Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.64 mg/L
Assessment factor:
50
Extrapolation method:
assessment factor
PNEC freshwater (intermittent releases):
6.4 mg/L

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.064 mg/L
Assessment factor:
500
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:
4.3 mg/kg sediment dw
Extrapolation method:
equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
0.43 mg/kg sediment dw
Assessment factor:
10
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:
0.48 mg/kg soil dw
Extrapolation method:
equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
PNEC oral
PNEC value:
10 mg/kg food
Assessment factor:
300

Additional information

The hydrolysis half-life of triethoxy(2,4,4-trimethylpentyl)silane is 43 h at pH 7, 20-25°C. The registered substance will therefore hydrolyse at a moderately fast rate in contact with water and atmospheric moisture to form (2,4,4-trimethylpentyl)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”. 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. It is considered that the substance will reside in a waste water treatment plant for a sufficient amount of time that the test substance will have undergone substantial hydrolysis before it is released into the environment. Therefore, the environmental hazard assessment is based on the properties of the parent substance, in accordance with REACH guidance. However, due to the moderate rate of hydrolysis, hazard assessment for the sediment and soil compartments due to water and moisture being present, is based on the properties of the silanol hydrolyssi product.

In addition, organisms in chronic studies are likely to be exposed to predominantly the hydrolysis products, and so this has been taken into account with the choice of read-across substance for the long-term test on invertebrates in order to take into account exposure to some of the parent. The assessment of the substance will be revised once the new data are available.

As described in Section 1.3 of the CSR and 4.8 of IUCLID, the silanol hydrolysis products may be susceptible to condensation reactions.

READ-ACROSS JUSTIFICATION

In order to reduce animal testing read-across is proposed to fulfil REACH Annex VII-X requirements for the registered substance 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.

The registered substance and the substances used as surrogate for read-across are part of a class of low-functionality compounds acting via a non-polar narcosis mechanism of toxicity. The group of organosilicon substances in this group contain alkyl, aryl, alkoxy or hydroxy groups attached to the silicon atom when present in aqueous solution. Secondary features may be present in the alkyl chain (e.g. halogen, nitrile, unsaturated bonds) that do not affect the toxicity of the substances. The silanol hydrolysis products may be susceptible to condensation reactions, see Section 1.3 of the CSR and 4.8 of IUCLID.

 

In moist medium, triethoxy(2,4,4-trimethylpentyl)silane hydrolyses at a moderate rate (half-life 43 h at pH 7, 20-25°C) to (2,4,4-trimethylpentyl)silanetriol and ethanol.(2,4,4-Trimethylpentyl)silanetriol is a silanetriol with a branched alkyl side chain and a log Kow of 0.9. The effects of ethanol are not thought to affect the outcome of the studies and are discussed below. Short-term data are available for the registered substance and a structural analogue of the registered substance. Long-term invertebrate data are available for the registered substance and an analogous substance that shares the same hydrolysis product as the registered substance. 

 

Additional information is given in a supporting report (PFA 2016y) attached in Section 13 of the IUCLID dossier.

In the following paragraphs the read-across approach for triethoxy(2,4,4‑trimethylpentyl)silane is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties. Table 1 presents relevant physico-chemical properties and the available ecotoxicological data.

Table 1: Physicochemical parameters and ecotoxicity data for the registered and surrogate substances

CAS Number

35435-21-3

2943-75-1

18379-25-4

Chemical Name

Triethoxy(2,4,4-trimethylpentyl)silane

Triethoxyoctylsilane

Trichloro(2,4,4-trimethylpentyl)silane

Si hydrolysis product

(2,4,4-trimethylpentyl)silanetriol

Octylsilanetriol

(2,4,4-trimethylpentyl) silanetriol

Molecular weight (parent)

276.5

276.5

247.7

Molecular weight (hydrolysis product)

192.3

192.3

192.3

log Kow (parent)

>6.5

6.4

n/a

log Kow (silanol hydrolysis product)

