Reaction mass of 1,1,1,5,5,5-hexamethyl-3-phenyl-3-((trimethylsilyl)oxy)trisiloxane and 1,1,1,7,7,7-hexamethyl-3,5-diphenyl-3,5-bis[(trimethylsilyl)oxy]tetrasiloxane and 1,1,1,9,9,9-hexamethyl-3,5,7-triphenyl-3,5,7-tris((trimethylsilyl)oxy)pentasiloxane

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:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

222 mg/kg food

Assessment factor:

90

Additional information

       

Phenyl silsesquioxanes (EC No. 939-487-8) is considered 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.

All constituents are poorly soluble (from 4.9E-05 to 4.94E-51 mg/l at 20°C) and have log K_ow values of (or limited to) 9.0. Aquatic organisms are therefore likely to be exposed to very low dissolved concentrations of the substance in the environment. In the sediment and soil compartments adsorption to particulate and organic matter is expected to be high.

The major and minor linear constituents of the substance are hydrolytically unstable with a half-lives for siloxane degradation of 630 – >63 000 h at pH 7 and 20-25°C. The minor cyclic constituents of the substance are also expected to be hydrolytically unstable. Hydrolysis half-lives for the ring-opening step are read-across from the structurally analogous methyl-substituted cyclic siloxane, and range from 0.4 - >10,000 h. Subsequent hydrolysis half-lives of the ring-opened structure are predicted to be of ≥10,000 h at pH 7 and 20-25°C.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registration substance from substances that have similar structure and physicochemical properties.

The behaviour of phenyl silsesquioxanes in the environment will be dominated by the high partition coefficient (log K_ow 9.0) and adsorption coefficient (log K_oc 6.0) properties of the constituents.

Phenyl silsesquioxanes and the substances used as surrogates for read-across are part of the Reconsile Siloxane Category. Substances in this group tend to have slow hydrolysis rates, low water solubility, high log K_ow, high adsorption coefficients and a tendency for high persistence in the sediment compartment.

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

In the following paragraphs the read-across approach for phenyl silsesquioxanes is assessed for each surrogate substance taking into account structure and physico-chemical properties of each constituent of the registration substance and of the surrogate substances. Table 7.0.1 presents relevant physicochemical properties and the available ecotoxicological data.

Table 7.0.1 Physicochemical parameters and ecotoxicity data for the registration and surrogate substances

