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Description of key information

The substance is a multiconstituent substance with two constituents, 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2) and 4,4,14,14-tetraethoxy-3,15-dioxa-8,9,10-trithia-4,14-disilaheptadecane (S3).

Since the registered substance is a multiconstituent substance; two aspects need to be considered in relation to physicochemical properties:

In the context of raw material handling and safe use it is important to understand the properties of the product/substance as a whole as discussed below.

1. The substance is a liquid at ambient temperature and pressure, with a measured melting point of <-20°C. The substance was observed to undergo thermal decomposition from about 267°C. It has a measured relative density of 1.0424 at 20°C and measured viscosity values of 7.820 mm2/s at 20°C and 4.383 mm2/s at 40°C. A measured vapour pressure of 9 Pa at 20°C is used for assessment purposes, which was determined for one individual constituent of the substance (S2).

 

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008, on the basis of a measured flash point of 128°C. It has a measured auto-ignition temperature of 235°C at 1014-1022.5 hPa, and is not explosive and not oxidising on the basis of structural examination. Also, neither of the constituents of the substance is expected to be surface active.

 

2. For the purposes of assessing environmental fate and behaviour, in the environment, individual constituents may behave independently of each other. It is therefore essential to consider the properties of individual constituents, and whole property data may have no real scientific meaning or significance. However, it has been concluded for the submission substance, that the chemical safety assessment should be conducted as if the substance is a mono-constituent substance because of the similar physicochemical properties for the constituents. The details considered in reaching that conclusion are presented below:

 

Each constituent of the parent substance has a measured vapour pressure of 9 Pa at 25°C (measured for the S2 constituent, read across to S3), a predicted water solubility of <1 mg/l at 20°C and a predicted log Kow of 5.2 at 20°C. Table 1.3.2 gives the predicted result for each constituent of the substance.

 

Table 1.3.2: Key physicochemical properties of constituents of the substance

Constituent No

Constituent

SMILES

Molecular Weight (g/mol)

Vapour Pressure* (Pa at 25°C)

Log Kow

Water solubility (mg/l)

Constituent 1 (S2)

4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane

CCO[Si](OCC)(OCC)CCCSSCCC[Si](OCC)(OCC)OCC

474.82

0.46

5.2

0.014

Constituent 2 (S3)

4,4,14,14-tetraethoxy-3,15-dioxa-8,9,10-trithia-4,14-disilaheptadecane

CCO[Si](OCC)(OCC)CCCSSSCCC[Si](OCC)(OCC)OCC

506.88

0.44

5.2

0.012

*The value reported in Table 1.3.2 is the predicted vapour pressure for individual constituents of the submission substance, however, a measured vapour pressure of 9 Pa at 20°C is used for assessment purposes as a worst-case approach.

 

In contact with water, the constituents react moderately to slowly, with half-lives of 0.8 – 1.2 h at pH 4, 0.5 – 1 h at pH 5, 40 - 90 h at pH 7 and 0.5 – 1 h at pH 9 for S2, and of 0.8 – 1.4 h at pH 4, 0.5 – 1.1 h at pH 5, 40 - 110 h at pH 7 and 0.5 – 1.1 h at pH 9 for S3 and 20-25°C according to the following equations:

 

S2: (CH3CH2O)3SiCH2CH2CH2SSCH2CH2CH2Si(OCH2CH3)3+ 3H2O → (OH)3SiCH2CH2CH2SSCH2CH2CH2Si(OH)+ 6CH3CH2OH

 

S3: (CH3CH2O)3SiCH2CH2CH2SSSCH2CH2CH2Si(OCH2CH3)3+ 3H2O → (OH)3SiCH2CH2CH2SSSCH2CH2CH2Si(OH)+ 6CH3CH2OH

The silanol hydrolysis products of S2 and S3, (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol and (3-{[3-(trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol respectively, each have a predicted vapour pressure of 2.9E-05 Pa at 25°C. The saturation concentration in water of the silanol hydrolysis products is limited by condensation reactions to approximately 1000 mg/l. However, it is very hydrophilic (calculated water solubility is 1.0E+06 mg/l using a QSAR method) and has a low log Kow (-3.0, predicted).

None of the hydrolysis products are expected to be surface active and they are not expected to undergo significant dissociation within the environmentally-relevant range. The first dissociation constant of a structurally analogous silanetriol (phenylsilanetriol) has been reported to be around pKa of 10.

The physicochemical properties of the silanol hydrolysis products are reported in Table 1.3.3

 Table 1.3.3: Key physicochemical properties of silanol hydrolysis product

Hydrolysis product

SMILES

Molecular Weight (g/mol)

Vapour Pressure (Pa at 25°C)

Log Kow

Water solubility (mg/l)

(3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol

O[Si](O)(O)CCCSSCCC[Si](O)(O)O

306.50

2.9E-05

-3.0

1E+06

(3-{[3-(trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol

O[Si](O)(O)CCCSSSCCC[Si](O)(O)O

338.56

2.9E-05

-3.0

1E+06

 

Therefore, since physicochemical properties of the constituents and the physicochemical properties of the silanol hydrolysis products are similar, environmental and human health exposure assessments were conducted as if the substance was a mono-constituent substance.

 

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