0.9 (QSAR prediction)

1.1 (QSAR prediction)

0.9 (QSAR prediction)

Water sol (parent)

<0.1 mg/l

<0.13 – 0.79 mg/l (measured and QSAR prediction, respectively)

n/a

Water sol (silanol hydrolysis product))

2.4E+05 mg/l (QSAR prediction) (limited to approximately 200 mg/l by condensation reactions)

59000 mg/l (QSAR prediction)

2.4E+05 mg/l (QSAR prediction) (limited to approximately 200 mg/l by condensation reactions)

Vapour pressure (parent)

0.14 Pa at 20°C and 0.22 Pa at 25°C

0.11 Pa at 25°C

(QSAR prediction)

45 Pa at 25°C

(QSAR prediction)

Vapour pressure (hydrolysis product)

1.2E-04 Pa at 25°C (QSAR prediction)

2.7E-05 at 25°C

(QSAR prediction)

1.2E-04 Pa at 25°C (QSAR prediction)

Hydrolysis t1/2 at pH 7 and 20-25°C

43 hours (QSAR prediction)

30 hours (QSAR prediction)

<1 min (read-across from methyltrichlorosilane)

Hydrolysis t1/2 at pH 4 and 20-25°C

0.9 hours (QSAR prediction)

0.7 hours (QSAR prediction)

<1 min (read-across from methyltrichlorosilane)

Hydrolysis t1/2 at pH 9 and 20-25°C

0.5 hours (QSAR prediction)

0.4 hours (QSAR prediction)

<1 min (read-across from methyltrichlorosilane)

Short-term toxicity to fish (LC50)

>100 mg/l (Greater than the limit of solubility)

>0.055 mg/l

>100 mg/l

Short-term toxicity to aquatic invertebrates (EC50)

12 mg/l (unfiltered test solution), <100 mg/l (filtered test solution)(Greater than the limit of solubility)

>0.049 mg/l

n/a

Algal inhibition (ErC50 and NOEC)

>1.2 mg/l and 0.28 mg/l (Greater than the limit of solubility)

>0.13 and ≥0.13 mg/l

n/a

Long-term toxicity to aquatic invertebrates (NOEC)

32 mg/l (nominal)

0.058 mg/l

(measured)*

 

n/a 

Testing proposed

32 mg/l (measured)

 

* The test in which these results were obtained is subject to some uncertainty. 

Read-across from triethoxyoctylsilane to triethoxy(2,4,4 trimethylpentyl)silane

Triethoxy(2,4,4-trimethylpentyl)silane (CAS 35435-21-3) and triethoxyoctylsilane (CAS 2943-75-1) both hydrolyse moderately rapidly in contact with water (43 h at pH 7, 20-25°C, and 30 h at pH 7, 20-25°C, respectively). The organosilicon hydrolysis products are (2,4,4-trimethylpentyl)silanetriol and octylsilanetriol respectively. Triethoxy(2,4,4-trimethylpentyl)silane and triethoxyoctylsilane are structural analogues; both are trialkoxysilanes with an octyl side chain, which is branched for the registration substance, linear for the read-across substance, and they have no other secondary feature. Both substances have very low water solubility (<0.1 mg/l and <0.13 - 0.79 mg/l at 20°C respectively), high log Kow (6.5 and 6.4 respectively), low vapour pressure (0.22 Pa and 0.11 Pa at 25°C respectively) and the same molecular weight (276.5). Their hydrolysis products, (2,4,4-trimethylpentyl)silanetriol and octylsilanetriol, are silanetriols with hydrocarbon side chains and have very similar physico-chemical properties: high water solubility (24400 and 59000 mg/l, respectively, predicted), low log Kow (0.9 and 1.1, respectively), low vapour pressure (2.7E-05 Pa and 1.2E-04 Pa at 25°C, respectively) and the same molecular weight (192.33). Therefore it is considered appropriate to read-across between the two substances.