CAS Number	No CAS number	2116-84-9	141-62-8	541-02-6	540-97-6
Chemical Name	Phenyl silsesquioxanes	1,1,5,5,5 -Hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane	Decamethyltetrasiloxane (L4)	Decamethylcyclopentasiloxane (D5)	Dodecamethylcyclohexasiloxane (D6)
Si hydrolysis product	n/a	n/a	Trimethylsilanol	Dimethylsilanediol	Dimethylsilanediol
Molecular weight (parent)	372.8 to1635.1	372.6	177.53	370.78	444.93
Molecular weight (hydrolysis product)	n/a	n/a	90.2	92.17	92.17
log Kow (parent)	9.0	9.0	8.2	8.0	8.9
log Kow (silanol hydrolysis product)	n/a	n/a	1.2	-0.38	-0.38
Water sol (parent)	4.9E-05 to 4.94E-51 mg/l	0.0066 mg/l	0.0067 mg/l	0.017 mg/l	0.005 mg/l
Water sol (silanol hydrolysis product))	n/a	n/a	995 mg/l	1.0E+06 mg/l	1E+06 mg/l
Vapour pressure (parent)	0.23 Pa at 20°C	0.0099 Pa	73 Pa	33.2 Pa	5 Pa
Vapour pressure (hydrolysis product)	n/a	n/a	1900 Pa	7 Pa	7 Pa
Hydrolysis t1/2 at pH 7 and 25°C	see section 4.1.1	approximately 630 h	728 h	1590 h	>1 year
Hydrolysis t1/2 at pH 2 and 25°C	see section 4.1.1	0.036 h at 20-25°C	n/a	9.3 h	n/a
Hydrolysis t1/2 at pH 9 and 25°C	see section 4.1.1	Approximately 232 h at 37.5°C	21.1 h	24.8 - 31.6 h	n/a
Short-term toxicity to fish (LC50)	n/a	n/a	>6.3 μg/l	n/a	n/a
Short-term toxicity to aquatic invertebrates (EC50)	n/a	n/a	n/a	n/a	n/a
Algal inhibition (ErC50 and NOEC)	n/a	n/a	EC50: >2.2 μg/l; NOEC: ≥2.2 μg/l	ErC50: >2.0 μg/; NOEC: ≥2.0 μg/l	EC50: >2.0 μg/l; NOEC: ≥2.0 μg/l
Long-term toxicity to fish (NOEC)	≥0.05 mg/l (nominal); ≥0.11 mg/l (mean measured)	≥4.4 μg/l	≥5.3 μg/l	≥4.4 μg/l	≥4.4 μg/l
Long-term toxicity to aquatic invertebrates (NOEC)	≥980 mg/l (loading rate WAF); ≥0.00605 mg/l (geometric mean measured)	n/a	≥4.9 μg/l	≥4.6 µg/l	≥4.6 µg/l
Long-term sediment toxicity (NOEC)	n/a	n/a	≥92 mg/kg dwt H. azteca ≥17 mg/kg dwt L. variegatus	70 mg/kg dwt, C. riparius ≥1272 mg/kg dwt L. variegatus 130 mg/kg dwt, H. azteca	≥420 mg/kg dwt, L. variegatus; ≥620 mg/kg dwt, C. riparius
Short-term terrestrial toxicity (L/EC50)	n/a	n/a	n/a	(IC50) 2050 mg/kg dwt,H. vulgare; >4054 mg/kg dwt, T. pratense	n/a
Long-term terrestrial toxicity (NOEC)	≥1000 mg/kg dwt, Eisenia fetida; EC25 and EC50 >100 mg/kg dwt, soil microorganisms.	n/a	≥1000 mg/kg dwt, Eisenia fetida; EC25 and EC50 >100 mg/kg dwt, soil microorganisms	≥4074 mg/kg dwt, E. Andrei 377 mg/kg dwt, F. candida	n/a

Constituent #1 of phenyl silsesquioxanes:

Constituent #1 of the registration substance is 1,1,1,5,5,5 -hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9, EC No. 218-320-6). This is a siloxane made up of a branched chain of four Si atoms linked by three oxygen atoms. The central Si has a phenyl group bound to it, the other three Si are bound to three methyl groups. See table 1.4.1 for a diagram of the chemical structure.

A long-term fish study is available with 1,1,1,5,5,5-hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane indicating that no mortalities were recorded at the highest concentration tested in a flow-through BCF study.

1,1,1,5,5,5 -Hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane is used as supporting data for the long-term toxicity to fish endpoint for constituent #1.

Read-across from decamethyltetrasiloxane (L4) to phenyl silsesquioxanes:

Decamethyltetrasiloxane (L4, CAS 141-62-8, EC No. 205-491-7) is a linear siloxane with four Si linked by oxygen, with methyl groups bound to silicon. Linear constituents 1-7 and cyclic constituents C3-C7 of the registration substance are all siloxanes with four to sixteen silicon atoms linked by oxygen and methyl and phenyl groups bound to silicon. Increase of the chain length results in decreased water solubility. Even though the surrogate substance and the constituents cannot be considered as close structural analogues, the property that will dominate the behaviour of the substance in the environment and its toxicity, in particular in the sediment compartment, is the high adsorption potential (log K_owand K_oc). Both the surrogate substance and the registration substance have high MW (310.7 and from 372.8 to 1635.1 respectively) very high log K_ow (8.2 and 9 respectively), and high log K_oc (5.16 and 6 respectively).

L4 and the registration substance have similar properties and the environmental behaviour and toxicity is driven by the high log K_ow and high log K_oc, thereforethe read-across of toxicity data is considered to be appropriate.

