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EC number: 213-668-5 | CAS number: 999-97-3
- Life Cycle description
- Uses advised against
- Endpoint summary
- Appearance / physical state / colour
- Melting point / freezing point
- Boiling point
- Density
- Particle size distribution (Granulometry)
- Vapour pressure
- Partition coefficient
- Water solubility
- Solubility in organic solvents / fat solubility
- Surface tension
- Flash point
- Auto flammability
- Flammability
- Explosiveness
- Oxidising properties
- Oxidation reduction potential
- Stability in organic solvents and identity of relevant degradation products
- Storage stability and reactivity towards container material
- Stability: thermal, sunlight, metals
- pH
- Dissociation constant
- Viscosity
- Additional physico-chemical information
- Additional physico-chemical properties of nanomaterials
- Nanomaterial agglomeration / aggregation
- Nanomaterial crystalline phase
- Nanomaterial crystallite and grain size
- Nanomaterial aspect ratio / shape
- Nanomaterial specific surface area
- Nanomaterial Zeta potential
- Nanomaterial surface chemistry
- Nanomaterial dustiness
- Nanomaterial porosity
- Nanomaterial pour density
- Nanomaterial photocatalytic activity
- Nanomaterial radical formation potential
- Nanomaterial catalytic activity
- Endpoint summary
- Stability
- Biodegradation
- Bioaccumulation
- Transport and distribution
- Environmental data
- Additional information on environmental fate and behaviour
- Ecotoxicological Summary
- Aquatic toxicity
- Endpoint summary
- Short-term toxicity to fish
- Long-term toxicity to fish
- Short-term toxicity to aquatic invertebrates
- Long-term toxicity to aquatic invertebrates
- Toxicity to aquatic algae and cyanobacteria
- Toxicity to aquatic plants other than algae
- Toxicity to microorganisms
- Endocrine disrupter testing in aquatic vertebrates – in vivo
- Toxicity to other aquatic organisms
- Sediment toxicity
- Terrestrial toxicity
- Biological effects monitoring
- Biotransformation and kinetics
- Additional ecotoxological information
- Toxicological Summary
- Toxicokinetics, metabolism and distribution
- Acute Toxicity
- Irritation / corrosion
- Sensitisation
- Repeated dose toxicity
- Genetic toxicity
- Carcinogenicity
- Toxicity to reproduction
- Specific investigations
- Exposure related observations in humans
- Toxic effects on livestock and pets
- Additional toxicological data
Endpoint summary
Administrative data
Description of key information
Additional information
Approach to the Terrestrial Chemical Safety Assessment
The registered substance will hydrolyse very rapidly (half-life <<1 min at pH 4, 7 and 9 at approximately 20°C) in contact with water and atmospheric moisture to trimethylsilanol and ammonia. 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”. TGD and ECHA guidance, (ECHA 2016) also suggest that when the hydrolysis half-life is less than 12 hours, the breakdown products, rather than the parent substance, should be evaluated for aquatic toxicity. 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 trimethylsilanol and ammonia since both contribute to the toxicity.
Therefore, in accordance with REACH guidance, the terrestrial chemical safety assessment for 1,1,1,3,3,3-hexamethyldisilazane is based on its hydrolysis products, trimethylsilanol and ammonia. These are assessed separately.
Trimethylsilanol
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 highly water soluble, is not readily biodegradable but has low bioavailability and low potential for adsorption (based on log Kow<3 (1.19) and log Koc 1.6). Low toxicity was observed in short-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.
Trimethylsilanol is classed as soil 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.
In the event that terrestrial invertebrate and plant studies need to be conducted, the definitive terrestrial risk characterisation would use a PNECsoil based on the lower of the two test results with an assessment factor of 50 (unless soil microorganism data are available as well, in which case, the assessment factor would be 10).
A confirmatory test would be conducted with the most sensitive organism group based on short-term aquatic testing. For this substance, invertebrates were the most sensitive organism, indicating a preference to conduct a confirmatory test with terrestrial invertebrates.
The PNECscreen(EQPM) for trimethylsilanol is derived from the short-term test results with invertebrates and has a value of 0.12 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 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3), the highest PECx10/PNECscreen(EQPM) for trimethylsilanol is 9.58.
Full testing in both terrestrial plants and invertebrates would result in a new value of 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. There is no basis to expect such toxicity for trimethylsilanol based on the absence of significant toxicity observed in aquatic tests.
In the case of trimethylsilanol, 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 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.
Ammonia:
Ammonia is assessed separately from trimethylsilanol. Data are available for the toxicity of ammonia to terrestrial plants. Concentrations of ammonia in soil resulting from use of 1,1,1,3,3,3-hexamethyldisilazane are minimal compared to the natural background and other anthropogenic sources (see Section 9 of the CSR).
The following results for terrestrial toxicity of ammonia are sourced from the OECD HPV Chemical Programme, SIDS Dossier Ammonia (CAS 7664-41-7) 2007.
The effects of ammonia gas were tested on terrestrial plant species.
4 hour LOECs for damage to leaf area ranged from 3 ppm to 40 ppm.
A 1 hour LC100 of 40 ppm was recorded for tomato and sunflower plants, where complete injury was caused.
High concentration tests from 4 to 8 minutes duration resulted in EC50s for foliar necrosis of 250 ppm NH3 (tomatoes) to 1000 ppm NH3 (buckwheat and tobacco).
A 4 hour exposure to 1000 ppm NH3 killed moist spring rye seeds, whereas radish seeds were still viable after 16 hours. 250 ppm NH3 for 16 hours reduced rye seed germination by half, but had no effect on radish seeds.
Treatment of barley roots with 1.6x10-3 M unionised ammonia reduced their respiration 23 percent and 3.3x10-3 M unionised ammonia caused a 78 percent inhibition after 4 hours. The respiration of barley roots treated with 1x10-3to 3x10-3 M unionised ammonia was reduced by 46 to 62 percent within 4 hours.
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