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EC number: 217-496-1
CAS number: 1873-88-7
The hydrolysis half-life of 1,1,1,3,5,5,5-heptamethyltrisiloxane
(H-L3) is 2.2 days at pH 7 and 25°C. The water solubility of the
substance is low (0.02 mg/l) and the log Kow is high (6.2).
It is therefore likely that, under the conditions of the short-term
aquatic toxicity tests with fish, invertebrates and algae that
exposure will have been to low concentrations of the registered
substance, rather than the hydrolysis products.
REACH guidance (ECHA 2016, R.16) states that “for substances
where hydrolytic DT50 is less than 12 hours, environmental
effects are likely to be attributed to the hydrolysis product rather
than to the parent itself”. TGD and ECHA guidance, (EC 2003, ECHA
2016) also suggest that when the hydrolysis half-life is less than 12
hours, the breakdown products, rather than the parent substance,
should be evaluated for aquatic toxicity. Due to the hydrolysis
half-life of 2.2 days, it is likely that, under the conditions
of the short-term aquatic toxicity tests with fish, invertebrates and
algae, exposure will have been to low concentrations of the registered
substance, rather than the hydrolysis products.
Therefore, the environmental hazard assessment, including
sediment and soil compartments due to water and moisture being
present, is based on the properties of the parent substance, in
accordance with REACH guidance.
In order to reduce animal 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.
The behaviour of 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3) in the
environment is dominated by the high partition coefficient (log Kow
6.2) and adsorption coefficient properties (log Koc 3.8).
The registered substance 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3,
CAS 1873-88-7) and the substances used as surrogates are part of the
Reconsile Siloxanes Category. Substances in this group tend to
have slow hydrolysis rates, low water solubility, high log Kow,
high adsorption coefficients and slow degradation in the sediment
Additional information is given in a supporting report (PFA 2017)
attached in Section 13 of the IUCLID 6 dossier.
In the following paragraphs the read-across approach for H-L3 is
assessed for the surrogate substance taking into account structure and
physico-chemical properties, presented in the following table.
Table: Summary of physico-chemical and ecotoxicological properties for
the registered and surrogate substances.
* Aquatic tests carried out on the parent substance. Due to test
methods/exposure regime/hydrolysis rates indication of whether the
test organisms were exposed to parent substance or hydrolysis product
is included in the end point summaries.
from L3 to H-L3:
The registration substance, 1,1,1,3,5,5,5-heptamethyltrisiloxane (H-L3)
is a siloxane of three silicon atoms linked by oxygen atoms. The
structurally analogous substance proposed for read-across is
octamethyltrisiloxane (L3, CAS 107-51-7), which is also a linear
siloxane made up of a chain of 3 silicon atoms linked by oxygen. In both
substances, methyl groups form the silicon side groups, however the
difference is that in H-L3 the middle silicone has an Si-H bond in place
of one of the methyl groups.
Significant reaction of the Si-H bond, present in the registration
substance but not in L3, would be expected to occur during long-term
aquatic toxicity tests (see section on hydrolysis for more information).
To exclude the possibility that by-products of this Si-H reaction might
bring unpredicted toxicity to aquatic organisms, data have been
read-across from another substance that contains a Si-H bond (see
“Read-across from trimethoxysilane to H-L3” below).
H-L3 and L3 are part of the Reconsile Siloxanes Category. They have low
water solubility (0.02 and 0.034 mg/l respectively), high log Kow (6.2
and 6.6 respectively) and slow hydrolysis rate (2.2 days at pH 7 and
20-25°C and 13.7 days at pH 7 and 25°C respectively) indicating
that organisms can only be exposed to very low concentrations of the
substance in solution. 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.
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, L3. L3 is used to read-across to long-term toxicity
to fish and invertebrates endpoints, for which a 90-d NOEC of ≥0.027
mg/l and a 21-d NOEC value of ≥0.015 mg/l, respectively, have been
determined. L3 is also used to read-across to sediment toxicity
endpoints. 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
from trimethoxysilane to H-L3:
Trimethoxysilane (CAS 2487-90-3) has been included as read-across in the
dataset for H-L3 because, in addition to the Si-OMe group, it has an
Si-H bond which will undergo the same hydrolysis reaction as the
registration substance. The Si-OMe hydrolyses rapidly (hydrolysis
half-life of ≤17 seconds at pH 4, 7 and 9, 2˚C). The Si-H bond is
expected to react in water, however the rate of this reaction is
uncertain. Long-term aquatic read-across is proposed for a substance
that does not contain an Si-H bond. The Si-H bond is expected to react
during the course of long-term aquatic toxicity tests. To exclude the
possibility that this reaction might cause toxicity to aquatic
organisms, trimethoxysilane has been included in the dataset for H-L3.
The short-term toxicity data for this substance report L(E)C50 values
in excess of 100 mg/l. The data indicate that the Si-H bond further
reactions are not likely to affect the toxicity.
Trimethoxysilane is used to read-across to short-term toxicity to fish,
aquatic invertebrates and algae endpoints. E(L)C50 values of
>100 mg/l for each species have been determined. Read-across of
trimethoxysilane is not used to fulfil any data-gap, it is just used to
demonstrate that reaction of the Si-H bond does not cause toxic effects.
The substance has reliable short-term E(L)C50 values of
>0.108 mg/l in fish, >0.030 mg/l in invertebrates and >0.019 mg/l in
algae. It has reliable NOEC ≥0.019 mg/l in algae, and long-term data
(read across from a structural analogue) give NOEC ≥0.027 mg/l in fish
and NOEC ≥0.015 mg/l in invertebrates.
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 at a moderate rate in water and is not readily
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.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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