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

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:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

no hazard identified

Sediment (marine water)

Hazard assessment conclusion:

no hazard identified

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential to cause toxic effects if accumulated (in higher organisms) via the food chain

Additional information

1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) has been predicted to be insoluble in water with a calculated solubility of 9E-11 mg/l. It has very high estimated log K_ow (9.0 at 20-25°C).

The substance is susceptible to hydrolysis with a half-life of >329 hours at pH 7 and 20-25°C (read-across from structural analogue). The reaction products are methyl(diphenyl)silanol (CAS 778-25-6) and methyl(phenyl)silanediol. The registration substance has a high adsorption potential, and once adsorbed, hydrolysis may not occur.

On the basis that the substance is insoluble, data waivers are in place for short-term toxicity to aquatic invertebrates and algae. The physicochemical properties of the registration substance indicate that maintaining stable aqueous test solutions will be extremely difficult. 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.

The registration substance, and the substances used as surrogate for read-across, are members of the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

Additional information is given in a supporting report (PFA 2017at) attached in Section 13 of the IUCLID dossier.

In addition:

- Above a log K_ow 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 K_ow of 8, long-term toxicity effects are not expected.¹

- 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.

- The physicochemical properties and very high value of K_oc (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.

Sediment and terrestrial testing are not an Annex VIII requirement for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. However, the low water solubility, high log K_owand high log K_ocof the substance indicate that it will adsorb to organic matter and partition to sludges in a waste water treatment plant, as mentioned above. As aquatic toxicity tests are not feasible, it may be more appropriate to consider sediment and terrestrial toxicity in order to assess the potential toxicity of the substance in the environment. Data are read-across from the analogous substances dodecamethylpentasiloxane (L5, CAS 141-63-9) for the sediment compartment, and phenyl silsesquioxanes (EC 939-487-8) for the terrestrial compartment.

¹The very high log K_ow (9.0) and very low limit of solubility in water mean the substance has very low bioavailability. For substances with these properties, chronic aquatic ecotoxicity is unlikely to be of concern. The principle of log K_ow-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 K_ow cut-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.

Consideration of low functionality side chains and reactive groups

Phenyl group: The ecotoxicity of phenyls is related to non-polar narcosis. The ecotoxicity of substances with phenyl groups increases with increasing log K_ow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log K_ow of the hydrolysis product is -0.2. Short-term toxicity data with trichlorophenylsilane determine no effects up to 100 mg/l (Springborn Smithers 2009a, b and c).

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to comply with REACH requirements for the assessment of risk and hazard for the registration substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

The registration substance (1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2)) and the substances used as surrogate for read-across are members of the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

Further details and the application of the Read-Across Assessment Framework (RAAF), are found in the siloxanes category supporting report (PFA 2017at) attached in Section 13 of the IUCLID dossier.

In the following paragraphs the read-across approach for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane is assessed for the surrogate substances taking into account structure, hydrolysis rate and physico-chemical properties.

Read across from dodecamethylpentasiloxane (L5, CAS 141-63-9) to 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2)

The registration substance, 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2), and the surrogate substance dodecamethylpentasiloxane (L5, CAS 141-63-9) are both linear siloxanes. CAS 3390-61-2 has three silicon atoms, connected by two oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. 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. L5 has 5 silicon atoms with methyl branches and alternated by oxygen. Cas 3390-61-2 and L5 possess similar physico-chemical properties: high molecular weight (546.89 and 348.9, respectively), low water solubility (both insoluble, at 9.0E-11 mg/l and 7.0E-05 mg/l, respectively), high log K_ow(9 and 9.3-9.5, respectively) and high log K_oc(6 and 6.3, respectively). Both substances have negligible biodegradability and slow hydrolysis rates. A comparison of the key physicochemical properties is presented in the table below.

Read-across from phenyl silsesquioxanes (EC 939-487-8) to 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2)

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 phenyl silsesquioxanes hydrolyses to form phenylsilanetriol and trimethylsilanol.

A comparison of the key physicochemical properties is presented in the table below.

Table 7.0.1. Summary of aquatic ecotoxicological and physico-chemical properties for the registered substance, its hydrolysis products and the surrogate substances.

CASNumber	3390-61-2	141-63-9	EC 939-487-8
Chemical Name	1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane	Dodecamethylpentasiloxane (L5)	Phenyl silsesquioxanes
Si hydrolysis product	methyl(diphenyl)silanol (CAS 778-25-6) and methyl(phenyl)silanediol	Dimethylsilanediol (CAS 1066-42-8) and trimethylsilanol (CAS 1066-40-6)	Phenylsilanetriol and trimethylsilanol
Molecular weight (parent)	546.89	384.85	372.8 to 1635.1
log Kow(parent)	9	9.41	9.0
Log Koc(parent)	6	6.3	6.0
Water sol (parent)	9.0E-11 mg/l	7.0E-05 mg/l at 23°C	0.0066 mg/l to 4.94E-51 mg/l
Vapour pressure (parent)	1.3E-08 Pa	7.8 Pa at 25°C (QSAR prediction)	0.23 Pa at 20°C
Hydrolysis t1/2at pH 7 and 25°C	>329 hours	>728 hours at 20-25°C, predicted	630 – >63 000 h
Short-term toxicity to fish (LC50)	n/a	>75 ng/l	n/a
Short-term toxicity to aquatic invertebrates (EC50)	n/a	n/a	n/a
Algal inhibition (ErC50and NOEC)	n/a	n/a	n/a
Long-term toxicity to fish (NOEC)	n/a	≥39 ng/l (reported in a bioconcentration study)	≥0.05 mg/l (nominal); ≥0.11 mg/l (mean measured)
Long-term toxicity to aquatic invertebrates (NOEC)	n/a	≥47 ng/l	≥980 mg/l (loading rate WAF); ≥0.00605 mg/l (geometric mean measured)
Sediment toxicity (NOEC) (normalised to 5% OC)	n/a	≥38 mg/kg dry weight,Lumbriculus variegatus; ≥27 mg/kg dry weight,Chironomus riparius; ≥59 mg/kg dry weight,Hyalella azteca.	n/a
Short-term terrestrial toxicity (L(EC)50)	n/a	n/a	n/a
Long-term terrestrial toxicity (NOEC)	n/a	EC10>1000 mg/kg dw,Eisenia fetida(OECD 222); EC10>100 mg/kg dw, soil microorganisms (OECD 216)	≥1000 mg/kg dwt, Eisenia fetida; EC25 and EC50 >100 mg/kg dwt, soil microorganisms

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.

Conclusion on classification

The substance has no reliable toxicity data: data is waived due to the insolubility of the test substance meaning testing is technically not feasible and toxic effects are not likely to be expressed at the limit of solubility.

It is therefore proposed that 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (CAS 3390-61-2) should not be classified in the EU for acute or chronic toxicity.

According to Regulation (EC) No 1272/2008 (CLP):

Aquatic Acute: Not classified

Aquatic Chronic: Not classified