Registration Dossier

Administrative data

Hazard for aquatic organisms

Hazard for air

Hazard for terrestrial organisms

Hazard for predators

Additional information

The hydrolysis half-life of tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9) is >200 h at pH 7 and 20-25°C (QSAR prediction). The water solubility of the substance is low (0.0066 mg/l at 23°C) and the log Kow is high (9.0, QSAR prediction). The ultimate products of hydrolysis are phenylsilanetriol and trimethylsilanol.

A long-term toxicity to fish study is available with tris(trimethylsiloxy)phenylsilane. There are no short-term data available with the registration substance therefore data have been read across from the analogous substance decamethyltetrasiloxane (L4, CAS 141-62-8) to fulfil the data gaps.

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex VII requirements for the registration 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 since after hydrolysis occurs the resulting product has different physicochemical properties and structure.

In aqueous media, tris(trimethylsiloxy)phenylsilane hydrolyses slowly in water (half-life >200 h at pH 7 and 20-25°C).

Tris(trimethylsiloxy)phenylsilane and the substance used as surrogate for read-across are part of a class of compounds called siloxanes. Substances in this group tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates in the sediment compartment. For substances with a log Kow of 6 and above no effects are seen with aquatic organisms due to the low water solubility limiting the effects. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

Additional information is given in a supporting report (PFA, 2017) attached in Section 13.

In the following paragraphs the read-across approach for tris(trimethylsiloxy)phenylsilane is assessed for the surrogate substances 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 registration and surrogate substance

CAS number

2116-84-9

141-62-8

Chemical Name

Tris(trimethylsiloxy)phenylsilane

Decamethyltetrasiloxane (L4)

Si hydrolysis product

Phenylsilanetriol and Trimethylsilanol: Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to phenylsilanetriol. For properties of trimethylsilanol, see data reported for L4 hydrolysis product in next column

Trimethylsilanol and dimethylsilanediol: Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol

Molecular weight (parent)

372.76

310.69

Molecular weight (hydrolysis product)

156.21

90.2

log Kow (parent)

9.0

8.21

log Kow (silanol hydrolysis product)

0.021

1.2

Log Koc (parent)

No data

5.16

Water sol (parent)

0.0066 mg/l

0.00674 mg/l

Water sol (silanol hydrolysis product)

No data

995 mg/l

Vapour pressure (parent)

0.09 Pa 25°C (QSAR prediction)

73 Pa

Vapour pressure (hydrolysis product)

6E-5 Pa  

25°C (QSAR prediction)

1900 Pa

Hydrolysis t1/2 at pH 7 and 20-25°C

>200 h

728 h

Hydrolysis t1/2 at pH 4 and 20-25°C

>1.9 h

14 h (pH 5)

Hydrolysis t1/2 at pH 9 and 20-25°C

>2.0 h

21.2 h

Short-term toxicity to fish (LC50)

No data

>6.3 µg/l

Short-term toxicity to aquatic invertebrates (EC50)

No data

n/a

Algal inhibition (ErC50 and NOEC)

No data

ErC50: >2.2 µg/l; NOEC: =2.2 µg/l

Long-term toxicity to fish (NOEC)

=4.4 mg/l (bioconcentration test)

=7.9 µg/l

Long-term toxicity to aquatic invertebrates (NOEC)

No data

=4.9 µg/l

Sediment toxicity (NOEC) (normalised to 5% OC)

No data

92 mg/kg dwt H. azteca

Short-term terrestrial toxicity (L(E)C50)

No data

n/a

Long-term terrestrial toxicity (NOEC)

No data

n/a

·        Read-across from decamethyltetrasiloxane (L4, CAS 141-62-8) to tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9).

Tris(trimethylsiloxy)phenylsilane (CAS 2116-84-9) and L4 are both tetrasiloxanes, with four silicon atoms linked by three oxygen atoms. Tris(trimethylsiloxy)phenylsilane is a branched siloxane with nine methyl substituents and one phenyl substituent; whereas L4 is a linear siloxane with ten methyl substituents.

Tris(trimethylsiloxy)phenylsilane and L4 possess similar physicochemical properties: high molecular weight (372.76 and 310.69, respectively), low water solubility (0.0066 mg/l and 0.00674 mg/l, respectively), and high log Kow (9.0 and 8.21, respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates.

No effects at the limit of solubility have been reported in short-term and long-term studies conducted with both substances.

Given the similar properties and structural similarities, it is considered valid to read across toxicity data from L4 to tris(trimethylsiloxy)phenylsilane.

L4 is used to read across to long-term toxicity to aquatic invertebrates and toxicity to aquatic algae endpoints.

Conclusion on classification

The substance has a reliable short-term E(L)C50 value of >2.2 µg/l in algae read across from the structurally related substance decamethyltetrasiloxane (L4, CAS 141-62-8). It has a reliable NOEC of =2.2 µg/l in algae and a reliable long-term NOEC of =4.9 µg/l in Daphnia, also read across from the structurally related substance decamethyltetrasiloxane (L4, CAS 141-62-8).

A long-term toxicity to fish NOEC, derived from a bioconcentration factor test, of =4.4 µg/l is available for the registration substance.

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 slowly 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.