Decamethyltetrasiloxane (L4) is used to read-across to long-term toxicity to fish and invertebrates endpoints as supporting data, as well as toxicity to algae endpoint.

Read-across from dodecamethylcyclohexasiloxane (D6) to phenyl silsesquioxanes (sediment toxicity)

Dodecamethylcyclohexasiloxane (D6, CAS 540-97-6, EC No. 208-762-8) is a cyclic siloxane with six Si linked by oxygen and with methyl groups bound to silicon. Linear constituents 1-7 and cyclic constituents C3-C7 of the registration substance are all siloxanes with four to sixteen silicon atoms linked by oxygen and methyl and phenyl groups bound to silicon. Increase of the chain length results in decreased water solubility. Even though the surrogate substance and the constituents cannot be considered as close structural analogues, the property that will dominate the behaviour of the substance in the environment, in particular the sediment compartment, is the high adsorption potential (log K_ow and K_oc). Both the registration substance and D6 have similar physico-chemical properties: high molecular weight (from 372.8 to 1635.1 and 444.9 respectively), high log K_ow(9 and 8.9 respectively) and high log K_oc (both 6). The major concern of toxicity for the registration substance is in the sediment compartment.

Because D6 and the registration substance have similar properties and the environmental behaviour and toxicity is driven by the high log K_ow and high log K_oc, the read-across of toxicity data is considered to be appropriate.

Dodecamethylcyclohexasiloxane (D6) is used to read-across to long-term toxicity to fish and invertebrates endpoints as supporting data, as well as toxicity to algae and sediment toxicity endpoints.

Read-across from decamethylcyclopentasiloxane (D5) to phenyl silsesquioxanes: 

Decamethylcyclopentasiloxane (D5, CAS 541-02-6, EC No. 208-764-9) is a cyclic siloxane with five Si linked by oxygen and with methyl groups bound to silicon. Linear constituents 1-7 and cyclic constituents C3-C7 of the registration substance are all siloxanes with six to sixteen silicon atoms linked by oxygen and methyl and phenyl groups bound to silicon. Increase of the chain length results in decreased water solubility. Even though the surrogate substance and the constituents cannot be considered as close structural analogues, the property that will dominate the behaviour of the substance in the environment is the high adsorption potential (log K_ow and K_oc). Both phenyl silsesquioxanes and D5 have similar physico-chemical properties: high molecular weight (from 372.8 to1635.1 and 370.8 respectively), high log K_ow (9 and 8.0 respectively) and high log K_oc (both 6).

Because D5 and the registration substance have similar properties and the environmental behaviour and toxicity is driven by the high log K_ow and high log K_oc, the read-across of sediment toxicity data is considered to be valid.

Consideration of low functionality side chains and reactive groups

Phenyl group: The toxicity of phenyls is also related to non-polar narcosis. The toxicity of substances with phenyl groups increases with increasing log K_ow. For example, trichlorophenylsilane (CAS 98-13-5, EC No. 202-640-8) 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 2009c, d and e).

Conclusion on classification

Experimental long-term invertebrate toxicity data are available with the registered substance. The data show no effect at the limit of solubility.

The rest of the data set at present has some uncertainty:

Ecotoxicity conclusions for the constituents of the registration substance are read across from structural analogues. These long-term no observed effect concentrations for aquatic organisms are therefore subject to some uncertainty in terms of equivalent NOECs for the registration substance in an experimental test.

Further testing of long-term aquatic effects of the registration substance on fish is currently underway.

Conclusions for classification and labelling for the environment for the registration substance will therefore be finalised in a future update, once the new long-term aquatic testing has been conducted.

PROPOSED NEW C&L

The substance has a reliable ErC₅₀ value of >2.2 μg/l and NOEC value of ≥2.2 μg/l in algae, read across from a suitable analogue. It has reliable long-term NOEC values of ≥0.00605 mg/l in Daphnia and ≥0.11 mg/l in fish.

The available short- and long-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.

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