Reaction mass of 1,1,1,5,5,5-hexamethyl-3-phenyl-3-((trimethylsilyl)oxy)trisiloxane and 1,1,1,7,7,7-hexamethyl-3,5-diphenyl-3,5-bis[(trimethylsilyl)oxy]tetrasiloxane and 1,1,1,9,9,9-hexamethyl-3,5,7-triphenyl-3,5,7-tris((trimethylsilyl)oxy)pentasiloxane

Endpoint summary

Additional information

Phenyl silsesquioxanes is considered a multiconstituent substance, and is a reaction mass of the monomer, dimer and oligomers of phenyltris(trimethylsiloxy)silane. The linear oligomers of phenyl silsesquioxanes contain a siloxane chain, where the terminal Si atoms are fully methyl substituted, and the repeating unit contains a Si atom substituted with a phenyl and a trimethylsiloxy group. The major constituents comprise 70-80% of the linear oligomers n=1-5; the minor constituents comprise 10-20% cyclic (n=3-7) and linear (n=6-7) oligomers. The combined purity of both linear and cyclic oligomers is about 85-95%. The remaining impurities are higher polymerised material and alkoxy substituted oligomers.

Phenyl silsesquioxanes is a liquid at ambient temperature and pressure, with a measured melting point of <-180°C and a measured boiling point of 322°C. It has a measured relative density of 0.985 at 22.1°C and a measured kinematic viscosity of 19.41 mm²/s at 25°C. The vapour pressure of phenyl silsesquioxanes as a whole substance was determined to be 0.23 Pa and 0.42 Pa at 20°C and 25°C, respectively.

The substance is not classified for flammability according to Regulation (EC) No 1272/2008, on the basis of a measured flash point of 130°C and measured boiling point of 322°C. It has a measured auto-ignition temperature of 380°C, and is not explosive and not oxidising on the basis of examination of the structure. Also, none of the constituents of the substance are expected to be surface active.

Phenyl silsesquioxanes is a multiconstituent substance; therefore, 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, it is essential to consider the properties of individual constituents, and whole property data have no real scientific meaning or significance.

The constituents of the parent substance have predicted vapour pressures of 9.3E-23 Pa to 0.096 Pa at 25°C, predicted water solubilities of 4.9E-05 to 4.94E-51 mg/l at 20°C and predicted log K_owvalues of 9 at 20°C. A measured water solubility value of 6.6 ± 0.43 ppb at 23 °C was determined for phenyltris(trimethylsiloxy) silane.

The major and minor linear constituents of the substance are hydrolytically unstable with half-lives for siloxane degradation of 630 – >63 000 h at pH 7 and 20-25°C.The minor cyclic constituents of the substance are also expected to be hydrolytically unstable. Hydrolysis half-lives for the ring-opening step are read-across from the structurally analogous methyl-substituted cyclic siloxane, and range from 0.4 - >10,000 h. Subsequent hydrolysis half-lives of the ring-opened structure are predicted to be ≥10,000 h at pH 7 and 20-25°C.The ultimate hydrolysis products of the reaction are phenylsilanetriol and trimethylsilanol.

The key physico-chemical properties are summarised in the Table below:

Table: Summary of key physico chemical properties

	Vapour pressure (Pa)	Water solubility (mg/l)	Log Kow	Hydrolysis half-life ( h)
Substance as a whole	2.3E-01 (measured)	n/a	n/a	n/a
Constituent 1	9.6E-02 (QSAR)	6.6E-03 (measured)	9.0 (QSAR)	630 (QSAR)
Constituent 2	1.1E-04 (QSAR)	4.6E-10 (QSAR)	9.0 (QSAR)	6300 (QSAR)
Constituent 3	4.5E-07 (QSAR)	3.7E-15 (QSAR)	9.0 (QSAR)	63000 (QSAR)
Constituent 4	2.5E-11 (QSAR)	2.82E-20(QSAR)	9.0 (QSAR)	>63000 (QSAR)
Constituent 5	1.8E-13 (QSAR)	2.04E-25(QSAR)	9.0 (QSAR)	>63000 (QSAR)