Reaction Mass of 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane

Endpoint summary

Description of key information

The substance is a multi-constituent substance with two main constituents, 3,3-diphenylhexamethyltrisiloxane and 3,3,5,5-tetraphenylhexamethyltetrasiloxane.

Since the substance is a multi-constituent substance; two aspects need to be considered in relation to physicochemical properties:

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

The substance is a liquid at room temperature and pressure, with a freezing temperature of <-68°C and a measured boiling range of 268°C to 341°C at 1013 hPa. It has a measured relative density of 0.9929 at 20°C. The vapour pressure of the whole substance was measured to be 0.068 Pa at 25°C, 0.26 Pa at 35°C and 0.64 Pa at 45°C. It has a calculated water solubility value of 2.2E-05 mg/L at 20°C.

The substance is not classified for flammability according to Regulation (EC) No. 1272/2008, on the basis of a measured flash point of 141°C and a measured boiling range of 268°C to 341°C. It has a measured auto-ignition temperature of 395°C at 968 mBar (968 hPa) and is not explosive and not oxidising on the basis of chemical structure.

 

2) For the purposes of assessing environmental fate and behaviour, 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 (CSA) should be conducted as if the substance is a mono-constituent substance because of the similar physicochemical properties of the constituents. The details considered in reaching the conclusion is in Table 1.3.2.

The predicted key physicochemical properties of the individual constituents are shown in Table 1.3.2.

Table 1.3.2: Predicted key physicochemical properties of constituents of the substance

Constituent Number	Constituent name	SMILES	Molecular weight (g/mol)	Vapour pressure (Pa ) at 25°C	log Kow at 20°C	Water solubility (mg/L) at 20°C
1	3,3-Diphenylhexamethyltrisiloxane	C[Si](O[Si](c1ccccc1)(c1ccccc1)O[Si](C)(C)C)(C)C	360.68	3.6E-03	9.0	2.8E-05
2	3,3,5,5-Tetraphenylhexamethyltetrasiloxane	C[Si](O[Si](c1ccccc1)(c1ccccc1)O[Si](c1ccccc1)(c1ccccc1)O[Si](C)(C)C)(C)C	558.98	1.3E-07	9.0	1.5E-10

 

In contact with water, the constituents react slowly with predicted hydrolysis half-lives below:

Constituent 1: >1.9 hours at pH 4, >200 hours at pH 7 and >2.0 hours at pH 9 and 20-25°C

Constituent 2: >3.6 hours at pH 4, >630 hours at pH 7 and >5.3 hours at pH 9 and 20-25°C.

The products of the hydrolysis reaction for both constituents are trimethylsilanol and phenylsilanediol.

Therefore, since physicochemical properties of the constituents are similar, (low vapour pressure, very high log K_ow, very low solubility in water (see Table 1.3.2) and slow hydrolysis in contact with water), environmental and human health exposure assessments were conducted as if the substance is a mono-constituent substance.