Registration Dossier

Diss Factsheets

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Additional information

Terrestrial testing is not an Annex VIII requirement for Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9; EC No. 451-620-7). However, the low water solubility, high log Kow and high log Koc of the substance indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant. No effects were observed in the short-term aquatic toxicity tests. As the substance is poorly water soluble and the aquatic data available are based on WAFs, it is more appropriate to use terrestrial data to assess the potential toxicity of the substance in the environment.

Screening Assessment and Testing Strategy for Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9)

The registered substance falls within soil hazard category 3 as defined in REACH R.7; high absorption (log Kow >5) but no indication that the substance is very toxic to aquatic organisms. As discussed in Section 6.0, for substances with a log Kow of ~6 or above, aquatic toxicity is not expected because the very limited aqueous solubility prevents toxic concentrations being achieved. Aquatic toxicity data show no effects at loading rates that are above the limit of water solubility of the substance. A true PNEC cannot be calculated from the aquatic test data because the EL50 values that have been determined are limit values and loading rate values should not be used for PNEC derivation. It is therefore not possible to conduct the screening assessment for terrestrial testing by calculating a PNECsoil using the equilibrium partitioning approach.

OECD TG 216 soil microorganism and OECD TG 208 seedling emergence and growth tests have been conducted with the registered substance in accordance with ECHA final decision TPE-D-2114497761-36-01/F. An OECD TG 222 earthworm reproduction test is also required and is currently in progress.

Until the results of the OECD 222 earthworm study are available, the chemical safety assessment cannot be finalised and PNECs cannot be derived from test data with the registered substance.

The substance dataset and risk assessment will therefore be updated further once the results of the earthworm study are available and the study report is finalised.

The definitive earthworm test began on 26th May 2021. Days 28 and 56 will be 23rd June and 21st July respectively. The final study report is expected to be available in October 2021 based on the schedule provided by the test laboratory. As soon as the final report is available the PNECs and chemical safety assessment will be updated. It is anticipated that the revised dossier will be completed by the end of December 2021.

Until the earthworm data become available, PNECsoil has been derived based on data read across from decamethylcyclopentasiloxane (D5, CAS 541-02-6) in order to perform risk characterisation.

Read-across justification

Read-across from decamethylcyclopentasiloxane (D5, CAS: 541-02-6) to Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9):

In the interim period, before the data with the registered substance become available, data can be read-across from D5 (CAS 541-02-6) for toxicity to soil macroorganisms except arthropods. Alongside existing data for toxicity to terrestrial arthropods and toxicity to terrestrial plants, the data for D5 are considered to be suitable for deriving an interim PNECsoil for the registered substance.

The registered substance, Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9), and the surrogate substance decamethylcyclopentasiloxane (D5, CAS 541-02-6) are both members of the siloxanes category. D5 is a cyclic siloxane with 5 silicon atoms with methyl branches and alternated by oxygen. The registration substance is a multi-constituent substance containing two main constituents 3,3-diphenylhexamethyltrisiloxane (55-70%) and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (15-25%). The main constituent has three silicon atoms with 2 phenyl groups and 6 methyl groups. The second constituent has 4 silicon atoms, with 4 phenyl groups and 6 methyl groups. Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9) and D5 have similar physicochemical properties: high molecular weight (360.68-558.98 g/mol and 370.78 g/mol respectively), low water solubility (2.8E-05 mg/L for Constituent 1, 1.5E-10 mg/L for Constituent 2 and 1.7E-03 mg/L for D5), high log Kow (9.0 and 8.07 respectively) and high log Koc (6.0 and 5.17-5.34, respectively). Both substances have negligible biodegradability and hydrolysis rates. A comparison of the key physicochemical properties is presented in the table in Section 6.0,

• A 28-day LC50 value of >4074 mg/kg dry weight and a 56-day NOEC of ≥4074 mg/kg dry weight have been determined for the effects of D5 on mortality and reproduction and growth respectively of the earthworm, Eisenia andrei.

• A 28-day LC50 value of 813 mg/kg dry weight and a 28-day IC50 value of 767 mg/kg dry weight have been determined for the effects of the D5 on mortality and reproduction of the springtail, Folsomia candida. A NOEC of 377 mg/kg dry weight has been determined by the reviewer on the basis of a visual examination of the data for both mortality and reproduction.

• A 14 day IC50value of 209 mg/kg dry weight has been determined for the effects of the test substance on root dry mass of Barley, Hordeum vulgare. IC50/EC50 values for effects on seedling emergence, root and shoot length and shoot dry mass determined in the same test were ≥248 mg/kg dry weight. 14 Day EC50 values of >4054 mg/kg dry weight have been determined for the effects of the test substance on seedling emergence, root and shoot length and root and shoot dry mass of Red Clover, Trifolium pratense. NOECs were not determined in the tests.

The studies are considered to be non-standard; they have been conducted by spiking surrogate biosolids mixed into natural soil rather than by direct spiking of soil. The registered substance and the surrogate substance share similar physicochemical properties but are not close structural analogues (linear and cyclic siloxanes).

Read-across of the terrestrial toxicity data for D5 to Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9) is considered to be suitable to derive an interim hazard and risk assessment under REACH until the data with the registered substance are available.

Categories Display