Reaction mass of bis(trimethoxysilylpropyl)methylphosphonate and (trimethoxysilylpropyl)methylmethyl phosphate

Ecotoxicological Summary

Additional information

The submission substance is a multi-constituent substance containing two constituents, (trimethoxysilylpropyl)methylmethylphosphonate (Constituent 1) and bis(trimethoxysilylpropyl)methylphosphonate (Constituent 2). Both constituents will hydrolyse rapidly in contact with water. The predicted half-life for Constituent 1 is 4.0 h at pH 7 and 20-25°C. The predicted half-life for Constituent 2 is 10.0 h at pH 7 and 20-25°C. Therefore, for the purposes of the environmental assessment, the hydrolysis products for assessment are:

• methyl 3-(trihydroxysilyl)propyl methylphosphonate (1 mole from Constituent 1; HP1)

• bis[3-(trihydroxysilyl)propyl] methylphosphonate (1 mole from Constituent 2; HP2)

• methanol (3 moles from Constituent 1; 6 moles from Constituent 2)

Over a long timescale, hydrolysis of the phosphonate ester groups may occur. The final hydrolysis products from both constituents are (3-hydroxypropyl)silanetriol (1 mole from Constituent 1; 2 moles from Constituent 2) and methylphosphonate (1 mole from each constituent).

The water solubilities of the constituents have been predicted using a validated QSAR estimation method. The predicted water solubilities are 1.0E+06 mg/L at 20°C and 4.3E+04 mg/L at 20°C for Constituent 1 and Constituent 2 respectively.

The silanol hydrolysis products, methyl 3-(trihydroxysilyl)propyl methylphosphonate (HP 1) and bis[3-(trihydroxysilyl)propyl] methylphosphonate (HP 2) are soluble, both with predicted water solubilities of 1.0E+06 mg/L. However, they may undergo condensation reactions in solution which are expected to become important at loadings above about 1000 mg/L for HP 1 and 100 -500 mg/L for HP 2 (see Section 4.8 for further discussion).

The predicted log K_ow using a validated QSAR estimation method for the constituents are 0.3 at 20°C and -0.2 at 20°C for Constituent 1 and Constituent 2 respectively. The predicted log K_ow for the silanol hydrolysis products are -2.8 at 20°C and -4.0 at 20°C for HP 1 and HP 2 respectively.

The other non-silanol hydrolysis product is methanol for both constituents.

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”. EU technical guidance for risk assessment (EC 2003) and REACH guidance R.16 (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. Therefore, the environmental chemical assessment (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis products, methyl 3-(trihydroxysilyl)propyl methylphosphonate and bis[3-(trihydroxysilyl)propyl] methylphosphonate, in accordance with REACH guidance. As described in Section 4.8, condensation reactions of the hydrolysis products are possible.

Consideration of the non-silanol hydrolysis product methanol

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 methanol are in excess of 1000 mg/L (OECD 2004 - 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.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ whole substance values of >100 mg/L in fish, >100 mg/L in invertebrates and >100 mg/L in algae, as well as a NOEC value of ≥100 mg/L in algae.

The constituents of the substance hydrolyse rapidly in water. The hydrolysis products are not considered to be readily biodegradable but have low potential for bioaccumulation (i.e. low log K_ow)

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.