3-(trimethoxysilyl)propyl isocyanate

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

The isocyanate group in 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6) will hydrolyse very rapidly, with a hydrolysis half-life less than that of the alkoxy groups, to form a primary amine, 3-(trimethoxysilyl)propylamine (CAS 13822-56-5) as an intermediate hydrolysis product, and carbon dioxide. The intermediate hydrolysis product hydrolyses further (half-life 2.6 h at pH 7 and 20-25°C, estimated) to form 3-aminopropylsilanetriol (CAS 58160-99-9) and methanol (CAS 67-56-1).

 

3-Aminopropylsilanetriol and methanol are both miscible in water and have low log K_ow (-4.0 and -0.82, respectively).

 

As described in the water solubility section (Section 1.4 of the CSR and 4.8 of IUCLID), condensation reactions of the silanetriol are possible.

 

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 minimum residency time in the wastewater treatment plant is approximately 7 hours (although this is a conservative figure and wastewater treatment time may be hours to days longer) 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.

Therefore, the environmental chemical safety assessment (aquatic, sediment and soil) is based on the properties of the silanol hydrolysis product, 3-aminopropylsilanetriol, in accordance with REACH guidance.

 

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 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) (PFA 2013z). 

 

READ-ACROSS JUSTIFICATION

There are only reliable available measured data for the registration substance for short-term fish and invertebrates. In order to reduce testing, the Annex requirements are fulfilled with data 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 selection of substances used for read-across (source substances) is based on the silanol hydrolysis products, the groups present on the side chains, as well as their log K_ow.

The analogue approach for fulfilling the data requirement by read-across is discussed in the ecotoxicity read-across justification report attached in Section 13 of IUCLID, according to the Read-across Assessment Framework (RAAF).

 

The registered substance (target substance) 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6), and the substance used as surrogate for read-across (source substance) for toxicity to aquatic algae and cyanobacteria, 3-aminopropyl(triethoxy)silane (CAS 919-30-2), are part of a class of alkoxysilane compounds which hydrolyse rapidly or moderately rapidly to produce the same Si hydrolysis product, 3-aminopropylsilanetriol, and another non-Si hydrolysis product.

 

In the context of the RAAF, the basis of the read-across hypothesis is “(Bio)transformation to common compound(s)”; Scenario 1 applies. The source substance hydrolyses to the same silicon containing hydrolysis product as the target substance, and the non-common hydrolysis products will not have an impact on the prediction of the ecotoxicological property.

 

Data have also been read across from trichloro(ethyl)silane (CAS 115-21-9) for long-term toxicity to aquatic invertebrates. In the context of the RAAF, the basis of the read-across hypothesis for this substance is “Different compounds have the same type of effect(s)”; Scenario 2 applies. Both the source and target substance hydrolyse to compounds with qualitatively similar properties. The non-common hydrolysis products will not have an impact on the prediction of the ecotoxicological properties.

Table: Overview of physico-chemical and ecotoxicity data for the registered substance and surrogate substances

CAS Number	15396-00-6	13822-56-5	919-30-2	115-21-9
Chemical Name	3-(Trimethoxysilyl)propyl isocyanate	3-(Trimethoxysilyl)propylamine (intermediate hydrolysis product)	(3-Aminopropyl)triethoxysilane	Trichloro(ethyl)silane
		Note, this substance is generated by the very rapid reaction of the registration substance, 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6)
Si hydrolysis product	3-(trimethoxysilyl)propylamine (CAS 13822-56-5) and subsequently 3-aminopropylsilanetriol. See next column for all properties of Si hydrolysis products.	3-aminopropylsilanetriol	3-aminopropylsilanetriol	ethylsilanetriol
Molecular weight (parent) g/mol	205.29	179.29	221.37	163.51
Molecular weight (hydrolysis product) g/mol	See next column	137.21	137.21	108.17
log Kow (parent)	n/a	0.2 (un-ionised); -2.8 at pH 7	1.7 (un-ionised); -1.3 at pH 7	n/a
log Kow (silanol hydrolysis product)	See next column	-2.9 (un-ionised); -4 at pH 7	-2.9 (un-ionised); -4 at pH 7	-1.9
Water solubility (parent)	n/a	5.7E+05 mg/L	1.7E+04 mg/L	n/a
Water solubility (silanol hydrolysis product))	See next column	1E+06 mg/L (limited to approximately 1000 mg/L by condensation reactions)	1E+06 mg/L (limited to approximately 1000 mg/L by condensation reactions)	1E+06 mg/L (but will be limited by condensation reactions)
Vapour pressure (parent)	7.1 Pa at 25°C	18 Pa at 25°C	8 Pa at 20°C	4780 Pa at 20°C
Vapour pressure (hydrolysis product)	See next column	2.5E-04 Pa at 25°C	2.5E-04 Pa at 25°C	0.02 Pa at 25°C
Hydrolysis t1/2 at pH 7 and 25°C	<5 minutes	2.6 hours	8.5 hours at 24.7°C	<5 minute
Hydrolysis t1/2 at pH 2 and 37.5°C	<5 minutes	5 seconds	5 seconds	<1 minute
Hydrolysis t1/2 at pH 7 and 37.5°C	<5 minutes	0.1 hours	3 hours	<1 minute
Short-term toxicity to fish (LC50)	>100 mg/l	1264 mg/L (R4)	>934 mg/L	n/a
Short-term toxicity to aquatic invertebrates (EC50)	>100 mg/L	302 mg/L (R4)	331 mg/L	n/a
Algal inhibition (ErC50 and NOEC)	n/a	n/a	>1000 and 863 mg/L	n/a

Conclusion on classification

The isocyanate functional group of 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6) undergoes a very rapid hydrolysis reaction (half-life of minutes or less at pH 7) to initially produce 3-(trimethoxysilyl)propylamine. A second rapid hydrolysis step then follows.

Reliable short-term toxicity tests results are available with the registration substance and additionally, read-across data are available for freshwater fish, invertebrates and algae. E(L)C₅₀ values for the three organisms were all >100 mg/L. A long-term invertebrate study is available with Daphnia magna (NOEC (21-d): >100 mg/L) read-across from a structurally related substance. 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.