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

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

100 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

8.9 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.89 mg/kg sediment dw

Assessment factor:

100

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

10.8 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

0.83 mg/kg food

Assessment factor:

300

Additional information

The hydrolysis half-life of 1,1,1,3,5,5,5-heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (M3T) is ca. 630 h at pH 7 and 25°C (predicted). The water solubility of the substance is low (1.9 µg/l) and the log K_owis high (8.2).It is therefore likely that, under the flow-through exposure conditions of the fish study that the test organisms will have predominantly been exposed to the registration substance. In the semi-static long-term invertebrate study and the static algal test it is likely that exposure will have been predominantly to the registration substance and a small proportion of its hydrolysis product.

The test substance is volatile but measures were taken to maintain exposure concentrations by conducting the key fish and invertebrate tests under flow-through conditions and the algal test under sealed conditions.

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to fulfil up to REACH Annex IX 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.

In aqueous media, M3T hydrolyses slowly in water (half-life approximately 630 hours at pH 7 and 25°C).

M3T and the substances used as surrogate for read-across are part of the Reconsile Siloxanes Category. Substances in this group tend to have low water solubility, high adsorption and partition coefficients and slow degradation in the sediment compartment. For substances with a log K_ow of 8 and above no long-term 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.

In the following paragraphs the read-across approach for M3T is assessed for the surrogate substance taking into account structure, hydrolysis rate and physicochemical properties.

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

Aquatic and sediment toxicity: Read-across from decamethyltetrasiloxane (L4) to M3T:

The registered substance (M3T) and read-across substance (L4, CAS 141-62-8) are methylated siloxanes containing Si atoms linked by oxygen. L4 is a linear chain of 4 Si-O units, whereas M3T is a tertiary branched structure of four Si atoms, with a central Si (see Section 1 for chemical structure). Neither of the substances contains any reactive functional groups. M3T and L4 are members of the Reconsile Siloxane Category, they have low water solubility (0.00189 and 0.00674 mg/l respectively), high log K_ow(both 8.2) and slow hydrolysis rates (630 h at pH 7 and 20-25°C and 728 h at pH 7 and 25°C respectively), indicating that organisms can only be exposed to very low concentrations of the substance in solution.. For substances with a log Kow of 8 and above no long-term toxicity effects are seen with aquatic organisms due to the substance low water solubility limiting the bioavailability of the substance in aquatic studies and toxicity is not expressed. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments. Both substances also are not readily biodegradable and have high adsorption to sediment potential.

It is known that high log K_ow values limit the long and short-term toxicity of a substance by limiting its bioavailability. This is true for substances acting by non-polar narcosis such as neutral organics and alcohols. For example, the programme ECOSAR v.1.11 (2012) presents chronic fish and invertebrate QSARs for neutral organics which are valid up to log K_ow8, indicating that no long-term toxicity effects can be expected for substances having log K_ow>8. The available data in the siloxanes Category indicates that neither short- nor long-term toxicity effects are recorded with substances having log K_ow≥6.6.

No effects at the limit of solubility have been reported in short-term and long-term studies in other trophic levels conducted with the surrogate substance, L4. Both the registration and read-across substances are expected to act via a non-polar narcotic mechanism of toxicity; neither substance has functional groups that affect the toxicity. Given the similar properties and structural similarities, it is considered valid to read-across data from decamethyltetrasiloxane (L4) to M3T.

Sediment toxicity: Read-across from octamethyltrisiloxane (L3) to M3T:

The registered substance (M3T) and read-across substance (L3, CAS 107-51-7) are methylated siloxanes containing Si atoms linked by oxygen. L3 is a linear chain of 3 Si-O units, whereas M3T is a tertiary branched structure of four Si atoms, with a central Si (see Section 1 for chemical structure). M3T and L3 are members of the Reconsile Siloxanes Category, they have low water solubility (0.00189 and 0.034 mg/l respectively), high log Kow (8.2 and 6.6 respectively) and slow hydrolysis rates (630 h at pH 7 and 20-25°C and 329 h at pH 7 and 25°C respectively). Both substances also are not readily biodegradable and have high potential for adsorption to sediment potential. Therefore read-across between the two substances is considered to be valid.

Terrestrial toxicity: Read-across from hexamethyldisiloxane (L2)and octamethyltrisiloxane (L3) to M3T:

The registration substance, M3T and the source substances hexamethyldisiloxane (L2, CAS 107-46-0) and octamethyltrisiloxane (L3, CAS 107-51-7) are members of the Siloxane Category. L2 and L3 are linear siloxanes with two silicon atoms and one oxygen atom, and three silicon and two oxygen atoms, respectively. Each of the silicon atoms are fully substituted with methyl groups. M3T is a tertiary branched structure of four Si atoms, with a central Si. Refer to Section 1 for more structural information on the registration substance including diagrams.