Triethoxyoctylsilane is used to read-across to short-term toxicity to fish, invertebrates and algae endpoints. E(L)C50 values of >0.005, >0.049 and >0.13 mg/l, respectively, have been determined. A long-term toxicity study test proposal with invertebrates is also read-across from triethoxyoctylsilane to the registered substance. A new test with the registered substance is not proposed because read-across is considered valid for the reasons outlined above, therefore minimising testing.

Read-across from trichloro(2,4,4-trimethylpentyl)silane to triethoxy(2,4,4 trimethylpentyl)silane

Triethoxy(2,4,4 trimethylpentyl)silane (CAS 35435-21-3) and trichloro(2,4,4-trimethylpentyl)silane (CAS 18379-25-4) both hydrolyse in contact with water (43 h at pH 7, 20-25°C, and <1-m at pH 7, 25°C respectively) to form the same organolsilicon hydrolysis product (2,4,4-trimethylpentyl)silanetriol. Hydrolysis rate is slowest at pH 7, and the rate increases with an increase or decrease in pH. Preparation for the 21-d invertebrate test on the registration substance involved stirring of the stock solution for 96 hours, and the pH of the test was 7.4 to 8.2. Therefore it is likely that test organisms were predominantly exposed to the hydrolysis products. As the hydrolysis rate for trichloro(2,4,4-trimethylpentyl)silane is so rapid, and test organisms will be exposed to its hydrolysis products, it is therefore considered appropriate to read-across between the two substances.

Trichloro(2,4,4-trimethylpentyl)silane is used to read-across to long-term toxicity invertebrates endpoint. A 21-d NOEC value of 32 mg/l has been determined.

Considerations on the non-silanol hydrolysis product ethanol:

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.

Considerations on the non-silanol hydrolysis product hydrochloric acid:

Chloride ions occur naturally (typically at levels 40 – 160 mg/l in environmental fresh waters). Standard test media contain chloride salts at levels equivalent to approximately 20 – 64 mg Cl-/l.

Effects on aquatic organisms arising from exposure to hydrochloric acid are thought to result from a reduction in the pH of the ambient environment (arising from an increase in the H+ concentration) to a level below their tolerable range. Aquatic ecosystems are characterized by their ambient conditions, including the pH, and resident organisms are adapted to these conditions. The pH of aquatic habitats can range from 6 in poorly-buffered ‘soft’ waters to 9 in well-buffered ‘hard’ waters. The tolerance of aquatic ecosystems to natural variations in pH is well understood and has been quantified and reported extensively in ecological publications and handbooks (e. g. OECD SIDS for CAS No. 7647-01-0, hydrochloric acid). It is not considered appropriate or useful to derive a single aquatic PNEC for hydrochloric acid because any effects will not be a consequence of true chemical toxicity and will be a function of, and dependent on, the buffering capacity of the environment. Physical hazards related to pH effects are considered in the risk management measures (e.g. neutralisation) for effluents/aqueous waste.

It is not appropriate for this substance to discuss the combined ecotoxicological potency of the silicon and non-silicon hydrolysis products because effects arising from exposure to HCl are related to changes in pH and not true chemical toxicity.

Reference:

ECHA 2016: European Chemicals Agency. Guidance on information requirements and chemical safety assessment Chapter R.16: Environmental Exposure Estimation. Version: 2.1, October 2012 (Section R.16.5.4.1. Hydrolysis).

Conclusion on classification

The substance has reliable short-term E(L)C50 values of >100 mg/l in fish, 12 mg/l in invertebrates and >1.2 mg/l in algae. It has reliable NOEC value of 0.28 mg/l for algae and reliable long-term NOEC of 32 mg/l for Daphnia.

The available short- and long-term aquatic toxicity data for the test substance indicate that there are no effects on aquatic organisms at the limit of solubility of the substance in water, as all the E(L)C50 and NOEC results exceed the water solubility of the test substance.

The substance hydrolyses moderately rapidly in water and is not readily biodegradable.

 

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.