2,4,6,8-tetramethylcyclotetrasiloxane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

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:

0.73 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.073 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:

0.03 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

2,4,6,8-Tetramethylcyclotetrasiloxane (HD4, CAS 2370-88-9) hydrolyses rapidly with a hydrolysis half-life at pH 7 of 2.2 minutes. This rapid half-life is attributed to the ring-opening hydrolysis of the cyclic siloxane as the initial mechanism of degradation. Full hydrolysis of the Si-H groups to give the final product is expected to be slower but still complete in approximately 1 day (see Figure 1 of the read-across justification document, attached in Section 13, showing the reaction scheme for the hydrolysis of HD4 for further details).

The substance is predicted to hydrolyse rapidly, with several intermediate products being formed transiently. These intermediate hydrolysis products are linear hydroxy siloxanes and methylsilanediol; hydrogen gas is produced as a by-product of the reaction. The intermediate hydrolysis products are then expected to hydrolyse further to the final hydrolysis product methylsilanetriol (CAS 2445-53-6). This reaction is expected to be complete within one day. At low concentrations relevant for the environment, the final hydrolysis product is presumed to be methylsilanetriol. However, at higher concentrations (such as may be found in laboratory studies) the condensation of methylsilanetriol or the intermediate hydrolysis products may mean that polymeric species are formed.

Short-term aquatic toxicity to Daphnia and algae tests have been conducted with the parent substance. The substance was allowed to hydrolyse for 48 hours prior to testing, therefore the test organisms are likely to have been exposed to the hydrolysis products. The EC₅₀s were found to be >100 mg/l and the NOECs ≥100 mg/l based on nominal concentrations.

Toxicity to fish, invertebrates and algae data with the intermediate hydrolysis products have been predicted using ECOSAR version 2.0. The predicted data indicate that the intermediates would not be toxic at concentrations relevant to ecotoxicity tests.

Due to the instability of the substance and that of its breakdown products, as well as the absence of short-term aquatic toxicity, it is not considered necessary to conduct further testing with the parent substance. Furthermore, the substance is only used in industrial settings in closed systems under controlled conditions and emissions to surface waters are unlikely.

The environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the final hydrolysis product methylsilanetriol in accordance with REACH guidance for Chemical Safety Assessment.

Methylsilanetriol is highly water soluble (1000000 mg/l at 20 °C although condensation reactions become important above approximately 1000 mg/l and can limit the concentration dissolved in water) and has low log K_ow (-2.4 at 20 °C) and low vapour pressure (0.053 Pa at 25 °C).

Methylsilanetriol is considered to be a low-functionality silanetriol with no chemical groups attached that affect the toxicity of the substance. It is part of a class of low functionality compounds acting via a non-polar narcosis mechanism of toxicity.  The organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the Si atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. The registered substance hydrolyses rapidly in water and therefore the selection of surrogate substances is based the resulting silanol. The silanol hydrolysis products may be susceptible to condensation reactions (see Section 4.8 of IUCLID and Section 1.4 of the CSR). Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID dossier.

Short-term toxicity to fish, invertebrates and algae data which assess the toxicity of methylsilanetriol have been read across.

A chronic 21 day Daphnia magna reproduction test is also available with methylsilanetriol. 2,4,6,8-Tetramethylcyclotetrasiloxane (HD4, CAS 2370-88-9) hydrolyses rapidly to produce methylsilanetriol. Methylsilanetriol is the relevant species for chemical safety assessment, therefore the data with methylsilantriol are directly relevant for assessing toxicity of HD4.

READ ACROSS JUSTIFICATION

Please see the report attached in Section 13 for the analogue approach to address ecotoxicity of 2,4,6,8-tetramethylcyclotetrasiloxane (HD4, CAS 2370-88-9).

Conclusion on classification

The substance rapidly hydrolyses in aquatic media. Chemical safety assessment and hazard classification are therefore based on the properties of the silanol hydrolysis product methylsilanetriol. The parent substance has reliable short-term E(L)C₅₀ values of >100 mg/l in invertebrates and >100 mg/l in algae. Reliable short-term E(L)C₅₀ values of >500 mg/l in fish, >500 mg/l in invertebrates and >500 mg/l in algae have also been read-across from a substance that rapidly hydrolyses to methylsilanetriol. It has a reliable NOEC of ≥500 mg/l in algae and also a reliable long-term NOEC of ≥10 mg/l in Daphnia.

 

The substance hydrolyses very rapidly. The silanol hydrolysis product is not readily biodegradable but has low log K_ow.

 

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Not classified.