Triethoxy(3-isocyanatopropyl)silane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.5 mg/L

Assessment factor:

50

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

2.05 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.05 mg/L

Assessment factor:

500

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

0.94 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

1.8 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.18 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:

0.069 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

no potential for bioaccumulation

Additional information

Discussion

The registered substance, triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5), contains an isocyanate group which hydrolyses very rapidly, with a half-life less than that of the alkoxy groups, to form a primary amine, 3-aminopropyl(triethoxy)silane (CAS 919-30-2). This hydrolyses further (8.5 hours at pH 7 and 25˚C, measured) giving 3-aminopropylsilanetriol and ethanol.

 

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.

 

The substance will be exposed to the environment through wastewater 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. In addition, hydrolysis rate is slowest at neutral pH. Any deviation away from neutral significantly increases the rate of reaction. Amines are basic, therefore they will alter the pH of the test medium, thus increasing hydrolysis rate. The practicality of performing ecotox tests on the parent substance with a hydrolysis half-life of only 8.5 hours at pH 7 would be very challenging. In particular, tests with algae, due to their static regime, would almost certainly reflect exposure predominantly to the hydrolysis products.

 

The environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is therefore based on the properties of the silanol hydrolysis product, in accordance with REACH guidance.

 

As described in Sections 4.8 of IUCLID, the silanol hydrolysis products may be susceptible to condensation reactions.

 

3-Aminopropylsilanetriol and ethanol are both miscible in water and have low log K_ow (-4.0 and -0.31 respectively).

 

Amines are basic, therefore once dissolved they will increase the pH of the test media. In addition, the un-ionised form of the amine is more toxic than the ionised form. This pattern of toxicity is consistent with ammonia, where the un-ionised form, NH₃, is more toxic than the ionised form, NH₄+. The concentration of the un-ionised form is higher at higher pH. The difference in toxicity is attributed to bioavailability, where the un-ionised form can cross cell membranes more readily than the ionised form. Many of the aquatic toxicity studies for amine-containing organosilicon substances were conducted at pH ~8 and in some cases the pH was not adjusted after the test substances were added to the test media. Therefore, for media and natural waters having a pH closer to 7 it is likely that ecotoxicity might be expected to be less severe than indicated by these test results (i.e. the value of E(L)C₅₀ may possibly be higher under pH-neutral conditions).

 

READ-ACROSS JUSTIFICATION

In order to reduce 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. Ecotoxicological studies are conducted in aquatic medium or in moist environments; therefore the hydrolysis rate of the substance is particularly important since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

 

The isocyanate functional group of triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5) undergoes a very rapid hydrolysis reaction (half-life in the order of minutes or less) to form the intermediate hydrolysis product 3-aminopropyl(triethoxy)silane (CAS 919-30-2). The triethoxy groups are also susceptible to hydrolysis, with a reliable measured half-life of 8.5 hours at pH 7 and 25˚C. Therefore, it is more relevant to assess the ecotoxicity of triethoxy(3-isocyanatopropyl)silane by reference to the properties of 3-aminopropyl(triethoxy)silane and its silanol hydrolysis product.

 

The analogue approach for fulfilling the data requirement by read-across is discussed according to the Read-across Assessment Framework (RAAF).

 

Please see the reports attached in Annexes 2 and 3 for the analogue approach to address ecotoxicity of triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5) by read-across from 3-aminopropyl(triethoxy)silane (CAS 919-30-2) and N-(3(trimethoxysilyl)propyl)ethylenediamine (CAS 1760-24-3).

 

Discussion of read-across for supporting data

• Read-across from 3-(trimethoxysilyl)propyl isocyanate to triethoxy(3-isocyanatopropyl)silane

The isocyanate functional group of triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5) undergoes a very rapid hydrolysis reaction (half-life <5 minutes at pH 7) to initially produce 3-aminopropyl(triethoxy)silane and CO₂. Hydrolysis of the ethoxy groups then proceeds rapidly (half-life 8.5 hours at pH 7, 25˚C). Similarly, the isocyanate functional group of 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6) undergoes a very rapid hydrolysis reaction (half-life <5 minutes at pH 7) to form the intermediate hydrolysis product 3-aminopropyl(trimethoxy)silane and CO₂. Hydrolysis of the methoxy groups then proceeds rapidly (half-life 2.6 hours at pH 7, 25˚C, estimated).

The ultimate silanol hydrolysis product in both cases is 3-aminopropylsilanetriol. The registration substance also produces ethanol; the read-across substance also produces methanol.

Conclusion on classification

The isocyanate functional group of triethoxy(3-isocyanatopropyl)silane (CAS 24801-88-5) undergoes a very rapid hydrolysis reaction (half-life <5 minutes at pH 7) to initially produce 3-aminopropyl(triethoxy)silane (CAS 919-30-2). A second rapid hydrolysis step then follows.

Reliable short-term toxicity tests results are available with 3-aminopropyl(triethoxy)silane (CAS 919-30-2) as a surrogate substance. Read-across data are available for freshwater fish, invertebrates and algae. E(L)C₅₀ values for the freshwater fish and invertebrate were >100 mg/l. A long-term invertebrate study is available for invertebrates (NOEC (21-d): >1 mg/l) within the analogue group. The registration substance hydrolyses rapidly to a silanetriol hydrolysis product, 3-aminopropylsilanetriol, which has a very low log K_ow, but may persist in the environment.

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.

 

These data are consistent with the following classification under Directive 67/548/EEC (as amended) (DSD): 

Not classified.

Ethanol is included in the United Nations Environment Programme (UNEP) Chemicals Branch, OECD SIDS list of harmonised substances and does not require classification for the environment under the CLP Regulation (Regulation (EC) No. 1272/2008).