Polysulfides, bis[3-(triethoxysilyl)propyl]

Endpoint summary

Description of key information

The submission substance is a multi-constituent substance with three constituents and a named impurity:

• 4,4,13,13-tetraethoxy-3,14-dioxa-8,9-dithia-4,13-disilahexadecane (S2, CAS 56706-10-6)


• 4,4,14,14-tetraethoxy-3,15-dioxa-8,9,10-trithia-4,14-disilaheptadecane (S3, CAS 56706-11-7) and


• 4,4,15,15-tetraethoxy-3,16-dioxa-8,9,10,11-tetrathia-4,15-disilaoctadecane (S4, CAS 40372-72-3).


• 4,4,16,16-Tetraethoxy-3,17-dioxa-8,9,10,11,12-pentathia-4,16-disilanonadecane (S5, named impurity)

 

The substance is a liquid at room temperature and pressure, with a melting point of < -20°C at 1013 hPa and an estimated boiling point ≥ 269°C at 1013 hPa. It has a measured relative density of 1.0813 at 20°C and a measured kinematic viscosity value of 10.74 mm²/s at 20°C. A measured vapour pressure of 9 Pa at 20°C which was determined for the S2 constituent of the substance is used for assessment purposes.

 

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008, on the basis of a measured flash point of > 110°C at 1013 hPa and predicted boiling point ≥ 269°C at 1013 hPa. It has a measured auto-ignition temperature of 250°C at 998 -1004 hPa and is not explosive and not oxidising based on examination of the structure. Also, none of the constituents or impurities with similar chemical structure of the substance are expected to be surface active.

 

Since the substance is a multi-constituent 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 as a whole.


• 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 very similar physicochemical properties for the constituents. The details considered in reaching that conclusion are presented below.

 

The individual constituents of the parent substance have estimated vapour pressures shown in Table 1.3.2, and the S2 constituent has a measured vapour pressure of 9 Pa at 25°C. Each constituent has predicted water solubilities of < 1 mg/l at 20°C, a predicted log K_ow of 5.2 at 20°C and predicted vapour pressure ≤ 0.46. Table 1.3.2 gives the predicted result for each constituent of the substance.

 

Table 1.3.2: Predicted physicochemical properties of constituents of the substance

Constituent No. (Name)	Constituent	SMILES	Molecular Weight (g/mol)	Vapour Pressure* (Pa at 25°C)	Log Kow	Water solubility (mg/l)
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
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	≤ 0.46	5.2	0.012
3 (S4)	4,4,15,15-Tetraethoxy-3,16-dioxa-8,9,10,11-tetrathia-4,15-disilaoctadecane	CCO[Si](OCC)(OCC)CCCSSSSCCC[Si](OCC)(OCC)OCC	538.94	≤ 0.46	5.2	0.010
Named impurity (S5)	4,4,16,16-Tetraethoxy-3,17-dioxa-8,9,10,11,12-pentathia-4,16-disilanonadecane	CCO[Si](OCC)(OCC)CCCSSSSSCCC[Si](OCC)(OCC)OCC	571.02	≤ 0.46	5.2	0.0086

*The values reported here is the predicted vapour pressure for individual constituents of the 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 predicted half-lives at 20-25°C of:

 

• S2 constituent: 0.8-1.2 h at pH 4, 40-80 h at pH 7 and 0.5-1.0 h at pH 9


• S3 constituent: 0.8-1.3 h at pH 4, 40-100 h at pH 7 and 0.5-1.1 h at pH 9


• S4 constituent: 0.8 – 1.4 h at pH 4, 40-120 h at pH 7 and 0.5-1.2 h at pH 9


• S5 named impurity: 0.8 – 1.6 h at pH 4, 40-140 h at pH 7 and 0.5-1.5 h at pH 9

 

The equations for the reactions are reported below:

 

S2:

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

 

S3:

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

 

S4:

(CH₃CH₂O)₃SiCH₂CH₂CH₂SSSSCH₂CH₂CH₂Si(OCH₂CH₃)₃ + 6H₂O → (OH)₃SiCH₂CH₂CH₂SSSSCH₂CH₂CH₂Si(OH)₃ + 6CH₃CH₂OH

 

S5:

(CH₃CH₂O)₃SiCH₂CH₂CH₂SSSSSCH₂CH₂CH₂Si(OCH₂CH₃)₃ + 6H₂O → (OH)₃SiCH₂CH₂CH₂SSSSSCH₂CH₂CH₂Si(OH)₃ + 6CH₃CH₂OH

 

 

The silanol hydrolysis products are bis[3-(trihydroxysilyl)propyl]polysulfides.

Bis[3-(trihydroxysilyl)propyl]polysulfides is the hydrolysis product of the multiconstituent substance, consisting of the silanol hydrolysis products:

For S2 constituent, (3-{[3-(trihydroxysilyl)propyl]disulfanyl}propyl)silanetriol

For S3 constituent, (3-{[3-(trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol

For S4 constituent, [3-({[3-(trihydroxysilyl)propyl]disulfanyl}disulfanyl)propyl]silanetriol

For S5 named impurity, (3-{[3-(trihydroxysilyl)propyl]pentasulfanyl}propyl)silanetriol

 

The silanols are predicted to have very low vapour pressures. The saturation concentration in water of the silanol hydrolysis products is limited by condensation reactions to approximately 100 mg/l. However, they are very hydrophilic (calculated solubility is 1.0E+06 mg/l using a QSAR method) and have a predicted low log K_ow (-3.0).

 

None of the hydrolysis products is expected to be surface active and they are not expected to undergo significant dissociation within the environmentally relevant range. The measured first dissociation constant, pK_a, of a structurally analogous silanetriol (phenylsilanetriol) is approximately 10.

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

 

Table 1.3.3: Predicted physicochemical properties of silanol hydrolysis products

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	6.3E-12	-3.0	1E+06
(3-{[3-(Trihydroxysilyl)propyl]trisulfanyl}propyl)silanetriol	O[Si](O)(O)CCCSSSCCC[Si](O)(O)O	338.56	6.3E-12	-3.0	1E+06
[3-({[3-(Trihydroxysilyl)propyl]disulfanyl}disulfanyl)propyl]silanetriol	O[Si](O)(O)CCCSSSSCCC[Si](O)(O)O	370.62	6.3E-12	-3.0	1E+06
(3-{[3-(Trihydroxysilyl)propyl]pentasulfanyl}propyl)silanetriol	O[Si](O)(O)CCCSSSSSCCC[Si](O)(O)O	402.7	6.3E-12	-3.0	1E+06

 

Since the three constituents and the named impurity have very similar physicochemical properties and they’re silanol hydrolysis products have very similar physicochemical properties, environmental and human health exposure assessments were conducted as if the substance were a mono-constituent substance.