 

 

Discussion of read-across for supporting data

Read-across from trimethoxy(vinyl)silane (CAS 2768-02-7) to N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine (CAS 7691-02-3):

To provide evidence that the vinyl side chain does not contribute to the toxicity of the registered substance it has been necessary to use read-across from trimethoxy(vinyl)silane (CAS 2768-02-7). The available dataset on silicon substances containing a vinyl fragment indicates that this functional group does not have any specific mode of action with respect to ecotoxicity. This is represented by the ecotoxicity data on trimethoxy(vinyl)silane (CAS 2768-02-7), which has L(E)C₅₀values in the range >89 to 191 mg/l and a 21-d NOEC for invertebrates of 28.1 mg/l. The low log K_owvalues of trimethoxy(vinyl)silane and its silanol hydrolysis product, vinylsilanetriol (1.1 and -2 respectively), indicate that the results are consistent with non-polar narcosis toxicity, as seen with other alkoxysilanes. All the available short- and long-term toxicity data are read across to the registered substance.

 

Considerations of the non-silanol hydrolysis product

Ammonia is a known toxicant for aquatic organisms, in particular to fish and invertebrates. This compound drives the toxicity of the substance and is assessed separately to the silanol hydrolysis product. Further details on the assessment of the substance are presented in the ammonia ecological information summary and in the endpoint summaries.

 

Conclusion and PNEC derivations

Given the rapid hydrolysis of N-(dimethylvinylsilyl)-1,1-dimethyl-1-vinylsilylamine (CAS 7691-02-3), it is appropriate to carry out risk characterisation on the basis of the properties of the two hydrolysis products. Therefore, the PNECs based on data with trimethylsilanol and ammonia are used.