Reaction mass of 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

Ecotoxicological Summary

Additional information

The registration substance, Reaction Mass of 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane, is a multi-constituent substance consisting mainly of two constituents: 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Vi5-D5, CAS 17704-22-2, Constituent 1) and 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane (Vi4-D4, CAS 2554-06-5, Constituent 2). The hydrolysis half-life of the whole substance is 63 - 1600 hours, therefore the environmental assessment will focus on the properties of the parent substances.

The water solubility of Constituent 1,Vi5-D5, is low (9.1E-06 mg/l (predicted)) and the log K_ow is high (9 (predicted)).

The water solubility of Constituent 2,Vi4-D4, is low (0.0073-0.0088 mg/l at 23°C) and the log K_ow is high (6.47).

READ-ACROSS JUSTIFICATION

In order to reduce testing, read-across is proposed to comply with REACH requirements for the assesment of risk and hazard 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.

The registration substance, and the substances used as surrogate for read-across, are members of the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates in the sediment compartment. For substances with a log K_ow of 6 and above no 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.

Additional information including application of the Read-Across Assessment Framework (RAAF) is given in a supporting report (PFA 2017at) attached in Section 13 of the IUCLID dossier.

For Constituent 1, Vi5-D5, data are read across from the structurally similar substance, decamethylcyclopentasiloxane (D5, 541-02-6).

For Constituent 2, Vi4-D4, some measured data are available with 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4-D4, CAS 2554-06-5) for the monoconstituent substance, and data are read across from octamethylcyclotetrasiloxane (D4, CAS 556-67-2) for other endpoints.

Read-across from decamethylcyclopentasiloxane (D5, CAS 541-02-6) to Constituent 1 Vi5 -D5

The registration substance, Reaction Mass of 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane, has two main constituents, constituent 1 Vi5-D5 and constituent 2 Vi4-D4. Constituent 1, Vi5-D5, and the first read-across substance, D5, are cyclic siloxanes made up of five silicon atoms linked by five oxygen atoms. In D5, each silicon atom has two methyl substituents, whereas in Vi5-D5 each silicon atom is substituted with one methyl and one vinyl group. Vi5-D5 and D5 have slow hydrolysis rates (1600 h at pH 7 and 20-25°C, predicted and 1590 h at pH 7 and 25°C respectively) and similar physico-chemical properties: high molecular weight (MW 431 and 370.8 respectively), low water solubility (9.1E-06 mg/l and 0.017 mg/l respectively) high log K_ow (9.0 (predicted) and 8.0 respectively) and high log K_oc (6 (predicted) and 5.2 respectively). D5 and Vi5-D5 are structural analogues with very similar properties. 

Data are read-across from D5 for the long-term toxicity to fish, short- and long-term toxicity to aquatic invertebrates, and toxicity to algae and cyanobacteria endpoints.

 Read-across from octamethylcyclotetrasiloxane (D4, CAS 556-67-2) to Constituent 2, Vi4-D4

Constituent 2, Vi4-D4, and the read-across substance, D4, are cyclic siloxanes made up of four silicon atoms linked by four oxygen atoms.In D4, each silicon atom has two methyl substituents, whereas in Vi4-D4 each silicon atom is substituted with one methyl and one vinyl group. Vi4-D4 and D4 have slow hydrolysis rates (63 h at pH 7 and 20-25°C (predicted) and 69-144 h at pH 7 and 25°C, respectively) and similar physico-chemical properties: high molecular weight (MW 344.7 and 296.6 respectively), low water solubility (0.056 mg/l and 0.0073 – 0.0088 mg/l respectively) high log K_ow (both 6.5) and high log K_oc (both close to 4).The environmental behaviour and toxicity is driven by the water solubility, high log K_ow and high log K_oc, therefore read-across of aquatic toxicity data is considered to be appropriate.

D4 is used to read-across to long-term toxicity to fish endpoint for constituent 2 of the registration substance. A NOEC value of >= 0.0044 mg/l has been determined.

Table: Summary of ecotoxicological and physicochemical properties for the registration substance and the surrogate substances.

