Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane

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:

PNEC sediment (freshwater)

PNEC value:

13.5 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

1.35 mg/kg sediment dw

Assessment factor:

100

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

19 mg/kg soil dw

Assessment factor:

10

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

The submission substance, Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9; EC No. 451-620-7), is a multi-constituent substance made up of two main constituents. Constituent 1: 3,3-Diphenylhexamethyltrisiloxane (CAS 797-7-3), and Constituent 2: 3,3,5,5-Tetraphenylhexamethyltetrasiloxane (CAS 18758-92-4). The constituents of the substance have predicted water solubility values of 2.8E-05 mg/L and 1.5E-10 mg/L for Constituent 1 and Constituent 2 respectively. The constituents of the substance are hydrolytically unstable with a half-life for siloxane degradation of >200 hours at pH 7 and 20-25°C for Constituent 1 and >630 hours at pH 7 and 20-25°C for Constituent 2. Both constituents are predicted to have very high log K_ow values of 9.

The hydrolysis rate of both constituents of the substance greatly increases with higher pH; 2 to 5.3 hours at pH 9. The aquatic tests were carried out at pH ranging from 7.7 – 8.9. It is therefore likely that, under the static exposure conditions of the fish, invertebrate and algae tests, the test organisms will have been exposed to a mixture of the parent substance and the hydrolysis products of each constituent of the tested substance.

Both constituents of the registered substance are siloxanes. Siloxanes 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.

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.

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 registered 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 registered substance (Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9)) 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 solubilities, 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.

Sediment and terrestrial testing are not standard Annex VIII requirements for the registered substance. However, the low water solubility, high log K_ow and high log K_oc of the substance indicate that it will adsorb to organic matter and partition to sludges in a wastewater treatment plant, as mentioned above. As no effects were observed in the short-term aquatic toxicity tests, it is appropriate to assess the potential toxicity of the substance in the sediment and soil compartments.

Terrestrial toxicity tests with the registered substance were required by ECHA Final Decision TPE-D-2114497761-36-01/F. OECD TG 222 earthworm reproduction, OECD TG 208 seedling emergence test and OECD TG 216 soil microorganisms test have been conducted.

No sediment toxicity data are available with the registered substance. Data have been read across from decamethylcyclopentasiloxane (D5, CAS 541-02-6; EC No. 208-764-9) and dodecamethylcyclohexasiloxane (D6, CAS 540-97-6; EC No. 208-762-8) for assessment of the sediment compartment.

 

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.

 

In the following paragraphs the read-across approach for Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9) is assessed for the surrogate substances taking into account structure, hydrolysis rate and physicochemical properties.

 

 

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)

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 Reconsile siloxanes category. D5 is a cyclic siloxane with 5 silicon atoms with methyl branches and alternated by oxygen. The registered 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 K_ow (9.0 and 8.07 respectively) and high log K_oc (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 below. The registered substance and the surrogate substance share similar physicochemical properties but are not close structural analogues (linear and cyclic siloxanes).

Long-term toxicity of D5 to Hyalella azteca data are read across to the sediment compartment.

 

Read across from dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) to Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9)

The registered substance, Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9), and the surrogate substance dodecamethylcyclohexasiloxane (D6, CAS 540-97-6) are both within the siloxanes category. The registered 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. D6 has 6 silicon atoms with methyl branches and alternated by oxygen. Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane (CAS 352230-22-9) and D6 have similar physicochemical properties: high molecular weight (360.68-558.98 g/mol and 444.93 g/mol respectively), low water solubility (2.8E-05 mg/L for Constituent 1, 1.5E-10 mg/L for Constituent 2 and 5.1E-03 mg/L for D6), high log K_ow (9.0 and 8.9 respectively) and high log K_oc (6.0 and 5.9 respectively). Both substances have negligible biodegradability and slow hydrolysis rates. The registered substance and the surrogate substance share similar physicochemical properties but are not close structural analogues (linear and cyclic siloxanes).

Long-term toxicity of D6 to Lumbriculus variegatus and Chironomus riparius data are read across to the sediment compartment.

