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Biodegradation in water: screening tests

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Description of key information

Biodegradation in water: screening tests: 2.2% in 28 days (O2 consumption, OECD 301F), based on read-across from a structurally-related substance.

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed

Additional information

There are no reliable ready biodegradation data available for 1,3,3,5 -tetramethyl-1,1,5,5-tetraphenyltrisiloxane (CAS No. 3982-82-9), therefore good quality data for the structurally-related substance, 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS No. 60111-47-9) has been read across.

1,3,3,5-Tetramethyl-1,1,5,5,tetraphenyltrisiloxane (CAS No. 3982-82-9) and 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS No. 60111-47-9) are structurally-similar substances.

The submission substance is a linear siloxane chain with three silicon atoms, connected by two oxygen atoms, in which each terminal silicon atom is substituted with one methyl and two phenyl groups, and the central silicon atom is substituted with two methyl groups. The read-across substance is a branched siloxane, with a chain of three silicon atoms connected by two oxygen atoms, and a further siloxane bond to the central silicon atom. Each of the terminal Si atoms is substituted by two methyl and one vinyl group. The central silicon atom is substituted by one phenyl group.

A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

Table: Key physicochemical properties of 1,3,3,5 -tetramethyl-1,1,5,5-tetraphenyltrisiloxane and surrogate substance 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane

Property

1,3,3,5 -Tetramethyl-1,1,5,5-tetraphenyltrisiloxane (3982-82-9)

3-[(Dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (60111-47-9)

Molecular weight (g/mol)

484.8

408.8

Log Kow

9.0

9.0

Log Koc

6.0

6.0

Water solubility (mg/l)

5.9E-09

1.6E-06

Vapour pressure at 25°C (Pa)

5.2E-07

0.01

Hydrolysis half- life at pH 7 (h)

> 200 h

> 200 h

The registration and read-across substances are within the Reconsile Siloxane Category of substances within which, in general, there is no evidence of any significant biodegradation. Therefore, in the context of the RAAF, Scenario 6 is considered to apply. The Category hypothesis is that read-across of “not readily biodegradable” between siloxanes is appropriate where data are not available for the substance itself. Additional information is given in a supporting report (PFA, 2017) attached in Section 13.

The table below presents ready biodegradation data for substances within the Siloxane Category.

Table 4.1.5 Ready biodegradation data for substances within theSiloxane Category

CAS

Name

Readily biodegradable? Yes/no

Result: Biodegradation after 28 days (%) *

Guideline

Test type

Reliability

107-46-0

Hexamethyldisiloxane

No

2%

OECD 301C

BOD

1

107-51-7

Octamethyltrisiloxane

No

-3.70%

OECD 310

CO2

1

540-97-6

Dodecamethylcyclohexasiloxane

No

4.5%

OECD 310

CO2

1

541-02-6

Decamethylcyclopentasiloxane

No

0.14%

OECD 310

CO2

1

541-05-9

Hexamethylcyclotrisiloxane

No

0.06%

OECD 310

CO2

1

556-67-2

Octamethylcyclotetrasiloxane

No

3.70%

OECD 310

CO2

1

1873-88-7

1,1,1,3,5,5,5-heptamethyltrisiloxane

No

0%

OECD 310/ ISO Guideline No 14593

CO2

1

2627-95-4

1,1,3,3-tetramethyl-1,3-divinyldisiloxane

No

0%

OECD 301D

BOD

1

17928-28-8

1,1,1,3,5,5,5-heptamethyl- 3-[(trimethylsilyl) oxy] trisiloxane

No

0%

OECD 301F

BOD

1

17955-88-3

1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane

No

10 – 32 % after 25 d

OECD 301B

CO2

 1

60111-47-9

3-[(dimethylvinylsilyl) oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane

No

2.20%

OECD 301F

BOD

1

69304-37-6

1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane

No

0% (BOD); 100% (GC)

OECD 301C

BOD

1

70940-00-0

1,1,7,7, -Tetramethyl-3,3,5,5, -Tetraphenyl-Tetrasiloxane

No

9.7 and 9.9%

OECD 301B

CO2

 1

180073-46-5

Pentasiloxane, 1,1,9,9-tetramethyl-3,3,5,5,7,7-hexaphenyl-

No

5.2 and 4.4%

OECD 301B

CO2

 1

352230-22-9

Siloxanes and Silicones, di-Ph, Ph (trimethylsilyl) oxy

No

0%

OECD 301F

BOD

1

It is therefore considered valid to read-across the results for3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxaneto fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017) attached in Section 13.

3-[(Dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane attained 2.2% biodegradation in 28 days (O2 consumption).

The study was conducted according to OECD 310 and is considered to be reliable. This is selected as the key study.