3-(dimethoxymethylsilyl)propyl methacrylate

Ecotoxicological Summary

Additional information

The hydrolysis half-life of 3-(dimethoxymethylsilyl)propyl methacrylate is 1.5 h at pH 7, 20-25°C. The registration substance will hydrolyse in contact with water and atmospheric moisture to 3-[dihydroxy(methyl)silyl]propyl methacrylate and methanol. REACH guidance (ECHA 2016b, 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 substance itself”. TGD and ECHA guidance, (EC 2003, ECHA 2016c, R.7b) 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. Therefore, the environmental hazard assessment is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance. As described in Section 4.8, the silanol hydrolysis products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing, read-across is proposed to fulfil REACH Annex VII requirements for the registration 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, because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour.

The registration substance and the substance used as surrogate for read-across both include a methacrylate structural feature, which can be a 'structural alert' for the possibility of toxicity caused by a specific mode of action. The silanol hydrolysis products are susceptible to condensation reactions, see Section 4.8.

The registration substance hydrolyses rapidly in water and therefore the selection of surrogate substance is based on log K_ow of the resulting silanols and the chemical groups present in them.

Additional information is given in a supporting report (PFA 2016y) attached in Section 13.

In the following paragraphs the read-across approach for 3-(dimethoxmethylsilyl)propyl methacrylate is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties. The Table below presents relevant physicochemical properties and the available ecotoxicological data.

Table: Physicochemical parameters and ecotoxicity data for the registration and surrogate substances

CAS Number	14513-34-9	24636-31-5
Chemical Name	3-[dimethoxy(methyl)silyl]propyl methacrylate	3-(Chlorodimethylsilyl)propyl methacrylate
Si hydrolysis product	3-[dihydroxy(methyl)silyl]propyl methacrylate	3-[hydroxy(dimethyl)silyl]propyl methacrylate
Molecular weight (parent) g/mol	232.4	220.8
Molecular weight (hydrolysis product) g/mol	204.3	202.3
log Kow (parent)	3.4 at 20°C (QSAR)	cannot be determined due to very rapid hydrolysis
log Kow (silanol hydrolysis product)	1.0 at 20°C (QSAR)	2.6 at 20°C (QSAR)
Water solubility (parent)	240 mg/L at 20°C (QSAR)	cannot be determined due to very rapid hydrolysis
Water solubility (silanol hydrolysis product))	35 mg/L (effective limit due to condensation phenomena)	30 mg/L (effective limit due to condensation phenomena)
Vapour pressure (parent)	1.3 Pa at 25°C (QSAR)	9.6 Pa at 25°C (QSAR)
Vapour pressure (hydrolysis product)	6.3E-04 Pa at 25°C (QSAR)	4.8E-02 Pa at 25°C (QSAR)
Hydrolysis t1/2 at pH 7 and 25°C	1.5 h	<1 min
Hydrolysis t1/2 at pH 4 and 25°C	0.1 h	<1 min
Hydrolysis t1/2 at pH 9 and 25°C	0.03 h	<1 min
Short-term toxicity to fish (LC50)	not available	not available
Short-term toxicity to aquatic invertebrates (EC50)	not available	EC50 (48 h): 0.573 mg/l
Algal inhibition (ErC50and NOEC)	not available	EC50 (72 h): 20.2 mg/l; ErC10 4.78 mg/l
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
Toxicity to microorganisms (EC50, EC10, NOEC)	not available	not available

•       Read-across from 3-(Chlorodimethylsilyl)propyl methacrylate (CAS 24636-31-5) to 3-(dimethoxmethylsilyl)propyl methacrylate

3-(Chlorodimethylsilyl)propyl methacrylate (CAS 24636-31-5) and 3-(dimethoxmethylsilyl)propyl methacrylate (CAS 14513-34-9)  both hydrolyse rapidly in contact with water (hydrolysis half-lives 1.5 h at pH 7, 20-25°C, and <1 minute at pH 7, 25°C respectively). The organosilicon hydrolysis products are 3-[dihydroxy(methyl)silyl]propyl methacrylate and 3-[hydroxy(dimethyl)silyl]propyl methacrylate respectively. 3-[dihydroxy(methyl)silyl]propyl methacrylate and 3-[hydroxy(dimethyl)silyl]propyl methacrylate are structural analogues, both containing a propylmethacrylate group bound to silicon, as well as one methyl group and two hydroxyl (silanol, Si-OH) groups and two methyl groups and one hydroxyl group respectively, such that the only structural difference is the presence of a silanol rather than a methyl group attached to the silicon atom. The additional methyl group in 3-[hydroxy(dimethyl)silyl]propyl methacrylate means only that it has a higher log Kow than the analogous silanediol; it also means that the condensation reactions of this silanol can only produce the disiloxane dimer and larger oligomeric siloxane condensation products, which could form in solutions of the registration substance, are not possible. They also share similar physicochemical properties: similar water solubility, and low MW (202.3 and 204.3 respectively). In general aquatic ecotoxicity tends to be positively correlated with the value of log Kow, hence in this case the read-across is probably conservative because the silanol product of the tested analogue has a higher log Kow value (log Kow 2.6) than that of the registration substance (log Kow 1.0).

The presence of dissolved and undissolved condensation products (siloxane dimer) present in the test media may have contributed to some extent to the effects observed in the studies with the read-across substance. It should be noted that at environmentally relevant concentrations of the registration substance, significant condensation is not expected based on its physicochemical properties.

3-(Chlorodimethylsilyl)propyl methacrylate (CAS 24636-31-5) is used to read-across to short-term toxicity to invertebrates and algae endpoints. E(L)C50 values of 0.573 and >20.2 mg/L, respectively, have been determined.

The aquatic toxicity data for the surrogate substance indicates that it is acutely toxic to aquatic organisms, attributed to the hydrolysis products (monomeric silanol and disiloxane dimer).

      

•       Considerations on the non-silanol hydrolysis products:

Methanol is well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC50 and LC50 values for this substance is in excess of 1000 mg/l (OECD 2004a - SIDS for methanol, CAS 67-56-1). Therefore, at the loading rates experienced in these tests it is unlikely that the presence of methanol would significantly affect the results of the tests.

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;

•       silanol and silanediol would not be expected to affect the pH of an aqueous solution based on predicted first dissociation constants around 10 for silanol structural features.

Reference:

ECHA 2016b: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Assessment. Version: 3.0, February 2016 (Section A.16-3.2.2 part (A): Hydrolysis)

ECHA 2016c: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.7b: Endpoint specific guidance. Version: 3.0, February 2016 (Section R.7.9.5.2, subsection on Assessment of the potential persistence of metabolites)

Conclusion on classification

The classification and labelling of the registration substance for the environment is based on read-across data for a structurally analogous substance, 3-(chlorodimethylsilyl)propyl methacrylate (CAS 24636-31-5).

The tested substance has reliable short-term E(L)C50 values of 0.573 mg/l in invertebrates and 20.2 mg/l in algae 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)). It has reliable ErC10 of 4.78 mg/l in algae.

The substance hydrolyses rapidly 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)