 

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

CAS Number	352230-22-9	541-02-6	540-97-6
Chemical Name	Reaction Mass of 3,3-diphenylhexamethyltrisiloxane; 3,3,5,5-tetraphenylhexamethyltetrasiloxane; 3-trimethylsiloxy-3,5,5-triphenylhexamethyltetrasiloxane	Decamethylcyclopentasiloxane (D5)	Dodecamethylcyclohexasiloxane (D6)
Molecular weight (parent) (g/mol)	Constituent 1: 360.68; Constituent 2: 558.98	370.78	444.93
log Kow (parent)	9.0 for Constituent 1 and Constituent 2	8.07	8.9
Log Koc (parent)	6.0 for Constituent 1 and Constituent 2	5.17	5.90
Water solubility (parent)	<0.5234 mg/L at 20°C (whole substance) (OECD 105) Constituent 1: 2.8E-05 mg/L at 20°C (QSAR) Constituent 2: 1.5E-10 mg/L at 20°C (QSAR)	0.017 mg/L	5.1E-03 mg/L
Vapour pressure (parent)	0.068 Pa at 25°C (whole substance) (OECD 104) 3.6E-03 Pa at 25°C (QSAR, Constituent 1) 1.3E-07 Pa at 25°C (QSAR, Constituent 2)	33.2 Pa	4.7 Pa
Hydrolysis t1/2 at pH 7 and 20-25°C	>200 hours for Constituent 1 >630 hours for Constituent 2	1590 hours	1 year
Short-term toxicity to fish (LC50)	LL50: >100 mg/L (WAF)	n/a	n/a
Short-term toxicity to aquatic invertebrates (EC50)	EL50: >100 mg/L (WAF)	n/a	n/a
Algal inhibition (ErC50 and NOEC)	ErL50: >100 mg/L; NOELR: ≥100 mg/L (WAF)	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	≥4.4 μg/L	n/a
Long-term toxicity to aquatic invertebrates (NOEC)	No data	≥4.6 µg/L	≥4.6 µg/L
Sediment toxicity (NOEC) (normalised to 5% OC)	No data	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	(IC50) 209 mg/kg dwt, H. vulgare; >4054 mg/kg dwt, T. pratense	n/a
Long-term terrestrial toxicity (NOEC)	>1000 mg/kg dw, terrestrial plants. >100 mg/kg dw, soil microflora. EC10 for reproduction: 62.62 mg/kg dw, Eisenia fetida.	≥4074 mg/kg dw, E. andrei; 377 mg/kg dw, F. candida	>1000 mg/kg dw, Eisenia fetida; >100 mg/kg dw, soil microflora

 

References:

Springborn Smithers 2009a: Trichloro(phenyl)silane: Acute toxicity to rainbow trout (Oncorhynchus mykiss) under static conditions. (study report), Testing laboratory: Springborn Smithers Laboratories (Europe) Seestrasse 21, Postfach CH-9326 Horn, Switzerland, Owner company; Reconsile Consortium, Avenue Edmond van Nieuwenhuyse 6, 1160 Bruxelles, Belgium, Study number: Springborn Smithers Laboratories (Europe) Study # 1142.002.103, Report date: Apr 13, 2010

Springborn Smithers 2009b: Trichloro(phenyl)silane: Acute toxicity to water fleas (Daphnia magna) under static conditions. (study report), Testing laboratory: Springborn Smithers Laboratories (Europe) Seestrasse 21, Postfach CH-9326 Horn, Switzerland, Owner company; Reconsile Consortium, Study number: Springborn Smithers Laboratories (Europe) Study # 1142.002.110.

Springborn Smithers 2009c: Trichloro(phenyl)silane: Growth inhibition test with Pseudokirchneriella subcapitata (syn. Selenastrum capricornutum) under static conditions (study report), Testing laboratory: Springborn Smithers Laboratories (Europe) Seestrasse 21, Postfach CH-9326 Horn, Switzerland., Owner company; Reconsile, Study number: Springborn Smithers Study # 1142.002.430.

Conclusion on classification

The substance has reliable short-term EL₅₀ values (measured) of >100 mg/L (loading rate WAF) in fish, Daphnia and algae. It also has a reliable NOELR value of ≥100 mg/L in algae. The available short-term aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water. The substance hydrolyses slowly in water and is not readily biodegradable.

For low solubility, highly adsorbing substances with short-term data only, the safety net classification Category Chronic 4 may apply. For the purposes of assessment, the log K_ow of the substance is limited to 9.0, however it is highly likely that the log K_ow >9. The very high log K_ow 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.

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Not classified.