Terrestrial toxicity studies with siloxanes such as M3T, L2 and L3 are technically difficult to conduct due to their high volatilisation potential (high Henry’s Law Constant and low octanol-air partition coefficient) and the potential for degradation in soil. Soil testing according to guideline methods does not allow for a renewal of the substrate and hence re-application of test substance. Therefore, there is potential for the organisms to not be exposed to the test material for a sufficiently long period of time for effects to be expressed, as well as the difficulty of quantifying actual exposure concentrations. The physical-chemical properties of L3 and L2 are generally consistent and similar to those of M3T, meaning this read-across to M3T is a robust interpolation. Homogeneous hydrolysis rates are comparable. The substances have similar molecular weight, relatively low water solubility and high log K_ow. See table 7.0.1 for relevant values. All three substances have negligible biodegradability.

Summary of aquatic ecotoxicological and physicochemical properties for the registered substance, its hydrolysis products and the surrogate substances.

CAS Number	17928-28-8	141-62-8	107-51-7	107-46-0
Chemical Name	1,1,1,3,5,5,5-Heptamethyl-3-[(trimethylsilyl)oxy]trisiloxane (M3T)	Decamethyltetrasiloxane (L4)	Octamethyltrisiloxane (L3)	Hexamethyldisiloxane
Final Si hydrolysis product(s)	Trimethylsilanol; methylsilanetriol	Trimethylsilanol; dimethylsilanediol	Trimethylsilanol; dimethylsilanediol	Trimethylsilanol
Molecular weight (parent)	310.69	310.69	236.54	162.38
Molecular weight (hydrolysis product(s))	90.2 (trimethylsilanol); 94.1 (methylsilanetriol)	90.2 (trimethylsilanol); 92.17 (dimethylsilanediol)	90.2 (trimethylsilanol); 92.17 (dimethylsilanediol)	90.2 (trimethylsilanol);
log Kow(parent)	8.2	8.2	6.60	5.06
log Kow(silanol hydrolysis product)	1.2 (trimethylsilanol); -2.4 (methylsilanetriol)	1.2 (trimethylsilanol); -0.41 (dimethylsilanediol)	1.2 (trimethylsilanol); -0.41 (dimethylsilanediol)	1.2 (trimethylsilanol);
Log Koc(parent)	5.3	5.16	4.34	3.0
Water solubility (parent) at 23°C	1.9E-03 mg/l	6.7E-03 mg/l	3.4E-02 mg/l	0.93 mg/l
Water sol (silanol hydrolysis product) at 25°C	9.9E+02 mg/l (trimethylsilanol); 1E+06 mg/l (methylsilanetriol)	9.9E+02 mg/l (trimethylsilanol); 1.0E+06 mg/l (dimethylsilanediol)	9.9E+02 mg/l (trimethylsilanol); 1.0E+06 mg/l (dimethylsilanediol)	9.9E+02 mg/l (trimethylsilanol);
Vapour pressure (parent) at 25°C	210 Pa	73 Pa	530 Pa	5500 Pa
Vapour pressure (hydrolysis product) at 25°C	1.9 hPa(trimethylsilanol); 0.05 Pa (methylsilanetriol)	1.9 hPa (trimethylsilanol); 7 Pa (dimethylsilanediol)	1.9 hPa (trimethylsilanol); 7 Pa (dimethylsilanediol)	1.9 hPa (trimethylsilanol);
Hydrolysis t1/2at pH 7 and 25°C	630 h	728 h	329 h	116 h
Short-term toxicity to fish (LC50)	LL50(96 h): > 100 mg/L (reliability 4)	>6.3 μg/l	>19.4 μg/l	460 μg/l
Short-term toxicity to aquatic invertebrates (EC50)	no data	no data	>20 μg/l	no data
Algal inhibition (ErC50and NOEC)	no data	EC50: >2.2 μg/l; NOEC: ≥2.2 μg/l	EC50: >9.4 μg/l; NOEC: ≥9.4 μg/l	ErC50>550 μg/l; EC10: 90 μg/l
Long-term toxicity to fish (NOEC)	no data	≥7.9 μg/l	≥27 μg/l	no data
Long-term toxicity to aquatic invertebrates (NOEC)	no data	≥4.9 μg/l	≥15 μg/l	80 μg/l
Sediment toxicity (NOEC)(normalised to 5% OC)	no data	≥ 92mg/kg dwt H. azteca  ≥34 mg/kg dwt (L. variegatus)	89 mg/kg dwt ,C. riparius ≥ 61 mg/kg dwt L. variegatus ≥ 95 mg/kg dwt, H. azteca	≥2.7 mg/kg dwt (L. variegatus)
Short-term terrestrial toxicity (L(EC)50)	no data	no data	no data	no data
Long-term terrestrial toxicity (NOEC)	testing not feasible	no data	testing not feasible	testing not feasible

 

Conclusion on classification

The submission substance is not classified for the environment in the EU according to the regulation (EC) No 1272/2008 because short- and long-term data available for the structurally analogous substance indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water.