1,1,1,3,5,5,5-heptamethyltrisiloxane

Endpoint summary

Additional information

1,1,1,3,5,5,5-Heptamethyltrisiloxane (H-L3) is a liquid at ambient temperature and pressure, with a measured melting point of <-20.15°C and a measured boiling point of 140 - 143°C. It has a predicted density of 0.89 g/cm³at 20°C, a predicted kinematic viscosity of 1.1 mm²/s at 20°C and a measured vapour pressure of approximately 850 Pa at 25°C.

The substance is classified as flammable (Flammable Liquid Category 3), according to Regulation EC (No) 1272/2008, on the basis of a measured flash point of 25°C at 1020.1 hPa and measured boiling point of 140 - 143°C. It has a measured auto-ignition temperature of 270°C, and is not explosive and not oxidising on the basis of structural examination.

In contact with water, the substance reacts moderately (half-life of 2.2 days at pH 7 and 25°C). H-L3 can undergo two separate reactions in water; hydrolysis of the siloxane (Si-O-Si) group and reaction of the Si-H bond. The latter generates hydrogen gas as co-product of hydrolysis. The evolution of gas was observed to occur over the time scale of the parent substance degradation but could not account for all the observed disappearance of parent substance. Therefore, it was concluded that degradation of parent substance was due to a combination of Si-O and Si-H bond hydrolysis. The likely final hydrolysis products are methylsilanetriol and trimethylsilanol. Hydrogen is produced as a co-product of hydrolysis.

The mechanism of the Si-H reaction is unknown; it may not be a true hydrolysis reaction. However, it is a chemical reaction with water and is referred to as hydrolysis in the remainder of this document.

The properties of H-L3 are therefore assessed based on the measured half-life of 2.2 days. The submission substance has a predicted log K_ow of 6.2 at 20°C and a measured water solubility of 0.02 mg/l at 22°C. The substance is not surface active based on structural examination.