Registration Dossier

Diss Factsheets

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
no hazard identified

Marine water

Hazard assessment conclusion:
no hazard identified

STP

Hazard assessment conclusion:
no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:
no hazard identified

Sediment (marine water)

Hazard assessment conclusion:
no hazard identified

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential to cause toxic effects if accumulated (in higher organisms) via the food chain

Additional information

The hydrolysis half-life of 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (M4Q, CAS 3555-47-3) is >200 h - 2000 h (predicted) at pH 7 and 25°C. The water solubility of the substance is low (0.0001507 mg/l) and the log Kow is high (9). It is therefore likely that, under the flow-through exposure conditions of the fish and invertebrate tests that the test organisms will have predominantly been exposed to the registered substance. In the static algal test with the read-across substance it is likely that exposure will have been predominantly to the test substance and a small proportion of its hydrolysis products.

 

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex VII requirements for the registered substance from substances that have similar structure and physicochemical properties. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

 

The registered substance, and the substances used as surrogate for read-across, are members of the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates. For substances with a log Kow of 6 and above no effects are seen with aquatic organisms due to the substance low water solubility limiting the effects seen. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

 

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

 

Read-across from 3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane (ViM4Q, CAS 60111-54-8) to 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (M4Q, CAS 3555-47-3)

The registered substance, M4Q, and the surrogate substance ViM4Q are quaternary-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.

M4Q and ViM4Q possess similar physicochemical properties: high molecular weight (384.85 and 432.89 respectively), low water solubility (both insoluble, at 0.0001507 mg/l and <0.00001 mg/l respectively), high log Kow (both 9) and high log Koc (both 6). Both substances have negligible biodegradability and similar slow hydrolysis rates. A comparison of the key physicochemical properties is presented in the table below. Given the similar properties and structural similarities, it is considered valid to read across data from ViM4Q to M4Q.

Data are read across from 3,3-bis[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-1,5-divinyltrisiloxane (ViM4Q, CAS 60111-54-8) for the toxicity to algae and cyanobacteria endpoint.

Read-across from dodecamethylpentasiloxane (L5, CAS 141-63-9) to 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (M4Q, CAS 3555-47-3)

The registered substance, M4Q, and the surrogate substance L5 are methylated siloxanes containing Si atoms linked by oxygen. M4Q is a quaternary-branched structure of five Si atoms, with four terminal Si atoms linked to a central Si atom by Si-O-Si bonds. The four terminal Si atoms are fully methyl-substituted. L5 is a linear chain of 5 Si-O units; each silicon atom is fully methyl-substituted.

M4Q and L5 possess similar physicochemical properties: high molecular weight (both 384.85), low water solubility (both insoluble, at 0.0001507 mg/l and 7.0E-05 mg/l respectively), high log Kow (9 and 9.41, respectively) and high log Koc (6 and 6.3, respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates. A comparison of the key physicochemical properties is presented in the table below. Given the similar properties and structural similarities, it is considered valid to read across data from L5 to M4Q.

Data are read across from L5 for the sediment and soil toxicity endpoints.

Read-across from phenyl silsesquioxanes (EC 939-487-8) to 1,1,1,5,5,5-hexamethyl-3,3-bis[(trimethylsilyl)oxy]trisiloxane (M4Q, CAS 3555-47-3) 

The read-across substance, 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. M4Q is a quaternary-branched structure of five Si atoms, with four terminal Si atoms linked to a central Si atom by Si-O-Si bonds. The four terminal Si atoms are fully methyl-substituted. Both phenyl silsesquioxanes and M4Q are high molecular weight, phenyl-substituted siloxanes, which have a low solubility, high log Kow and high log Koc. Both substances have negligible biodegradability and hydrolyse very slowly.

 

Data are read across from phenyl silsesquioxanes (EC 939-487-8) for the toxicity to aquatic microorganisms endpoint..

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

CAS Number

3555-47-3

60111-54-8

141-63-9

EC 939-487-8

Chemical Name

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

(M4Q)

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

(ViM4Q)

Dodecamethylpentasiloxane (L5)

Phenyl silsesquioxanes

Main ultimate Si hydrolysis product

Trimethylsilanol

Dimethylvinylsilanol

Dimethylsilanediol (CAS 1066-42-8) and trimethylsilanol (CAS 1066-40-6)

 

Molecular weight (parent)

384.85

432.89

384.85

372.8 to1635.1

log Kow (parent)

 9 (QSAR prediction)

9  (QSAR prediction)

9.41

9  (QSAR prediction)

Water sol (parent)

0.0001507 mg/l at 23°C

<0.00001 mg/l at 20°C

7.0E-05 mg/l at 23°C

4.9E-05 to 4.94E-51 mg/l

Vapour pressure (parent)

15 Pa at 25°C

<10 Pa at 25°C

0.23 Pa at 20°C

0.23 Pa at 20°C

Hydrolysis t1/2 at pH 7 and 25°C

>200 h - 2000 h (predicted)

 >200 h - 2000 h (predicted)

630 – >63 000 h

200 - 630 h

Hydrolysis t1/2 at pH 4 and 25°C

1.9 h - 6.6 h (predicted)

1.9 h - 6.6 h 

(predicted)

6.6 h

1.9 -3.3 h

Hydrolysis t1/2 at pH 9 and 25°C

2.0 h - 13.9 h 

(predicted)

2.0 h - 13.9 h 

(predicted)

14 h

2.0 - 5.3 h

Short-term toxicity to fish (LC50)

96-h: >182 ng/l;

14-d: >159 ng/l

>LoS (>14.2 ng/l)

>75 ng/l

No data

Short-term toxicity to aquatic invertebrates (EC50)

No data

>LoS (>19 ng/l)

No data

No data

Algal inhibition (ErC50 and NOEC)

No data

>LoS (>15 ng/l)

No data

No data

Long-term toxicity to fish (NOEC)

No data

No data

>=39 ng/l (reported in a bioconcentration study)

≥0.05 mg/l (nominal); ≥0.11 mg/l (mean measured)

Long-term toxicity to aquatic invertebrates (NOEC)

 

>=

191 ng/l

No data

>=47 ng/l

≥980 mg/l (loading rate WAF); ≥0.00605 mg/l (geometric mean measured)

Long-term toxicity to sediment organisms (NOEC)

 

>=

56 mg/kg sediment dry weight

No data

>=38 mg/kg dry weight, Lumbriculus variegatus;  

>=27 mg/kg dry weight, Chironomus riparius;  

>=59 mg/kg dry weight, Hyalella azteca.

No data

Short-term terrestrial toxicity (L/EC50)

No data

No data

No data

No data

Long-term terrestrial toxicity (NOEC)

No data

No data

EC10>1000 mg/kg dw, Eisenia fetida (OECD 222);  

EC10>100 mg/kg dw, soil microorganisms (OECD 216)

≥1000 mg/kg dwt, Eisenia fetida;

EC25 and EC50 >100 mg/kg dwt, soil microorganisms

Conclusion on classification

The registered substance has reliable measured short-term E(L)C50 values of >159 ng/l (14 days) and >182 ng/l (96 hours) in fish and a read-across EC50 (72 hours) >15 ng/l and NOEC ≥15 ng/l in algae. The registration substance also has a reliable long-term NOEC of  ≥

191 ng/l in Daphnia (highest concentration tested). The available short- and long-term aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water. The substance hydrolyses slowly in water and is not readily biodegradable.

These data are consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified.

Chronic toxicity: Not classified.