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Approach to the Terrestrial Chemical Safety Assessment


The isocyanate group in 3-(trimethoxysilyl)propyl isocyanate (CAS 15396-00-6) will hydrolyse very rapidly (<4.88 minutes at pH 7) 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 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.

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 terrestrial chemical safety assessment for 3-(trimethoxysilyl)propyl isocyanate is based on its silanol hydrolysis product 3-aminopropylsilanetriol (CAS 58160-99-9).


3-Aminopropylsilanetriol (CAS 58160-99-9):

No terrestrial toxicity data are available with the registered substance. 

In accordance with Column 2 of REACH Annex X, there is no need to further investigate the effects of 3-aminopropylsilanetriol in a long terrestrial toxicity to invertebrates/higher plants study because, as indicated in guidance R.7.11.6 (ECHA 2017), the quantitative chemical safety assessment (conducted according to Annex I of REACH) indicates that the Risk Characterisation Ratio is below 1 and therefore the risk is already adequately controlled and further testing is not justifiable.


3-Aminopropylsilanetriol is highly water soluble, is not readily biodegradable but has low potential for bioaccumulation and low potential for adsorption (based on log Kow <3 (-4) and log Koc -0.6). Low toxicity was observed in short- and long-term aquatic tests, and there is no reason to expect any specific mechanism of toxicity beyond narcosis. Therefore, the occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would be considered unlikely.


3-Aminopropylsilanetriol is classed as Hazard Category 3 for the terrestrial environment (Table R.7.11-2 of ECHA guidance R7.c, 2017) based on potential for high persistence (DT50 >180 days), lack of ready biodegradability and low toxicity to aquatic organisms (EC/LC50 not <1 mg/l).


In this situation, a screening approach is applied: a confirmatory long-term terrestrial test is usually appropriate, in addition to the equilibrium partitioning approach with an extra factor of ten, in order to determine whether further full tests are necessary.


The PNECscreen(EQPM) for 3-aminopropylsilanetriol is derived from the algal NOEC, generating a PNECsoil value of 0.069 mg/kg dwt.  For the purpose of the screening assessment comparison only, an extra factor of ten is applied (PECx10/PNECscreen(EQPM)). Based on the exposure assessment of the registered substance, the highest PECx10/PNECscreen(EQPM) for 3-aminopropylsilanetriol is 0.000887 x 10 = RCR 0.00887.


As can be seen from the PNECscreen (EQPM), the RCRs for the terrestrial compartment are very low. Due to the high water solubility of the substance and the low log Koc, the compartment of concern is the aquatic compartment rather than the terrestrial. The aquatic PNEC and aquatic RCRs are therefore likely to be protective of the terrestrial environment.


If one confirmatory long-term terrestrial test were to be conducted, an assessment factor of 100 would be applied to the results of that test to derive PNECsoil. The terrestrial organisms would have to exhibit a dose response of <6.9 mg/kg dw to be more conservative than the current PNECsoil of 0.069 mg/kg dw derived using the equilibrium partitioning method. There is no basis to expect such toxicity for 3-aminopropylsilanetriol based on the absence of significant toxicity observed in aquatic tests and the low potential for bioaccumulation.  


In the case of 3-aminopropylsilanetriol, the Registrant considers that a long-term terrestrial study is unlikely to affect the outcomes of the chemical safety assessment. As such the Registrant proposes that further testing (including the confirmatory study) is not necessary.


In addition,the technical feasibility of testing the silanol hydrolysis product, 3-aminopropylsilanetriol is uncertain. To achieve the test concentrations required by terrestrial testing guidance (up to 1000 mg/kg), aqueous stock solutions would need to be prepared at very high concentrations, well above 1000 mg/l. Silanetriols are susceptible to condensation reactions. Condensation of 3-aminopropylsilanetriol is expected to become important at loadings above about 1000 mg/l causing the formation of insoluble polymeric particles (sols) and gels over time. It therefore may not be possible to test terrestrial organisms at high enough test concentrations to meaningfully assess the terrestrial toxicity of 3-aminopropylsilanetriol. Prior to any testing, the feasibility of dosing at concentrations up to 1000 mg/kg dw would have to be assessed.


Overall it is concluded that the risk characterisation conclusion is sufficiently conservative and therefore further in vivo testing is not considered necessary.


Details on how the PNEC and the risk characterisation ratio have been derived can be found in IUCLID Section 6.0, CSR Section 7, and Chapters 9 and 10 of the Chemical Safety Report, respectively.



EC (2003). Technical Guidance Document on Risk Assessment in support of Commission Directive 93/67/EEC on Risk Assessment for new notified substances, Commission Regulation (EC) No 1488/94 on Risk Assessment for existing substances, Directive 98/8/EC of the European Parliament and of the Council concerning the placing of biocidal products on the market. Part IV. EUR 20418 EN/4


ECHA (2016). REACH Guidance on Information Requirements and Chemical Safety Assessment Chapter R16: Environmental Exposure Assessment Version: 3.0. February 2016.


ECHA (2017). Guidance on Information Requirements and Chemical Safety Assessment. Chapter R.7b: Endpoint specific guidance. Version 4.0 June 2017.