Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
no hazard identified

Marine water

Hazard assessment conclusion:
no hazard identified

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
1 mg/L
Assessment factor:
100
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
no hazard identified

Sediment (marine water)

Hazard assessment conclusion:
no hazard identified

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
3.8 mg/kg soil dw
Assessment factor:
100
Extrapolation method:
assessment factor

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential to cause toxic effects if accumulated (in higher organisms) via the food chain

Additional information

The hydrolysis half-life of tetradecamethylhexasiloxane (L6, CAS 107-52-8) is 6300 h at pH 7 and 25 °C. The water solubility of the substance is low (0.0000023 mg/l) and the log Kow is high (>9.41 at 25 °C, read across from L5). It is therefore likely that, under the flow-through exposure conditions of the aquatic toxicity tests reported in the following sections, that the test organisms will have been predominately exposed to very low concentrations of the registration substance.

READ-ACROSS JUSTIFICATION

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

In aqueous media, tetradecamethylhexasiloxane hydrolyses very slowly in water (half-life 6300 hours at pH 7 and 25 °C).

Tetradecamethylhexasiloxane 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 in the sediment compartment. For substances with a log Kow of 8 and above no long-term toxicity effects are seen with aquatic organisms due to the substance low water solubility limiting the effects seen. In the environment the substances will adsorb to particulate matter and will partition to soil and sediment compartments.

In the following paragraphs the read-across approach for tetradecamethylhexasiloxane is assessed for the surrogate substance taking into account structure, hydrolysis rate and physico-chemical properties.

Additional information is given in a supporting report (PFA, 2015i) attached in Section 13.

Read-across from L4 to L6:

The registered substance, tetradecamethylhexasiloxane (L6, CAS 107-52-8) and the surrogate substance decamethyltetrasiloxane (L4, CAS 141-62-8) are linear siloxanes with 6 and 4 silicon atoms respectively, with methyl branches and alternated by oxygen. L6 and L4 possess similar physico-chemical properties: high molecular weight (459 g/mol and 310.69 g/mol respectively), low water solubility (both insoluble, at 2.3E-06 mg/l and 6.7E-03 mg/l respectively), higher log Kow (>9.41 at 25°C (by read across from L5) and 8.21 at 25.1°C respectively) and high log Koc (6.0 and 5.16 respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates at environmentally relevant temperature and pH.

No effects have been reported at the limit of solubility in short-term and long-term studies in fish, Daphnia and algae conducted with L4. Given the similar properties and structural similarities, it is considered valid to read-across data from decamethyltetrasiloxane (L4) to tetradecamethylhexasiloxane (L6).

 

Read-across from L5 to L6:

The registration substance, tetradecamethylhexasiloxane (L6, CAS 107-52-8) and the surrogate substance dodecamethylpentasiloxane (L5, CAS 141-63-9) are linear siloxanes with 6 and 5 silicon atoms respectively, with methyl branches and alternated by oxygen. L6 and L5 possess similar physico-chemical properties: high molecular weight (459 g/mol and 384.85 g/mol respectively), low water solubility (both insoluble, at 2.3E-06 mg/l and 7.0E-05 mg/l respectively), high log Kow (both >9.0 at 20°C; the log Kow of L5 is 9.41 at 25°C, (measured), and this result is read across to L6) and high log Koc (6.0 and 5.16 respectively). Both substances have negligible biodegradability and similar slow hydrolysis rates at environmentally relevant temperature and pH.

No effects have been reported at the limit of solubility in short-term and long-term studies in fish, Daphnia and algae conducted with L5. Given the similar properties and structural similarities, it is considered valid to read-across data from dodecamethylpentasiloxane (L5) to tetradecamethylhexasiloxane (L6).

 

Table 3 Summary of aquatic ecotoxicological and physico-chemical properties for the registered substance, its hydrolysis products and the surrogate substances.

