[3-(2,3-epoxypropoxy)propyl]trimethoxysilane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.45 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

0.45 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.045 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

8.2 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

1.6 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.16 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

0.063 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

The hydrolysis half-life of [3-(2,3-epoxypropoxy)propyl]trimethoxysilane (CAS 2530-83-8) is 6.5 h at pH 7 and 24.5°C. This half-life reflects hydrolysis of the methoxy groups. It is considered that the epoxide ring opening is not significant for the exposure assessment at any relevant pH, and the chemical safety assessment considers the hydrolysis products [3-(2,3-epoxypropxy)propyl]silanetriol and methanol. This is a conservative position in the absence of clear evidence, as the epoxide structural feature is likely to be more toxic than the corresponding diol (this is discussed further below).

 

The registered substance will hydrolyse in contact with water and atmospheric moisture to [3-(2,3-epoxypropoxy)propyl]silanetriol and methanol. 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”. ECHA Guidance Chapter R.7b (ECHA, 2017) states that where degradation rates fall between >1 hour and <72 hours, testing of parent and/or degradation product(s) should be considered on a case-by-case basis.

 

[3-(2,3-epoxypropoxy)propyl]trimethoxysilane has a measured degradation half-life of 6.5 hours at pH 7 and 24.5°C, as indicated above. The substance will be exposed to the environment through waste-water treatment plant (WWTP) effluent only. The residency time in the waste-water treatment plant is approximately 15 hours with an average temperature of 15°C (assumed to be at neutral pH). Significant degradation by hydrolysis would be expected before the substance is released to the receiving waters. The aquatic exposure and chemical safety assessment therefore focus on the degradation products [3-(2,3-epoxypropxy)propyl]silanetriol and methanol. 

 

The proportion of the substance that adsorbs to the WWTP sludge will either be burnt or spread on soil, depending on the practices of the WWTP. The sediment is exposed to the substance in the environment via the effluent water released from the WWTP. The sediment will therefore be exposed to the same species as the receiving waters. Accordingly, exposure and chemical safety of the sediment is based on the hydrolysis products.

 

Exposure of soil is via the partitioning of the substance to the WWTP sludge. The sludge may then be spread onto soil, depending on the practices of the WWTP. The substance will have undergone significant hydrolysis by the time it reaches the soil, therefore exposure and chemical safety assessment of soil is based on the degradation products.

 

Consideration of the non-Si hydrolysis product

The non-silicon-containing hydrolysis product of [3-(2,3-epoxypropoxy)propyl]trimethoxysilane is methanol. Methanol is well characterised in the public domain literature and is not hazardous at the concentrations relevant to the aquatic toxicity studies; the short-term EC₅₀ and LC₅₀ values for the substance are in excess of 1000 mg/l (OECD, 2004a - SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18 -20 October 2004, Methanol, CAS 67-56-1).

 

In conclusion, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolysis product, in accordance with REACH guidance. As described in Sections 4.8 and 5.1.2 of IUCLID, the silanol hydrolysis products may be susceptible to condensation reactions as well as subsequent hydrolysis reactions of the epoxide functional group.

 

The registration substance [3-(2,3-epoxypropoxy)propyl]trimethoxysilane (CAS 2530-83-8) has a predicted water solubility of 1.1E+05 mg/L and predicted log K_ow of 0.5.

 

There are two characteristics of [3-(2,3-epoxypropoxy)propyl]trimethoxysilane that are thought to confer toxicity to the substance: the presence of the epoxy group in the molecular structure and the log K_ow of the substance. Hydrolysis of the substance can occur at the methoxy group and the epoxy group. The hydrolysis of the methoxy group will generate a silanetriol with a lower log K_ow than the parent and thus a less bioavailable form of the substance. The epoxy group will still be present as the methoxy group will hydrolyse more quickly than the epoxy group. The hydrolysis of the epoxy group will result in an open ring, which is less reactive and less toxic.

 

The intermediate hydrolysis product 3-(2,3-epoxypropoxy)propyl)silanetriol is expected to be less toxic than the parent substance because even though the epoxy ring is still present it has a lower log K_ow value. Likewise, it is expected to be more toxic than the ultimate hydrolysis product because the epoxy ring is absent from the ultimate hydrolysis product.

 

Consideration of the intermediate hydrolysis product [3-(2,3-epoxypropoxy)propyl]silanetriol:

[3-(2,3-Epoxypropoxy)propyl]silanetriol has a low log K_ow of -2.6 and a high predicted water solubility of 1E+06 mg/L, although this is limited by condensation reactions (see section 4.8 of the IUCLID), and a molecular weight of 194.26. The substance contains an epoxy group which is a chemical structure that might confer toxicity (REACH guidance, ECHA 2008, R.10). However, this intermediate hydrolysis product is expected to be less toxic than the parent substance, because even though the epoxy ring is still present, the substance has a lower log K_ow value.

 

Consideration of the final hydrolysis product 3-[3-(trihydroxysilyl)propoxy]propane-1,2-diol:

3-[3-(trihydroxysilyl)propoxy]propane-1,2-diol is the final silanol hydrolysis product of [3-(2,3-epoxypropoxy)propyl]trimethoxysilane. It has a molecular weight of 212.28, a predicted log K_ow of -3.8 and predicted water solubility of 1E+06 mg/L. It is expected to be less toxic than the parent substance and the intermediate hydrolysis product, because the epoxy ring is absent and the log K_ow is lower.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀values of 55 mg/l in fish, 324 mg/l in invertebrates and 350 mg/l in algae. It has a reliable NOEC of 130 mg/l in algae and reliable long-term NOEC of >100 mg/l in Daphnia. The substance hydrolyses rapidly in water and is not readily biodegradable but the log K_ow of the hydrolysis product is very low.

According to Regulation (EC) No 1272/2008 (as amended) (CLP), where adequate chronic toxicity data are not available for three trophic levels, classification should be based on the most stringent outcome of the criteria set out in Table 4.1.0(b)(i) or (ii), and Table 4.1.0(b)(iii) of the Regulation. In this case, the lowest acute E(L)C50 is between >10 and =100 mg/L, and the substance is not rapidly biodegradable.

These data are therefore consistent with the following classification under Regulation (EC) No 1272/2008 (as amended) (CLP):

Acute toxicity: Not classified

Chronic toxicity: Classified as Aquatic Chronic 3, H412: Harmful to aquatic life with long lasting effects.