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Ecotoxicological information

Endpoint summary

Administrative data

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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). 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 may be more appropriate to use terrestrial data to assess the potential toxicity of the substance in the environment.

No terrestrial toxicity data are available for the registration substance; a category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017at).

There is a limited amount of terrestrial toxicity data available with siloxanes. An integrated testing strategy for the category is proposed. In view of the high potential to adsorb to soil for siloxane substances, and the lack of terrestrial toxicity testing across the Category, it is concluded that further testing is required.

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

The registration 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. Read-across of terrestrial toxicity data is proposed from the structural analogue silsesquioxanes, phenyl (CAS 70131-69-0). Terrestrial testing proposals with silsesquioxanes, phenyl, are awaiting ECHA approval.

Read-across justification

Silsesquioxanes, phenyl (CAS 70131-69-0) is a multi-constituent substance, and is a reaction mass of the monomer, dimer and oligomers of phenyltris(trimethylsiloxy)silane. The linear oligomers of phenyl silsesquioxanes contain a siloxane chain, where the terminal Si atoms are fully methyl substituted, and the repeating unit contains a Si atom substituted with a phenyl and a trimethylsiloxy group. The major constituents comprise 70-80% of the linear oligomers n=1-5; the minor constituents comprise 10-20% cyclic (n=3-7) and linear (n=6-7) oligomers. The combined purity of both linear and cyclic oligomers is about 85-95%. The remaining impurities are higher polymerised material and alkoxy substituted oligomers. The submission substance, Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9), is also 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 Silsesquioxanes, phenyl (CAS 70131-69-0) have similar physicochemical properties: high molecular weight (360.68-558.98 g/mol and 372.8 to 1635.1 g/mol respectively), low water solubility (2.8E-05 mg/L for Constituent 1, 1.5E-10 mg/L for Constituent 2 and 4.9E-05 to 4.94E-51 mg/L for Silsesquioxanes, phenyl) high log Kow (both 9.0) and high log Koc (both 6.0). Both substances have negligible biodegradability and hydrolysis rates.

The source substance, Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9), and the target substance, silsesquioxanes, phenyl, are members of the Siloxanes Category. Terrestrial toxicity studies with siloxanes are considered to be difficult to conduct due to their high volatilisation potential (high Henry’s Law Constant and low octanol-air partition coefficient) and the potential for degradation in soil. This is based both on chemical properties, and challenges encountered in studies of fate and ecotoxicity conducted to date with substances within this Category. Soil testing according to guideline methods does not allow for a renewal of the substrate and hence re-application of test substance. Therefore, there is potential for the organisms to not be exposed to the test material for a sufficiently long period of time for effects to be expressed, as well as the difficulty of quantifying actual exposure concentrations.

A stability/recovery test under OECD TG 222 conditions performed with the structurally-related substance (L4) demonstrated significant loss of test item from the test system over a five-week period (37% remaining radioactivity after 35 days), ascribed to volatilisation losses. However, it is considered that it is possible that measurable concentrations will remain in the soil at the end of the eight-week test period for the definitive OECD TG 222 study. Silsesquioxanes, phenyl is expected to be more stable in soil, therefore an OECD TG 222 study is proposed. The need for further terrestrial toxicity tests (OECD TG 208) will be reviewed once the results of this study are available.

An OECD TG 216 toxicity to soil microorganisms study is also proposed for the silsesquioxanes, phenyl.

 

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 silsesquioxanes, phenyl become available, data can be read-across from D5 (CAS 541-02-6) for toxicity to soil macroorganisms except arthropods and toxicity to terrestrial arthropods. The data are considered to be suitable for deriving an interim PNECsoil for the registration substance.

The registration 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 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 8.0 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.

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 for the registration substance.