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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 phenyl silsesquioxanes, therefore good quality data for the structurally-related substance, 3-[(dimethylvinylsilyl) oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9), have been read across.

Phenyl silsesquioxanes and 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS No. 60111-47-9) are structurally-similar substances, both are siloxanes with phenyl functionality. Phenyl silsesquioxanes 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.The source substance 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS No. 60111-47-9) is a branched structure of four Si atoms,the longest siloxane chain contains three silicon atoms and two oxygen atoms, with a Si-O branch on the central silicon atom in the chain. The terminal silicon atoms are each substituted with two methyl and one vinyl group, whilst the central silicon atom has one phenyl group attached.

Both substances share structurally similar ultimate silanol hydrolysis products: 3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane hydrolyses to form dimethylvinylsilanol and phenylsilanetriol, and phenyl silsesquioxanes hydrolyses to form phenylsilanetriol and trimethylsilanol. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

Phenyl silsesquioxanes and 3-[(dimethylvinylsilyl) oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9) are within the Reconsile Siloxane Category of substances within which, in general, there is no evidence of any significant biodegradation.

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

Table: Ready biodegradation data for substances within the Siloxane Category

CAS

Name

Readily biodegradable? Yes/no

Result: Biodegradation after 28 days (%)

Guideline

Test type

Reliability

000107-46-0

Hexamethyldisiloxane

No

2%

OECD 301C

BOD

1

000107-51-7

Octamethyltrisiloxane

No

-3.70%

OECD 310

CO2

1

000540-97-6

Dodecamethylcyclohexasiloxane

No

4.5%

OECD 310

CO2

1

000541-02-6

Decamethylcyclopentasiloxane

No

0.14%

OECD 310

CO2

1

000556-67-2

Octamethylcyclotetrasiloxane

No

3.70%

OECD 310

CO2

1

001873-88-7

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

No

0%

OECD 310/ ISO Guideline No 14593

CO2

1

002627-95-4

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

No

0%

OECD 301 D

BOD

1

017928-28-8

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

No

0%

OECD 301F

BOD

1

017955-88-3

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

No

10 – 32 % after 25 d

OECD 301B

CO2

 1

060111-47-9

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

No

2.20%

OECD 301F

BOD

1

070940-00-0

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

It is therefore considered valid to read-across the results for 3-[(dimethylvinylsilyl) oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane (CAS 60111-47-9) to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017) attached in Section 13 of IUCLID.

3-[(Dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane attained 2.2% biodegradation in 28 days (O2consumption).

Read-across of this result is applicable to phenyl silsesquioxanes.