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

Biodegradation in water: screening tests

Description of key information

Biodegradation in water: screening tests: 0% biodegradation in 28 days (OECD 301 F) (based on read-across from M3T)

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Additional information

Discussion (screening testing)

There are no reliable ready biodegradation data available for M4Q, therefore good quality data for the structurally-related substances, M3T (CAS 17928-28-8) and ViM4Q (CAS 60111-54-8), have been read across.The choice of read-across substance is based on structural similarities between the surrogate and target substance and similarities in their physicochemical properties. M4Q, M3T and ViM4Q are structurally-similar substances.

M4Q and the source substance M3T are branched chain linear siloxanes with five silicon atoms and four silicon atoms respectively, alternated by oxygen atoms. M3T is a tertiary branched methylated siloxane structure of four Si atoms; the longest siloxane chain consists of three silicon atoms and two oxygen atoms, with a Si-O branch on the central silicon atom in the chain. The silicon atoms are fully substituted by methyl groups. M4Q is a quaternary branched methylated siloxane structure of five Si atoms; the longest siloxane chain consists of three silicon atoms and two oxygen atoms, with two Si-O branches on the central silicon atom in the chain. The silicon atoms are fully substituted by methyl groups. M4Q and M3T possess similar physicochemical properties.  

The registered substance, M4Q, and the source substance ViM4Q are branched structures of five Si atoms, with four terminal Si atoms linked to a central Si atom by Si-O-Si bonds. In the registered substance, the four terminal Si atoms are fully methyl-substituted, whereas in ViM4Q each of the four terminal Si atoms is substituted by one vinyl and two methyl groups. Evidence within the siloxanes group and from the low functionality organosilicon group suggests that the vinyl group does not confer additional toxicity to substances.

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 M4Q and surrogate substances M3T and ViM4Q

Property	M4Q (3555-47-3)	M3T (17928-28-8)	ViM4Q (60111-54-8)
Molecular weight	384.85	310.69	432.89
Log Kow	9.0	8.2	9.0
Log Koc	6.0	5.3	6.0
Water solubility (mg/l)	0.1507 µg/l at 23°C	1.9E-03 mg/l	<10 ng/L
Vapour pressure	15 Pa  at 25°C	210 Pa at 25°C	<10 Pa  at 25°C
Hydrolysis half- life at pH 7 (h)	>200-2000	630	>200 – 2000

 

M4Q, M3T and ViM4Q 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.

It is therefore considered valid to read-across the results for M3T and ViM4Q to fill the data gap for the registered substance.

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
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

In the study with M3T, 0% biodegradation was observed in 28 days. The study was conducted according to OECD 301F and is considered to be reliable.

A biodegradation rate of 1.5% in 28 days (CO2 evolution) was determined for ViM4Q in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.