Registration Dossier

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Additional information

Terrestrial testing is not an Annex VIII requirement for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2). 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. As aquatic toxicity testing is not feasible, it may be more appropriate to use terrestrial tests in order to look into 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, 2017).

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 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane

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, 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 testing is waived as it is technically not feasible to conduct an aquatic study due to the insolubility of the substance (predicted water solubility of 9E-11 mg/l) and very high log Kow (9). 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 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

Phenyl silsesquioxanes is a multiconstituent 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 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane is also a mono-constituent substance consisting of >95% of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxaneand <5% identified impurities (phenyl-siloxane). In 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, each terminal silicon atom is substituted with one methyl and two phenyl groups, whilst the central silicon atom is substituted with one methyl and one phenyl group.

Both substances share structurally similar ultimate silanol hydrolysis products: 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane hydrolyses to form methyl(diphenyl)silanol and methyl(phenyl)silanediol, and silsesquioxanes, phenyl hydrolyses to form phenylsilanetriol and trimethylsilanol.

The source substance, 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, 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 target substance.

Read-across from decamethylcyclopentasiloxane (D5, CAS: 541-02-6) to 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2):

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

The registration substance, 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2), and the surrogate substance dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) are both within the siloxanes category. CAS 3390-61-2 has three silicon atoms, connected by two oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. D6 has 6 silicon atoms with methyl branches and alternated by oxygen. CAS 3390-61-2 is a linear siloxane while the surrogate substance is a cyclic siloxane. CAS 3390-61-2 and D5 have similar physicochemical properties: high molecular weight (546.89 and 370.78, respectively), low solubility (both insoluble at 9.0E-11 mg/l and 0.017 mg/l, respectively), high log Kow (9.0 and 8.0, respectively) and high log Koc (6 and 5.17, respectively). Both substances have negligible biodegradability and slow hydrolysis rates. A comparison of the key physicochemical properties is presented in the table in Section 6, Ecotoxicological Information discussion.

The testing has been carried out with species that are representative of three taxonomic groups; earthworms, springtails and plants:

  • 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-d LC50 value of >41 mg/kg and a NOEC of 41 mg/kg (mean measured concentration) (highest concentration tested), have been determined for effects of D5 on survival and weight change of adult earthworms. In the same test a 56-d LC50 value of >41 mg/kg and a NOEC of 41 mg/kg (mean measured concentration) (highest concentration tested), have been determined for effects of D5 on reproduction of the earthworm Eisenia fetida.
  • 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 IC50 value of 209 mg/kg dry weight has been determined for the effects of D5 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 D5 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 physico-chemical properties but are not close structural analogues (linear and cyclic siloxanes).

Read-across of the terrestrial toxicity data for D5 to 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) is considered to be suitable to derive an interim hazard and risk assessment under REACH for the registration substance.