Registration Dossier
Registration Dossier
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EC number: 260-350-7 | CAS number: 56706-10-6
Endpoint summary
Administrative data
Description of key information
Additional information
Approach to the Terrestrial Chemical Safety Assessment
The registered substance, 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2) (CAS 56706-10-6, EC 260-350-7), will hydrolyse moderately rapidly (half-life 40-80 hours at pH 7 at 20-25°C) in contact with water and atmospheric moisture to (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol 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. Therefore, in accordance with REACH guidance, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of both the parent substance, 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2) and the silanol hydrolysis product (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol, in accordance with REACH guidance.
Therefore, in accordance with REACH guidance, the terrestrial chemical safety assessment for (S2) is based on both the registration substance and its silanol product, (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol. The non-silanol hydrolysis product, ethanol, is not expected to contribute to toxicity to terrestrial organisms.
Parent: 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2)
In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in a long- or short-term 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.
The substance is poorly water soluble and is not readily biodegradable, is not bioaccumulative (based on BCF = 146 to 715 (see IUCLID Section 5.3 for further details) and has some potential for adsorption (log Koc 3.1); therefore S2 is expected to partly partition to the terrestrial compartment. No toxicity was observed in short-term aquatic tests at, or below the limit of solubility (0.014 mg/l at 20°C) and the occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would be considered unlikely.
S2 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 confirmatory test would be conducted with the most sensitive organism group based on aquatic testing.
The PNECscreen(EQPM) for S2 is derived from the short-term test results with algae and has a value of 25 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, the highest agricultural soil RCR (0.023), the corresponding PEC value of 0.057 mg/kg dw, the (PECx10)/PNECscreen(EQPM) for S2 is (0.057 *10)/25 = 0.0228.
In addition, a confirmatory long-term terrestrial toxicity test is therefore required in accordance with the recommendations for hazard category 3 substances. If a confirmatory long-term terrestrial test were to be conducted, an assessment factor of 100 would be applied to derive PNECsoil from one long-term test. A confirmatory test with either terrestrial plants or invertebrates would result in a new value for PNECsoil. This value could only be more conservative than the value of PNECscreen(EQPM) in the situation that standard testing in terrestrial plants or invertebrates exhibited a dose response with a NOEC/EC10 ≤ 2500 mg/kg dw (and applying an assessment factor of 100). There is no basis to expect such toxicity for S2, based on the absence of significant toxicity observed in aquatic tests.
In the case of S2, the registrants consider that a long-term terrestrial study is unlikely to affect the outcomes of the chemical safety assessment. As such the registrants propose that further testing (including the confirmatory study) is not necessary.
Overall, it is concluded that the risk characterisation conclusion is sufficiently conservative and therefore further 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.
Hydrolysis product: (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol
In accordance with Column 2 of REACH Annex IX, there is no need to further investigate the effects of this substance in a long or short-term 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.
The substance is not readily biodegradable but is highly water soluble and has a low potential for adsorption (based on log Kow <3 (-3.0) and log Koc -3.2). No toxicity was observed in short-term aquatic tests at, or below the limit of solubility (0.014 mg/l at 20°C) and the occurrence of more severe toxic effects in the terrestrial compartment that were not expressed in the aquatic studies would be considered unlikely.
(3-{[3-(Trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol 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 confirmatory test would be conducted with the most sensitive organism group based on aquatic testing.
The PNECscreen(EQPM) for (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol is derived from the short-term test results with algae (and applying an appropriate molecular weight conversion, to represent the silanol hydrolysis product) and has a value of 0.06 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, the highest agricultural soil RCR (0.916), the corresponding PEC value of 0.055 mg/kg dw, the (PECx10)/PNECscreen(EQPM) for (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol is (0.055*10)/0.06 = 9.2.
In addition, a confirmatory long-term terrestrial toxicity test is therefore required in accordance with the recommendations for hazard category 3 substances. If a confirmatory long-term terrestrial test were to be conducted, an assessment factor of 100 would be applied to derive PNECsoil from one long-term test. A confirmatory test with either terrestrial plants or invertebrates would result in a new value for PNECsoil. This value could only be more conservative than the value of PNECscreen(EQPM) in the situation that standard testing in terrestrial plants or invertebrates exhibited a dose response with a NOEC/EC10 ≤ 6 mg/kg dw (and applying an assessment factor of 100). There is no basis to expect such toxicity for (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol based on the absence of significant toxicity observed in aquatic tests.
In the case of (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol, the registrants consider that a long-term terrestrial study is unlikely to affect the outcomes of the chemical safety assessment. As such the registrants propose that further testing (including the confirmatory study) is not necessary.
In addition, prior to terrestrial toxicity tests being carried out, the technical feasibility of testing the silanol hydrolysis product, (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol, would need to be assessed. 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-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol is expected to become important at loadings above about 100 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-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol. 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 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.
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