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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Biodegradation in water (screening tests):3.7% in 28 days (OECD 310), read-across from a structurally related substance. 

Key value for chemical safety assessment

Additional information

There are no ready biodegradation data available for 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4), therefore good quality data for the structurally-related substance, octamethylcyclotetrasiloxane (D4, CAS 556-67-2), have been read across.

The registration substance has an average purity of >80% Vi4-D4, with <15% 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Vi5-D5; CAS 17704-22-2; Impurity 1) and <10% 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane (Vi3-D3; CAS 3901-77-7; Impurity 2) present as impurities. Read-across studies are in place as supporting studies, to consider the properties of the impurities. Data for Vi5-D5 are read-across from decamethylcyclopentasiloxane (D5; CAS 541-02-6), data for Vi3-D3 are read-across from hexamethyltrisiloxane (D3; CAS 541-05-9).

D4 is a cyclic siloxane chain with four silicon atoms, connected by four oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. Similarly, Vi4-D4 is a cyclic siloxane chain with four silicon atoms, connected by four oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are substituted with one methyl group and one vinyl group. D5 and Vi5-D5, and D3 and Vi3-D3 are structurally analogous in the same way.

The source and target substances possess similar physicochemical properties.  A comparison of the key physicochemical properties is presented in the table below.


Table: Summary of physicochemical and ecotoxicological properties for the constituents of the registered substance and their surrogate substances.

CAS Number

2554-06-5

556-67-2

17704-22-2

541-02-6

3901-77-7

541-05-9

Chemical Name

2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4)

Octamethylcyclotetrasiloxane (D4)

2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Impurity 1 (Vi5-D5))

Decamethylcyclopentasiloxane (D5)

2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane (Impurity 2 (Vi3-D3))

Hexamethylcyclotrisiloxane (D3)

Ultimate Si hydrolysis product

Methylvinylsilanediol

Dimethylsilanediol

Methylvinylsilanediol

Dimethylsilanediol

Methylvinylsilanediol

Dimethylsilanediol

Molecular weight (parent)

344.66

296.62

431

370.8

259

223

Molecular weight (hydrolysis product)

104.18

92.17

104.18

92.17

104.18

92.17

log Kow(parent)

6.47

6.49

9.0

8.0

6.7

4.4

Water sol (parent)

0.0073 – 0.0088 mg/l at 23°C

0.056 mg/l

9.1E-06 mg/l

0.017 mg/l

0.13 mg/l

2 mg/l

Vapour pressure (parent)

93.5 Pa

132 Pa

0.6 Pa

33 Pa

67 Pa

671 Pa

Hydrolysis t1/2at pH 7 and 25°C

approximately 63 hours

69-144 hours

1600 hours

1590 hours

0.6 hours

23 minutes


 

Vi4-D4 and D4, and Vi5-D5 and D5, 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.

Vi3-D3 and D3 do not fall within the Siloxane Category definition, as the hydrolysis half-life of each substance is less than 1 hour at pH 7 and 25°C. However, biodegradation data available for D3 is in agreement with the hypothesis applied to the Category of “not readily biodegradable”, and it is concluded that read-across of this result to Vi3-D3 is appropriate.

Additional information is given in a supporting report (PFA, 2017) attached in Section 13 of the IUCLID 6 dossier.

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 day

Guideline

Reliability

000107-46-0

Hexamethyldisiloxane

no

2%

OECD 301C

1c

000107-51-7

Octamethyltrisiloxane

no

-3.7% (i.e. 0)

OECD 310

1a

000540-97-6

Dodecamethylcyclohexasiloxane

No

0.045%

OECD 310

1a

000541-02-6

Decamethylcyclopentasiloxane

No

0.14%

OECD 310

1a

000541-05-9

Hexamethylcyclotrisiloxane

No

0.06%

OECD 310

1a

000556-67-2

Octamethylcyclotetrasiloxane

No

3.7%

OECD 310

1a

001873-88-7

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

No

0%

OECD 310/ ISO Guidline No 14593

1a

002374-14-3

2,4,6-trimethyl-2,4,6-tris(3,3,3-trifluoropropyl)cyclotrisiloxane

No

0.0%

OECD 301B

1a

002627-95-4

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

No

0%

OECD 301D

1a

017928-28-8

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

No

0%

OECD 301F

1a

017955-88-3

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

No

10 - 32 % after 25 d

OECD 301B

1a

060111-47-9

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

No

2.2%

OECD 301F

1a

069304-37-6

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

No

0% (BOD); 100% (GC)

OECD 301C

1a

070940-00-0

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

No

9.7 and 9.9%

OECD 301B

1a

180073-46-5

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

No

5.2 and 4.4%

OECD 301B

1a

352230-22-9

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

No

0%

OECD 301F

1a