Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
no hazard identified

Marine water

Hazard assessment conclusion:
no hazard identified

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
100 mg/L
Assessment factor:
10
Extrapolation method:
assessment factor

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:
PNEC soil
PNEC value:
3.77 mg/kg soil dw
Assessment factor:
100
Extrapolation method:
assessment factor

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 Kow (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 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 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 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.

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 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 mentioned above. As aquatic toxicity tests are not feasible, it may be more appropriate to attempt sediment and terrestrial tests in order to look into potential toxicity of the substance in the environment. For this reason, testing proposals have been read-across from structurally similar substances dodecamethylpentasiloxane (L5, CAS 141-63-9) for the sediment compartment, and silsesquioxanes, phenyl (CAS 70131-69-3) for the terrestrial compartment.

Until these data become available, interim read-across is in place for assessing hazard to the soil and sediment compartments.

Data from L5, decamethyltetrasiloxane (L4, CAS 141-62-8) and dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) are read across to the sediment compartment.

Data from decamethylcyclopentasiloxane (D5, CAS 541-02-6) are read across to the terrestrial compartment.

1The very high log Kow (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 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 Kow 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 Kow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log Kow 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 Kow (9 and 9.3-9.5, respectively) and high log Koc (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.

Toxicity of L5 to Lumbriculus variegatus data are read across to the sediment compartment. Testing proposals with L5 for two other sediment species,Chironomus ripariusandHyalella azteca,are also read across.

 

Read across from decamethyltetrasiloxane (L4, CAS 141-62-8) 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 decamethyltetrasiloxane (L4, CAS 141-62-8) 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. L4 has 4 silicon atoms with methyl branches and alternated by oxygen. Cas 3390-61-2 and L4 possess similar physico-chemical properties: high molecular weight (546.89 and 310.69, respectively), low water solubility (both insoluble, at 9.0E-11 mg/l and 6.7E-03 mg/l, respectively), high log Kow (9.0 and 8.2, respectively) and high log Koc (6 and 5.16, respectively). Both substances have negligible biodegradability and slow hydrolysis rates. A comparison of the key physicochemical properties is presented in the table below.

Toxicity of L4 to Hyalella azteca data are read across to the sediment compartment as an interim measure until data with L5 become available. 

 

Read across from dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) 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 dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) are both within the siloxanes category.CAS 3390-61-2has 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 D6 have similar physicochemical properties: high molecular weight (546.89 and 444.93, respectively), low solubility (both insoluble at 9.0E-11 mg/l and 5.1E-03 mg/l, respectively), high log Kow (9.0 and 8.9, respectively) and high log Koc (6.0 and 5.9, respectively). Both substances have negligible biodegradability and slow hydrolysis rates.

Toxicity of D6 to  Chironomus riparius data are read across to the sediment compartment as an interim measure until data with L5 become available.

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

CAS Number

3390-61-2

141-63-9

141-62-8

541-02-6

540-97-6

Chemical Name

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

Dodecamethylpentasiloxane (L5)

Decamethyltetrasiloxane (L4)

Decamethylcyclopentasiloxane (D5)

Dodecamethylcyclohexasiloxane (D6)

Si hydrolysis product

methyl(diphenyl)silanol (CAS 778-25-6) and methyl(phenyl)silanediol

Dimethylsilanediol and trimethylsilanol. Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to dimethylsilanediol. For properties of trimethylsilanol, see next column)

Dimethylsilanediol and Trimethylsilanol. Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol. For properties of dimethylsilanediol, see previous column)

Dimethylsilanediol

Dimethylsilanediol

Molecular weight (parent)

546.89

348.9

310.69

370.78

444.93

Molecular weight (hydrolysis product)

214.34

92.2

90.2

92.2

92.2

log Kow(parent)

9

9.3-9.5

8.2

8.0

8.9

log Kow(silanol hydrolysis product)

No data

-0.4

1.2

-0.4

-0.4

Log Koc(parent)

6

6.3

5.16

5.17

5.90

Water sol (parent)

9.0E-11 mg/l

7.0E-05 mg/l

6.7E-03 mg/l

0.017 mg/l

5.1E-03mg/l

Water sol (silanol hydrolysis product))

No data

1.0E+06 mg/l

995 mg/l

1.0E+06 mg/l

1.0E+06 mg/l

Vapour pressure (parent)

1.3E-08 Pa

7.8 Pa

73 Pa

33.2 Pa

4.7 Pa

Vapour pressure (hydrolysis product)

No data

7 Pa

1900 Pa

7 Pa

7 Pa

Hydrolysis t1/2 at pH 7 and 25°C

>329 hours

728 hours

728 hours

1590 hours

1 year

Short-term toxicity to fish (LC50)

No data

>75 ng/l

6.3 μg/l

n/a

n/a

Short-term toxicity to aquatic invertebrates (EC50)

No data

n/a

n/a

n/a

n/a

Algal inhibition (ErC50 and NOEC)

No data

n/a

EC50: >2.2 μg/l; NOEC: ≥2.2 μg/l

ErC50: >2.0 μg/l; NOEC: ≥2.0 μg/l

EC50: >2.0 μg/l; NOEC: ≥2.0 μg/l

Long-term toxicity to fish (NOEC)

No data

≥39 ng/l

≥5.3 μg/l 

≥4.4 μg/l

n/a

Long-term toxicity to aquatic invertebrates (NOEC)

No data

≥47 ng/l

≥4.9 μg/l

≥4.6 µg/l

≥4.6 µg/l

Sediment toxicity (NOEC) (normalised to 5% OC)

No data

n/a

92 mg/kg dwt, Hyalella azteca

110 mg/kg dwt, C. riparius; ≥1719 mg/kg dwt, L.variegatus; 135 mg/kg dwt, H. azteca

≥969 mg/kg dwt, C. riparius; ≥656 mg/kg dwt, L. variegatus 

Short-term terrestrial toxicity (L(EC)50)

No data

n/a

n/a

(IC50) 209 mg/kg dwt, H. vulgare; >4054 mg/kg dwt, T. pratense

n/a

Long-term terrestrial toxicity (NOEC)

No data

n/a

n/a

≥4074 mg/kg dwt, E. andrei; 377 mg/kg dwt, F. candida

n/a

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