Reaction mass of 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

Biodegradation in water: screening tests

Description of key information

Biodegradation in water (screening tests):

Constituent 1 (Vi5-D5): 0.14% in 28 days (OECD 310), read-across from a structurally related substance.

Constituent 2 (Vi4-D4): 3.7% in 28 days (OECD 310), 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 ready biodegradation data available for the registration substance or for the individual constituents of the registration substance. For Constituent 2, 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4), good quality data for the structurally-related substance, octamethylcyclotetrasiloxane (D4, CAS 556-67-2), have been read across; for Constituent 1, 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Vi5-D5; CAS 17704-22-2) data are read-across from decamethylcyclopentasiloxane (D5; CAS 541-02-6),

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 is a cyclic siloxane chain with five silicon atoms, connected by five oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. Similarly, Vi5-D5 is a cyclic siloxane chain with five silicon atoms, connected by five 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.

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
Chemical Name	Constituent 1 of the registration substance: 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4)	Octamethylcyclotetrasiloxane (D4)	Constituent 2 of the registration substance: 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Vi5-D5)	Decamethylcyclopentasiloxane (D5)
Ultimate Si hydrolysis product	Methylvinylsilanediol	Dimethylsilanediol	Methylvinylsilanediol	Dimethylsilanediol
Molecular weight (parent)	344.66	296.62	431	370.8
Molecular weight (hydrolysis product)	104.18	92.17	104.18	92.17
log Kow(parent)	6.47 (meas.)	6.49 (meas.)	9.0 (QSAR)	8.02 (meas.)
Water sol (parent)	0.0073 – 0.0088 mg/l at 23°C (meas.)	0.056 mg/l (meas.)	9.1E-06 mg/l (QSAR)	0.017 mg/l (meas.)
Vapour pressure (parent)	93.5 Pa (meas.)	132 Pa (meas.)	0.6 Pa (QSAR)	33.2 Pa (meas.)
Hydrolysis t1/2at pH 7 and 25°C	approximately 63 hours (QSAR)	69-144 hours (meas.)	1600 hours (QSAR)	1590 hours (meas.)

 

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.

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

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

 

Based on read-across of data for structural analogues to the two constituents of the registration substance, it is concluded that the registration substance is not readily biodegradable. As described above, in the target substances each silicon atom is substituted by one methyl and one vinyl group, whereas in the source substances, the silicon atoms are fully methyl substituted. Studies with other vinyl-substituted siloxane substances within the Category also conclude 'not readily biodegradable'.

PFA (2017at) Siloxane Category Report for Environmental Endpoints, PFA.404.114.001