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EC number: 217-496-1
CAS number: 1873-88-7
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
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 Kow 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
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