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2,4,6-Trimethyl-2,4,6-tris(3,3,3-trifluoropropyl)cyclotrisiloxane (CAS 2374 -14 -3, F-D3) has an extremely low estimated water solubility (1.3E-06 mg/l) and very high estimated log Kow (9).

The substance is susceptible to hydrolysis with a half-life of 6 days at pH 7 and 25°C, and while the reaction products (fluorinated siloxanediols) in principle have higher water solubility than F-D3, preliminary evidence from the long-term Daphnia test suggests that precipitation of insoluble oligomers occurs at loading rates above the solubility of the test substance (refer to IUCLID Sections 4 and 5.1.2 for further discussion on condensation of the silanol hydrolysis product). However, the registered substance has a high adsorption potential, and once adsorbed, hydrolysis may not occur.

Regarding approach to filling data gaps: These physicochemical properties indicate that maintaining stable aqueous test solutions will be extremely difficult (as confirmed by the attempted long-term study with Daphnia magna). Furthermore, the substance will have the tendency to adsorb onto the glassware under experimental conditions. This contributes to technical difficulties making reliable experimental studies very difficult to conduct.

Acute aquatic toxicity studies were not conducted due to the very low water solubility of the substance.  However, a long-term Daphnia test performed at the limit of functional water solubility showed no acute effects.

In addition:

- Above a log Kow of 6.0 short-term toxicity effects are not expected to occur, because the very limited aqueous solubility prevents toxic concentrations being achieved in exposure media. Similarly, above log Kow of 8, long-term toxicity effects are not expected1.

Regarding environmental exposure: the chemical properties indicate the substance will have the tendency to adsorb onto dissolved organic matter in the natural environment. Therefore, aqueous exposure is unlikely.

In addition:

- The substance is classified for severe human health effects; therefore measures are in place to minimise exposure. These control measures to prevent human health exposure will also control environmental exposure.

- The physicochemical properties and very high value of Koc (6.0) means that if the substance were to enter a wastewater treatment plant in influent waste waters, it would partition mainly to sludges (a small proportion may be volatilised), with a negligible amount passing to water. The result is that exposure of freshwater or marine aquatic compartments is negligible.

Furthermore, the substance is not manufactured or used as such in the EU and is registered in accordance with the REACH Regulation (Article 6) in order to support import of polymers. Residual unbound monomer in polymer is very low and the substance is of extremely low water solubility, such that unintentional release to the aquatic environment from end use of the polymers is negligible.

1The very high log Kow(>7.5) and very low limit of solubility in water mean the substances have very low bioavailability. For substances with these properties, chronic aquatic ecotoxicity is unlikely to be of concern. The principle of log Kow-based cut-off values for toxic effects has been embraced by the ICCA and OECD high Production Volume (HPV) chemical programmes (e.g. UNEP, 2000). The US EPA has also accepted them in its Pollution Prevention Framework (http://www.epa.gov/oppt/p2framework/docs/casestu.htm). The ECOSAR QSAR applies a log Kowcut-off value of 8.0 beyond which chronic toxicity of neutral organics would not be expected because of limitations in bioavailability and uptake of the substance (Clements, 1996). These cut-offs are empirically derived and based on experimental test data.

References:

Clements (1996). Estimating toxicity of industrial chemicals to aquatic organisms using structure-activity relationships. Edited by: Richard G. Clements. Contributors: R.G. Clements, J.V. Nabholz, M. Zeeman, Environmental Effects Branch, Health and Environmental Review Division, Office of Pollution Prevention and Toxics, U.S. Environmental Protection Agency, Washington, DC 20460, August 30, 1996.

UNEP (2000). OECD SIDS Alfa Olefins(CAS N°:592-41-6, 111-66-0, 872-05-9,112-41-4, 1120-36-1).UNEP Publications.