1,1,3,3-tetramethyl-1,3-divinyldisiloxane

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

no hazard identified

Marine water

Hazard assessment conclusion:

no hazard identified

STP

Hazard assessment conclusion:

no hazard identified

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

8.9 mg/kg sediment dw

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

0.89 mg/kg sediment dw

Assessment factor:

100

Extrapolation method:

assessment factor

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

no hazard identified

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

14.44 mg/kg food

Assessment factor:

90

Additional information

The hydrolysis half-life of 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (CAS 2627-95-4, Vi2-L2) is 3.7 h at pH 5, 5.8 days at pH 7 and 2.6 h at pH 9 and 25°C. The water solubility of the substance is low (0.21 mg/l) and the log K_ow is high (5.4).

REACH Guidance (ECHA 2017, R.7b) indicates that the parent substance should be tested if the DT₅₀ is greater than 3 days. In addition, 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 REACH guidance (EC 2003) and R.16 (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. Due to the slow hydrolysis half-life, the environmental Chemical Safety Assessment is therefore based on the properties of the parent substance.

Due to the low water solubility and high log K_ow, aquatic organisms are likely to be exposed to low dissolved concentrations of the substance in the environment.

In the sediment and soil compartments adsorption to particulate and organic matter will be high.

It is therefore likely that, under the flow-through exposure conditions of the fish study that the test organisms will have predominantly been exposed to the registration substance. In the semi-static long-term invertebrate study and the static algal test it is likely that exposure will have been predominantly to the registration substance and a small proportion of its hydrolysis products. The test substance is volatile but measures were taken to maintain exposure concentrations by conducting the key fish test under flow-through conditions, the chronic invertebrate test under semi-static conditions and the algal test under sealed conditions.

READ-ACROSS JUSTIFICATION

In order to reduce testing read-across is proposed to fulfil up to REACH Annex IX requirements for the registered substance from substances that have similar structure and physicochemical properties. Risk characterisation for all substances needs sediment to be assessed and therefore a reliable PNEC is needed regardless of Annex requirements.

The registration substance Vi2-L2 (CAS 2627-95-4) and the substances used as surrogates are members of the Reconsile Siloxane Category. Substances in this category tend to have slow hydrolysis rates, low water solubility, high log K_ow, high adsorption coefficients and slow degradation in the sediment compartment.

Additional information is given in a supporting report (PFA 2017) attached in Section 13 of the IUCLID 6 dossier. The Siloxane Category report (PFA, 2017) addresses the Assessment Elements of the RAAF and, in addition to describing the siloxanes category, forms the RAAF justification report.

In the following paragraphs the category read-across approach for the registered substance is outlined, taking into account structure and physicochemical properties.

Read-across from hexamethyldisiloxane (L2) and octamethyltrisiloxane (L3) to 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (CAS 2627-95-4, Vi2-L2):

The registration substance, 1,1,3,3-tetramethyl-1,3-divinyldisiloxane (CAS 2627-95-4, Vi2 -L2), and the source substances hexamethyldisiloxane (L2, CAS 107-46-0) and octamethyltrisiloxane (L3, CAS 107-51-7) are members of the Siloxane Category. L2 and L3 are linear siloxanes with two silicon atoms and one oxygen atom, and three silicon and two oxygen atoms, respectively. Each of the silicon atoms are fully substituted with methyl groups. Vi2-L2 is a linear siloxane with two silicon atoms and one oxygen atom. One vinyl and two methyl groups are attached to each silicon atom with a total of four methyl and two vinyl groups. Refer to Section 1 for more structural information on the registration substance including diagrams.

Terrestrial toxicity studies with siloxanes such as Vi2-L2, L2 and L3 are technically difficult to conduct due to their high volatilisation potential (high Henry’s Law Constant and low octanol-air partition coefficient) and the potential for degradation in soil. Soil testing according to guideline methods does not allow for a renewal of the substrate and hence re-application of test substance. Therefore, there is potential for the organisms to not be exposed to the test material for a sufficiently long period of time for effects to be expressed, as well as the difficulty of quantifying actual exposure concentrations. OECD TG 222 acknowledges that the test method may not be applicable to substance for which the air-soil partition coefficient is greater than one, or to substance with vapour pressure exceeding 300 Pa at 25°C. Terrestrial toxicity testing with L2 and L3 has shown this to be the case (see discussion in IUCLID Section 6.3 and CSR Section 7.2). L2 and L3 meet both of these criteria (for L2, K_air-soil = 3.3 at 20°C (temperature for terrestrial testing), VP 5500 Pa at 25°C; for L3, Kair-soil = 1.3 at 20°C, VP 530 Pa at 25°C). The air-soil partition coefficient for Vi2-L2 is 3.3 at 20°C and the vapour pressure is 1700 Pa at 25°C.

The physical-chemical properties of L3 and L2 are generally consistent and similar to those of Vi2-L2, meaning this read across to Vi2-L2 is a robust interpolation. Homogeneous hydrolysis rates are comparable. The substances have similar molecular weight, relatively low water solubility and high log K_ow. See table 7.0.1 for relevant values. All three substances have negligible biodegradability.

 

Table 7.0.1 Relevant Physicochemical Properties of the Registration and Proposed Test Substances

Substance	Vi2-L2	L2	L3
Vapour Pressure (Pa)	1700	5500	530
Log Koc	3.2	3.0	4.3
Hydrolysis (hours) (pH7, 20-25˚C)	140	116	329
Molecular Weight	186.40	162.38	236.54
Water Solubility (mg/l)	0.207	0.93	0.034
Log Kow	5.4	5.06	6.60

 

L2 and L3 data are used for read-across to the sediment compartment. In addition, soil microorganisms tests with L2 and L3, and a terrestrial stability test with L3 under OECD 222 conditions, have been read across.

Conclusion on classification

The substance has reliable short-term E(L)C₅₀ values of >0.13 mg/l in fish, >0.1 mg/l in invertebrates and >0.12 mg/l in algae. It has reliable NOEC of ≥0.12 mg/l in algae and reliable long-term NOEC of ≥0.12 mg/l in Daphnia magna.

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.