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

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
PNEC oral
PNEC value:
222 mg/kg food
Assessment factor:
90

Additional information

        

Silsesquioxanes, phenyl 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 Kowvalues 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 consitutents 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 analoguous 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.

For the purposes of environmental exposure assessment and risk characterisation,

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 Silsesquioxanes, Phenylin the environment will be dominated by the high partition coefficient (log Kow9.0) and adsorption coefficient (log Koc6.0) properties of the constituents.

Silsesquioxanes, Phenyl (CAS70131-69-0) and the substances used as surrogate 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 Kow, 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 the IUCLID 6 dossier.

In the following paragraphs the read-across approach for Silsesquioxanes, Phenylis 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

70131-69-0

2116-84-9

141-62-8

541-02-6

540-97-6

Chemical Name

Silsesquioxanes, Phenyl

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 t1/2at 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 t1/2at 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 (ErC50and 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)

n/a

≥4.4 μg/l

≥5.3 μg/l

≥4.4 μg/l

≥4.4 μg/l

Long-term toxicity to aquatic invertebrates (NOEC)

n/a

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

70 mg/kg dwt,C. riparius
≥1272 mg/kg dwtL. 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)

n/a

n/a

n/a

≥4074 mg/kg dwt,
E. Andrei

377 mg/kg dwt,
F. candida

n/a

 


Constituent #1 of Silsesquioxanes, Phenyl:

Constituent #1 of the registration substance is 1,1,1,5,5,5-Hexamethyl-3-phenyl-3-[(trimethylsilyl)oxy]trisiloxane (CAS 2116-84-9). 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 Silsesquioxanes, Phenyl:

Decamethyltetrasiloxane (CAS 141-62-8, L4) 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 Kowand Koc). Both the surrogate substance and the registration substance have high MW (310.7 and from 372.8 to1635.1respectively) very high log Kow(8.2 and 9 respectively), and high log Koc(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 Kowand high log Koc, 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 well as toxicity to algae endpoint.

Read-across from dodecamethylcyclohexasiloxane (D6) to Silsesquioxanes, Phenyl (sediment toxicity)

Dodecamethylcyclohexasiloxane (CAS 540-97-6, D6) 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 Kowand Koc). Both the registration substance and D6 have similar physico-chemical properties: high molecular weight (from 372.8 to1635.1and 444.9 respectively), high log Kow(9 and 8.9 respectively) and high log Koc(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 Kowand high log Koc,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 well as toxicity to algae endpoint and sediment toxicity.

Read-across from decamethylcyclopentasiloxane (D5) to Silsesquioxanes, Phenyl: 

Decamethylcyclopentasiloxane (CAS 541-02-6, D5) 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 Kowand Koc). Both silsesquioxanes, phenyl and D5 have similar physico-chemical properties: high molecular weight (from 372.8 to1635.1and 370.8 respectively), high log Kow(9 and 8.0 respectively) and high log Koc(both 6).

Because D5 and the registration substance have similar properties and the environmental behaviour and toxicity is driven by the high log Kowand high log Koc,the read-across of sediment toxicity data is considered to be valid. Read-across of terrestrial toxicity data is considered to be appropriate to derive an interim risk characterisation.

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 Kow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log Kowof 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

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 has been proposed to resolve this uncertainty.

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.