CAS Number	17704-22-2	2554-06-5	541-02-6	556-67-2
Chemical Name	Vi5-D5: 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane	Vi4-D4: 2,4,6,8-tetramethyl-2,4,6,8-tetravinyl cyclotetrasiloxane	Decamethylcyclopentasiloxane (D5)	Octamethylcyclotetrasiloxane (D4)
Si hydrolysis product	Methylvinylsilanediol	Methylvinylsilanediol	Dimethylsilanediol	Dimethylsilanediol
Molecular weight (parent)	431	344.66	370.78	296.62
Molecular weight (hydrolysis product)	104.18	104.18	92.2	92.2
log Kow (parent)	9.0	6.47	8.0	6.49
log Kow(silanol hydrolysis product)	-0.1	-0.1	-0.4	-0.4
Log Koc (parent)	6.0	4.1	5.17	4.2
Water sol (parent)	9.1E-06 mg/l	0.0073 – 0.0088 mg/l	0.017 mg/l	0.056 mg/l
Water sol (silanol hydrolysis product))	5.0E+05 mg/l at 20°C. Limited to approximately 1000 mg/l due to condensation reactions	5.0E+05 mg/l at 20°C. Limited to approximately 1000 mg/l due to condensation reactions	1.0E+06 mg/l  Limited to approximately 1000 mg/l due to condensation reactions	1.0E+06 mg/l  Limited to approximately 1000 mg/l due to condensation reactions
Vapour pressure (parent)	0.6 Pa	93.5 Pa	33.2 Pa	132 Pa
Vapour pressure (hydrolysis product)	2.1 Pa at 20°C	2.1 Pa at 20°C	7 Pa	7 Pa
Hydrolysis t1/2 at pH 7 and 25°C	1600 hours	63 hours	1590 h	69-144 hours
Short-term toxicity to fish (LC50)	No data	LL­50 >1000 mg/l loading rate (WAFs), equivalent to >7.3 mg/l*	n/a	96-hour >22 µg/l; 14-day 10 µg/l
Short-term toxicity to aquatic invertebrates (EC50)	No data	EL50 272 mg/l loading rate (WAFs)*	n/a	>15 µg/l
Algal inhibition (ErC50 and NOEC)	No data	EL50 (70.5 h): >988 mg/l; NOELR: ≥988 mg/l*	ErC50: >2.0 μg/l; NOEC: ≥2.0 μg/l	>22 µg/l
Long-term toxicity to fish (NOEC)	No data	No data	≥14 μg/l	≥4.4 µg/l
Long-term toxicity to aquatic invertebrates (NOEC)	No data	≥0.00378 mg/l	≥15 µg/l	7.9 µg/l (survival); >15 µg/l (reproduction)
Sediment toxicity (NOEC) (normalised to 5% OC)	No data	No data	110 mg/kg dwt, C. riparius; ≥1719 mg/kg dwt, L. variegatus; 135 mg/kg dwt, H. azteca	13 mg/kg dry weight, Lumbriculus variegatus; 44 mg/kg dry weight, Chironomus riparius
Short-term terrestrial toxicity (L(EC)50)	No data	No data	(IC50) 209 mg/kg dwt, H. vulgare; >4054 mg/kg dwt, T. pratense	No data
Long-term terrestrial toxicity (NOEC)	No data	No data	≥4074 mg/kg dwt, E. Andrei; 377 mg/kg dwt, F. candida	No data

* The studies have been conducted at loading rates above the solubility limit of the substance. The studies indicate that the substance is not toxic at the limit of solubility

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values for each main constituent (measured and read-across).

Constituent 1 (Vi5-D5) has EC₅₀ values of >2.9 µg/l and >12 µg/l in Daphnia and algae, respectively. It has a reliable NOEC value of ≥12 µg/l in algae. These are read-across from D5.

Constituent 2 (Vi4-D4) has measured EC₅₀ values of 272 mg/l and >988 mg/l in Daphnia and algae, respectively. It has a reliable NOEC value of ≥988 mg/l in algae.

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. The substance hydrolyses slowly in water and is not readily biodegradable.

No long-term aquatic toxicity data are available for the registration substance. Although long-term testing is not a requirement at this annex level, long-term tests should be considered because the substance is poorly water soluble. If only short-term data are provided, a “safety net” classification Category Chronic 4 could apply because the substance is poorly soluble and shows no acute toxicity up to the limit of water solubility, is not rapidly degradable and has a log K_ow >4. Long-term aquatic toxicity studies have therefore been included.

A long-term toxicity to invertebrates study is available with Vi4-D4, which relevant for Constituent 2, Vi4-D4, of the registration substance.

Long-term toxicity to fish and invertebrate data have been read across from D5 for Vi5-D5.

Long-term toxicity to fish data have been read across from D4 for Vi4-D4.

A toxicity to fish testing proposal with Vi4D4 has also been included in this submission.

No effects were observed in the long-term invertebrate test with Vi4-D4.

No effects were observed in the long-term fish and invertebrate tests with D5.

No effects were observed in the long-term fish test with D4.

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.