4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

0.7 mg/L

Assessment factor:

1 000

Extrapolation method:

assessment factor

PNEC freshwater (intermittent releases):

7 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

0.07 mg/L

Assessment factor:

10 000

Extrapolation method:

assessment factor

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

1 mg/L

Assessment factor:

100

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

91 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

9.1 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:

29 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

6.7 g/kg food

Assessment factor:

300

Additional information

The hydrolysis half-life of 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane is ca. 40 - 90 h at pH 7, 20-25˚C. The registration substance will hydrolyse in contact with water and atmospheric moisture to (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol and ethanol. Therefore, the environmental hazard assessment, including sediment and soil compartments, is primarily based on the properties of the registration substance, in accordance with REACH guidance (R16).

As described in Section 1.3 of the CSR, the silanol hydrolysis products may be susceptible to condensation reactions.

It is likely that, because of the prolonged test medium preparation phases, the test organisms were predominantly exposed to the hydrolysis products of the substance, with the exception of one algal study in which direct addition to test medium was used (in this case, based on the static exposure regime, it is likely that the organisms would have been exposed to a mixture of the dissolved and undissolved parent substance, as well as hydrolysis products). The results suggest that, although the substance itself appears to be non-toxic at the solubility limit, the hydrolysis products have non-classifiable effects which under the conditions of the test may have been exacerbated by or caused mainly by physical effects of undissolved or precipitated test materials or by-products.

In light of the properties of the registration substance, it is possible that short-term studies may have under-estimated the potential for toxicity. Therefore, although there is no clear evidence of effects attributable to the parent substance, it is appropriate to derive PNECs and conduct risk characterisation to ensure risks are appropriately managed.

Exposure assessment and risk characterisation will be based on the properties of the registration substance. Since there is significant uncertainty on the half life range, exposure and risk for the soil and sediment protection targets will also be conducted using the properties of the hydrolysis product for comparison.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing, read-across is proposed to fulfil REACH Annex VII-X requirements for the registered substance using weight of evidence 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, because after hydrolysis occurs the resulting product has different structural features, physicochemical properties and behaviour, particularly at higher loading concentrations at which oligomerisation and cross-linking can be expected to occur.

In the following paragraphs the read-across approach for 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties. Table 1 presents relevant physico-chemical properties and Table 2 presents the available ecotoxicological data.

Table 1:  Physicochemical properties for the S2, S3 and S4 constituents

Property	S2	S3	S4
Name	4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane	4,4,14,14-tetraethoxy-3,15-dioxa-8,9,10-trithia-4,14-disilaheptadecane	4,4,15,15-tetraethoxy-3,16-dioxa-8,9,10,11-tetrathia-4,15-disilaoctadecane
CAS Number	56706-10-6	56706-11-7	40372-72-3
Hydrolysis product	(3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol	(3-{[3-(trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol	[3-({[3-(trihydroxysilyl)propyl]disulfanyl}disulfanyl)propyl]silanetriol
Molecular weight	474.8 g mol‑1	506.9 g mol-1	538.9 g mol-1
log Kow (parent)	5.2 (predicted)	5.2 (predicted)	5.2 (predicted)
log Kow (silanol hydrolysis product)	-3.0 (predicted)	-3.0 (predicted)	-3.0 (predicted)
Water solubility (parent)	≤ 1 mg/l (measured); 0.01 mg/l (predicted)	≤1 mg/l (read-across from S2 and S2/S3/S4 reaction mass); 0.01 mg/l (predicted)	≤1 mg/l (read-across from S2 and S2/S3/S4 reaction mass); 0.01 mg/l (predicted)
Water solubility (silanol hydrolysis product)	1 000 000 mg/l (predicted) (will be limited by formation of condensation products)	1 000 000 mg/l (predicted) (will be limited by formation of condensation products)	1 000 000 mg/l (predicted) (will be limited by formation of condensation products)
Vapour pressure (parent)	9 Pa (measured); 0.44 Pa (predicted)	9 Pa (read-across from S2); 0.44 Pa (predicted)	9 Pa (read-across from S2); 0.44 Pa (predicted)
Hydrolysis t1/2 at pH 7 and 20-25°C	40 -90 h (predicted)	40 -110 h (predicted)	40 -130 h (predicted)

