2,2,4,4,6,6-hexamethylcyclotrisilazane

Ecotoxicological Summary

Additional information

The hydrolysis half-life of 2,2,4,4,6,6-hexamethylcyclotrisilazane is << 1 min at 25°C and pH 4, 7 and 9 (OECD 111) (read-across from an analogous organosilazane).The registered substance will hydrolyse in contact with water and atmospheric moisture to dimethylsilanediol and aqueous ammonia/ammonium ions. 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, (EC 2003, 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, the environmental hazard assessment, including sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolysis product and ammonia, in accordance with REACH guidance. As described in IUCLID Section 4.8, the silanol hydrolysis products may be susceptible to condensation reactions.

• Considerations on the non-silanol hydrolysis products:

The registration substance also produces ammonia as a hydrolysis product. Ammonia is a known toxicant for aquatic organisms, in particular to fish and invertebrates. This compound drives the aquatic toxicity of the substance and is assessed separately to the silanol hydrolysis product. Further details on the assessment of the substance are presented in the toxicity test results section that follows.

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because:

• dimethylsilanediol has its predicted first dissociation constant of 11.9 and so does not significantly affect the pH of an aqueous solution;
• the silicon-containing hydrolysis products are not toxic to aquatic organisms at 100 mg/l in short-term studies.

Table: Comparison of properties of registration substance and read-across substances

CAS Number	1009-93-4	1066-42-8	7664 -41 -7
Chemical Name	2,2,4,4,6,6 -Hexamethylcyclotrisilazane	Dimethylsilanediol	Ammonia
Si hydrolysis product	Dimethylsilanediol	n/a	n/a
Molecular weight (parent)	219.51	92.17	17.03 (*)
Molecular weight (hydrolysis product)	92.17	n/a	n/a
log Kow (parent)	not applicable	-0.38	0.23 (**)
log Kow (silanol hydrolysis product)	-0.38	-	-
Water sol (parent)	not applicable	Limited to ca. 1000 mg/l by condensation reactions	531000 mg/l (**)
Water sol (silanol hydrolysis product)	Limited to ca. 1000 mg/l by condensation reactions	Limited to ca. 1000 mg/l by condensation reactions	-
Vapour pressure (parent)	35 Pa at 25°C	7 Pa at 25°C	861100 Pa or 786.7 kPa (*)
Vapour pressure (hydrolysis product)	7 Pa at 25°C	n/a	n/a
Hydrolysis t1/2 at pH 7 and 25°C	<<1 minute	not applicable	-
Hydrolysis t1/2 at pH 4 and 25°C	<<1 minute	not applicable	-
Hydrolysis t1/2at pH 9 and 25°C	<<1 minute	not applicable	-
Short-term toxicity to fish (LC50)	not available	>126 mg/l	0.083 mg NH3/l (*)
Short-term toxicity to aquatic invertebrates (EC50)	not available	>117 mg/l	0.16 mg NH3/l (**)
Algal inhibition (ErC50 and NOEC)	not available	>118 mg/l and ≥118 mg/l	2.0 mg NH3/l and 3.0 mg NH3/l (LOEC) (**)
Long-term toxicity to fish (NOEC)	not available	not available	0.014 mg NH3/l (*)
Long-term toxicity to aquatic invertebrates (NOEC)	not available	not available	0.066 mg NH3/l (**)
Long-term sediment toxicity (NOEC)	not available	not available	-
Short-term terrestrial toxicity (L(E)C50)	not available	not available	Plants (LOEC) 3 -40 ppm (*)
Long-term terrestrial toxicity (NOEC)	not available	not available	-

* Source: Ammonia data sourced from SIAR for the Ammonia category (OECD, 2007; sponsored by USA, shared partnership with European Fertilizers Manufacturing Association (EFMA-Europe) and The Fertilizer Institute (TFI-US)).

**Source: Environment Agency Proposed EQS for Water Framework Directive Annex VIII Substances: Ammonia (unionised) 2007.

Additional information about the aquatic ecotoxicity of ammonia

Ammonia is known to be toxic to the aquatic environment and has been reviewed by a number of different regulatory programmes including the SIAR for the Ammonia category (OECD, 2007; sponsored by USA, shared partnership with European Fertilizers Manufacturing Association (EFMA-Europe) and The Fertilizer Institute (TFI-US)), the Canadian Environmental Protection Act, 1999, Priority Substance List Assessment Report, Ammonia in the Aquatic Environment, and the UK Environment Agency Proposed EQS for Water Framework Directive Annex VIII Substances: Ammonia (unionised) 2007. The following information is taken from these sources; further review of the data has not been considered necessary. For the purpose of this hazard assessment, data has been taken from the UK Environment Agency (EA) EQS, where the EA have taken a thorough look at data which the authors of the present report believe is adequate for hazard assessment for REACH.

Ammonium salts dissociate in water to give ammonia/ammonium ion and, therefore, data for these salts may be used to assess the toxicity of ammonia.

The ammonia/ammonium ion in aqueous solution exists in equilibrium between NH₃and NH₄⁺, depending on the pH. The unionised species is by far the most toxic; the ammonium ion is assumed to be essentially non-toxic. Therefore, toxicity due to ammonia increases with increasing pH (Clement Associates, 1990; USEPA, 1999), because the fraction of unionised ammonia increases according to the following equation (Emerson, 1975):

Fraction unionised= 1/(10^pKa-pH+1)

At pH 8.5, the proportion of unionised ammonia is approximately 10 times that at pH 7.5. The concentration of unionised ammonia will be lower at higher ionic strengths of very hard fresh water or salt water environments. This effect can be significant in estuarine and marine waters. (Environment Canada, 2001b). Moreover, the pK_ais reciprocally related to temperature. For every 9°C increase in temperature, the proportion of unionised ammonia approximately doubles (EA, 2007).

Conclusion on classification

The registration substance hydrolyses very rapidly in water to products which are not readily biodegradable.

The silanol hydrolysis product of the registration substance has reliable short-term E(L)C₅₀ values of >117 mg/l in invertebrates (no effects at highest concentration tested) and >118 mg/l in algae (highest concentration tested). It has reliable NOEC ≥118 mg/l in algae (highest concentration tested). The available short-term aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water.

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 (EC) No 67/548/EEC (as amended) (DSD):

Not classified.

The ammonia hydrolysis product of the registration substance carries a harmonised classification and labelling in CLP as follows:

Index number: 007-001-01-2

EC number: 215-647-6

CAS number: 1336-21-6

International Chemical Identification: ammonia ....%

Harmonised environmental classification under Regulation (EC) No 1272/2008 (as amended) (CLP): Aquatic Acute 1, H400. (No M-factor is stated).