Registration Dossier

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Hazard for aquatic organisms

Hazard for air

Hazard for terrestrial organisms

Hazard for predators

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.


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 registration 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.


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

CAS Number



Chemical Name





Main ultimate Si hydrolysis product



Molecular weight (parent)



Molecular weight (silanol hydrolysis product)



log Kow (parent)

 9 (QSAR prediction)

9  (QSAR prediction)

log Kow (silanol hydrolysis product)


1.5  (QSAR prediction)

Water sol (parent)

0.0001507 mg/l 

<0.00001 mg/l

Water sol (silanol hydrolysis product))

995 mg/l

3700 mg/l (QSAR prediction)

Vapour pressure (parent)

27.4 hPa

<10 Pa

Vapour pressure (hydrolysis product)

1290 Pa and 1900 Pa (20°C and 25°C respectively)

250 Pa at 25°C (QSAR prediction) 

Hydrolysis t1/2 at pH 7 and 25°C

>200 h - 2000 h (predicted)

 >200 h - 2000 h (predicted)

Hydrolysis t1/2 at pH 4 and 25°C

1.9 h - 6.6 h (predicted)

1.9 h - 6.6 h 


Hydrolysis t1/2 at pH 9 and 25°C

2.0 h - 13.9 h 


2.0 h - 13.9 h 


Short-term toxicity to fish (LC50)

96-h: >182 ng/l;

14-d: >159 ng/l

>LoS (>14.2 ng/l)

Short-term toxicity to aquatic invertebrates (EC50)

No data

>LoS (>19 ng/l)

Algal inhibition (ErC50 and NOEC)

No data

>LoS (>15 ng/l)

Long-term toxicity to fish (NOEC)

No data

No data

Long-term toxicity to aquatic invertebrates (NOEC)

191 ng/l

No data

Long-term toxicity to sediment organisms (NOEC)

56 mg/kg sediment dry weight

No data

Short-term terrestrial toxicity (L/EC50)

No data

No data

Long-term terrestrial toxicity (NOEC)

No data

No data

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.