Table 2: Ecotoxicological data available for the three substances in the group

CAS Number	56706-10-6	211519-85-6
Chemical Name	4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane	Reaction mass of 4,4,15,15-tetraethoxy-3,16-dioxa-8,9,10,11-tetrathia-4,15-disilaoctadecane and 4,4,14,14-tetraethoxy-3,15-dioxa-8,9,10-trithia-4,14-disilaheptadecane and 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane
Short-name	S2	Polysulfides, S2, S3, S4
Short-term toxicity to fish (LL50)	>1000 mg/l	>1000 mg/l
Short-term toxicity to aquatic invertebrates (EL50)	<1 mg/l*	>10 mg/l
Algal inhibition (EL50 and NOELR)	EL50: 500 mg/l and NOELR: 32 mg/l	EL50: 700 mg/l and EL10: 60 mg/l

 * It is not possible to determine whether the effects observed in this study are due to the presence of undissolved test material or to the toxicity of the substance.

• Read-across from Polysulfides, bis[3-(triethoxysilyl)propyl] to 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane 

Polysulfides, bis[3-(triethoxysilyl)propyl] (CAS 211519-85-6) is also known as Reaction mass of 4,4,15,15-tetraethoxy-3,16-dioxa-8,9,10,11-tetrathia-4,15-disilaoctadecane and 4,4,14,14-tetraethoxy-3,15-dioxa-8,9,10-trithia-4,14-disilaheptadecane and 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane. It is a multiconstituent substance with three constituents. Each constituent is a bis[3-triethoxysilyl)propyl)polysulfide, where the number of sulfur atoms is 2 to 4. The constituents may be denoted as S2, S3 and S4 respectively. The registration substance is a constituent of Polysulfides, bis[3-(triethoxysilyl)propyl]. For the three constituents S2, S3 and S4, hydrolysis half-lives at 20-25°C are ca. 40 - 90 h, ca. 40 - 110 h and ca. 40 - 130 h at pH 7 respectively. The hydrolysis products are (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol, (3-{[3-(trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol, [3-({[3-(trihydroxysilyl)propyl]disulfanyl}disulfanyl)propyl]silanetriol and ethanol.

The registration substance and the constituents of polysulfides, bis[3-(triethoxysilyl)propyl] (CAS 211519-85-6) all hydrolyse at a moderate or moderate - slow rate in contact with water (ca. 40 - 90 h at pH 7, 20-25⁰C, and ca. 40 - 90 h, ca. 40 - 110 h and ca. 40 - 130 h at pH 7, 20-25⁰C respectively). 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane and constituents of Polysulfides, bis[3-(triethoxysilyl)propyl] are structural analogues, all containing a polysulfide bridge connecting two 3-(triethoxysilyl)propyl groups. They also share similar physicochemical properties: low water solubility (approximately 0.01 mg/l in all cases, quantified prediction; <1 mg/l, measured for S2 and polysulfides, bis[3-(triethoxysilyl)propyl] ), high log K_ow (approximately 5.2 in all cases) and relatively high MW (474 and 474-539 g/mol respectively).

The aquatic toxicity data for both the registration substance and surrogate substance indicates that they are not acutely toxic to aquatic organisms at the limit of solubility although physical effects occur when the solubility limit is exceeded and when cross-linking of the silanols can occur.

• Considerations on the non-silanol hydrolysis products:

Ethanol is well characterised in the public domain literature and is not hazardous at the concentrations relevant to the studies; the short-term EC₅₀ and LC₅₀ values for this substance is in excess of 1000 mg/l (OECD 2004b - SIDS for ethanol, CAS 64-17-5).Therefore, at the loading rates experienced in these tests it is unlikely that the presence of ethanol would significantly affect the results of the tests.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >1000 mg/l in fish, <1 mg/l in invertebrates and >500 mg/l in algae. It has reliable NOEC 32 mg/l in algae. The available short- and long-term aquatic toxicity data indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water.

The substance hydrolyses moderately to slowly in water and is not readily biodegradable. The substance is hydrolytically unstable and, therefore, does not require classification in Chronic Category 4. The hydrolysis products are non-toxic at a loading rate of 100 mg/l.

 

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 67/548/EEC (as amended) (DSD): 

Not classified.