CAS Number

107-52-8

141-63-9

141-62-8

Chemical Name

Tetradecamethylpentasiloxane (L6)

Dodecamethylpentasiloxane (L5)

Decamethyltetrasiloxane (L4)

Si hydrolysis product

Dimethylsilanediol (CAS 1066-42-8) and trimethylsilanol (CAS 1066-40-6) Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to dimethylsilanediol. For properties of trimethylsilanol, see column for L4)

Dimethylsilanediol and trimethylsilanol Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to dimethylsilanediol. For properties of trimethylsilanol, see next column)

Dimethylsilanediol and Trimethylsilanol Note: the properties reported below in rows for ‘silanol hydrolysis product’ relate to trimethylsilanol. For properties of dimethylsilanediol, see previous column)

Molecular weight (parent) g/mol

459

384.85

310.69

Molecular weight (hydrolysis product) g/mol

92.2

92.2

90.2

log Kow (parent)

>9.41 (read across from L5)

9.41

8.21

log Kow (silanol hydrolysis product)

-0.4

-0.4

1.2

Log Koc (parent)

6.0

5.16

5.16

Water sol (parent)

2.3E-06 at 20°C

7.0E-05 mg/l at 23°C

6.7E-03 mg/l at 23°C

Water sol (silanol hydrolysis product)

1.0E+06 mg/l

1.0E+06 mg/l

995 mg/l

Vapour pressure (parent)

 0.27 Pa at 20°C

7.8 Pa at 25°C (QSAR prediction)

73 Pa at 25°C

Vapour pressure (hydrolysis product) at 25°C

7 Pa

7 Pa

1900 Pa

Hydrolysis t1/2 at pH 7

6300 hours at 20 -25°C, predicted

630 hours at 20-25°C, predicted

2000 hours at 20-25°C, predicted

Short-term toxicity to fish (LC50)

n/a

>75 ng/l

>6.3 μg/l

Short-term toxicity to aquatic invertebrates (EC50)

n/a

n/a

n/a

Algal inhibition (ErC50 and NOEC)

n/a

n/a

EC50: >2.2 μg/l; NOEC: ≥2.2 μg/l

Long-term toxicity to fish (NOEC)

n/a

≥39 ng/l

≥5.3 μg/l 

Long-term toxicity to aquatic invertebrates (NOEC)

n/a

≥47 ng/l

≥4.9 μg/l

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

n/a

 n/a

92 mg/kg dwt Hyalella azteca

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

n/a

n/a

n/a

Long-term terrestrial toxicity (NOEC)

n/a

n/a

n/a

Read-across from decamethylcyclopentasiloxane (D5) to tetradecamethylhexasiloxane (L6): 

Decamethylcyclopentasiloxane (CAS 541-02-6, D5) is a cyclic siloxane with five Si linked by oxygen and with methyl groups bound to silicon. Tetradecamethylhexasiloxane (L6) is a siloxane with six silicons linked by oxygen and with methyl groups bound to silicon. Increase of the chain length results in decreased water solubility. The surrogate substance and the registration substance are close structural analogues, and the property that will dominate the behaviour of the substance in the environment is the high adsorption potential (log Kowand Koc). Both the registration substance and D5 have similar physico-chemical properties: high molecular weight (459 and 370.8 respectively), high log Kow(9.0 and 8.0 respectively) and high log Koc(6 for both structures).

Because D5 and the registration substance have similar properties and the environmental behaviour and toxicity is driven by the high log Kowand high log Koc,the read-across of terrestrial toxicity data is considered to be appropriate to derive an interim risk characterisation.

Soil data with D5 indicate that the substance is of low toxicity to terrestrial organisms. However it is recognised that the read-across from D5 to the registration substance needs to be revisited in future when additional testing data are available and in light of further developments for this read-across analogue group. A testing strategy is considered within the siloxanes group.

Decamethylcyclopentasiloxane (D5) is used to read-across to toxicity to soil macroinvertebrates except arthropods, terrestrial arthropods and terrestrial plants endpoints. Further testing is planned for terrestrial microorganisms.

Conclusion on classification

The substance has reliable short-term E(L)C50 values of >0.000075 mg/l in fish and >0.0022 mg/l in algae based on read-across from structural analogues. It has a reliable NOEC of ≥0.0022 mg/l in algae and reliable long-term NOECs of ≥0.000047 mg/l in Daphnia and ≥0.0079 mg/l in fish.

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.