(1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate

Ecotoxicological Summary

Additional information

(1,1,3,3-Tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate hydrolyses slowly under neutral conditions, with half-lives 0.9 h at pH 5, 63 h at pH 7 and 0.8 h at pH 9 and 20-25°C (QSAR) (for reaction of siloxane groups). The registration substance will hydrolyse to 3-[hydroxy(dimethyl)silyl]propyl methacrylate. The water solubility of the registration substance is low (1.5E-03 mg/l) (QSAR), it has a high log Kow of 8.1 (QSAR) and its predicted vapour pressure is 0.005 Pa at 25°C.

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil REACH Annex VII requirements for the registration substance from substances that have similar structure and physicochemical properties. The environmental behaviour of (1,1,3,3 -tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate is expected to be dominated by its high partition coefficient and hence potential to absorb. In the following paragraphs the read-across approach for (1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate is assessed for the surrogate substance taking into account structure and physico-chemical properties, presented in the Table below.

Both the registered substance and the substance used as surrogate for read-across include the methacrylate group, which can indicate the potential for toxicity by a specific mode of action.

Table: Physicochemical parameters and ecotoxicity data for the registered and source substances

CAS Number	18547-93-8	24636-31-5
Chemical Name	(1,1,3,3-Tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate	3-(Chlorodimethylsilyl)propyl methacrylate
Si hydrolysis product	3-[Hydroxy(dimethyl)silyl]propyl methacrylate	3-[Hydroxy(dimethyl)silyl]propyl methacrylate
Molecular weight (parent)	386.64 g/mol	220.77 g/mol
Molecular weight (hydrolysis product)	202.33 g/mol	202.33 g/mol
log Kow(parent)	8.1 (QSAR)	Very rapidly reactive
log Kow(silanol hydrolysis product)	2.6 (QSAR)	2.6 (QSAR)
Water sol (parent)	1.5E-03 mg/l (QSAR)	Very rapidly reactive
Water sol (silanol hydrolysis product))	Above approximately 30 mg/l (QSAR) condensation reactions can occur over time, limiting the concentration dissolved in water.	Above approximately 30 mg/l (QSAR) condensation reactions can occur over time, limiting the concentration dissolved in water.
Vapour pressure (parent)	0.005 Pa at 25°C (QSAR)	9.6 Pa at 25°C (QSAR)
Vapour pressure (hydrolysis product)	4.8E-02 Pa at 25°C (QSAR)	4.8E-02 Pa at 25°C (QSAR)
Hydrolysis t1/2at pH 7 and 25°C	63 hours	<1 minute
Hydrolysis t1/2at pH 4 and 25°C	0.9 hours at pH 5	<1 minute
Hydrolysis t1/2at pH 9 and 25°C	0.8 hours	<1 minute
Short-term toxicity to fish (LC50)	Not available	Not available
Short-term toxicity to aquatic invertebrates (EC50)	Not available	0.573 mg/l (geometric mean measured concentrations of identified hydrolysis products and condensation products)
Algal inhibition (ErC50and NOEC)	72 hr ErC50 >11 mg/l, ErC10 >11 mg/l and NOEC >=11 mg/l (geometric mean measured) (highest concentration tested) (OECD TG 201), in terms of the substance as tested. The observations in this study are attributed to the exposure of test organisms to the parent substance in the test system and the test results are interpreted to indicate no effects up to the limit of solubility.	ErC50: 20.2 mg/l; ErC10: 4.78 mg/l; NOEC: 2.13 mg/l (geometric mean measured concentrations of identified hydrolysis products and condensation products) There is some evidence of undissolved material (precipitated products) in the study report for this test, and therefore physical effects may have contributed to some extent to the growth rate effects in the test.
Long-term toxicity to fish (NOEC)	Not available	Not available
Long-term toxicity to aquatic invertebrates (NOEC)	Not available	Not available
Long-term sediment toxicity (NOEC)	Not available	Not available
Short-term terrestrial toxicity (L(E) C50)	Not available	Not available
Long-term terrestrial toxicity (NOEC)	Not available	Not available

•       Read-across from 3-(chlorodimethylsilyl)propyl methacrylate to (1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate

The basis for read-across is that the test organisms were exposed to a combination of the target substance and to the common hydrolysis product of both the source and target substance, in the study with the source substance.

The target substance (registration substance), (1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate, is a siloxane in which two silicon atoms, each having two methyl substituents and a propyl methacrylate group, are connected by an oxygen atom.

The target substance will hydrolyse slowly under environmentally relevant conditions to give the silanol, 3-[hydroxy(dimethyl)silyl]propyl methacrylate. This silanol hydrolysis product undergoes condensation reactions in water to reform the target substance, (1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate (CAS 18547-93-8, EC 242-419-3). A dynamic equilibrium is established, with the position of the equilibrium dependent on nominal loading, temperature, and pH of the system, as well as what else is present in the solution. It may also change over time.

The source substance, 3-(chlorodimethylsilyl)propyl methacrylate, is a monochlorosilane with two methyl substituents and a propyl methacrylate group attached to the silicon.  It contains one chlorosilane group, which reacts extremely rapidly in aqueous solution to give the silanol (3-[hydroxy(dimethyl)silyl]propyl methacrylate) and hydrochloric acid. Therefore, the source and target substances share a common hydrolysis product, 3-[hydroxy(dimethyl)silyl]propyl methacrylate. Hydrolysis of the source substance followed by condensation of the product results in formation of the target substance.

In toxicity to acute invertebrates study conducted with 3-(chlorodimethylsilyl)propyl methacrylate (the source substance), test organisms were exposed to both 3-[hydroxy(dimethyl)silyl]propyl methacrylate (the hydrolysis product) and 1,1,3,3- tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate (the target substance) in the test system. The read-across is therefore considered to be valid.

• Considerations on the non-silanol hydrolysis products:

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e. g. OECD SIDS for CAS No. 7647-01-0, hydrochloric acid). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. Physical hazards related to pH effects are considered in the risk management measures (e. g. neutralisation) for effluents/aqueous waste.

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because effects arising from exposure to HCl are related to changes in pH and not true chemical toxicity.

Conclusion on classification

The substance has reliable short-term E(L)C50 values of 0.573 mg/l in invertebrates based on geometric mean measured concentration of the hydrolysis products, sum of 3-[hydroxy(dimethyl)silyl]propyl methacrylate (monomeric silanol) and 1,1,3,3-tetramethyldisiloxane-1,3-diyl)dipropane-1,3-diyl dimethacrylate (CAS 18547-93-8, EC 242-419-3) (siloxane dimer)), and >11 mg/l (> limit of solubility) in algae. It has reliable ErC10 >11 mg/l (> limit of solubility) in algae.

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: Category Acute 1 (M-factor 1)

Chronic toxicity: Category Chronic 1 (M-factor 1)