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EC number: 228-204-7 | CAS number: 6166-86-5
2,4,6,8,10-Pentamethylcyclopentasiloxane is a liquid at standard temperature and pressure, with a melting point of <-20°C, and a boiling point of 168 to 169°C. It has a density of 1.0 g/cm3at 20°C and moderate viscosity (19.6 mm²/s static at 20°C). The measured vapour pressure is 91 Pa at 20°C.
The substance is classified as flammable on the basis of a flash point of 49°C. It has a self-ignition temperature of 235°C, is not explosive and is not oxidising.
In contact with water it reacts very rapidly (half-life 4.2 min at 22.5°C). The end product of the hydrolysis relevant to the chemical safety assessment is methylsilanetriol, with several intermediate products being formed transiently. The intermediate products of hydrolysis are linear siloxanols and methylsilanediol; hydrogen gas is produced as a by-product of the reaction.
Methylsilanetriol is predicted to be very soluble in water; however, at concentrations above approximately 1000-2000 mg/l condensation reactions can occur, forming dimers and higher oligomers, and limiting the concentration of methylsilanetriol dissolved in water. It has low log Kow(-2.4, predicted), is not surface active and does not undergo significant dissociation within the environmentally-relevant range. It has a lower vapour pressure than the parent substance (vapour pressure = 0.05 Pa, predicted).
Predicted values for the key physicochemical properties of the intermediate hydrolysis products are shown in Table 1.
Table1: Key physicochemical properties of some expected hydrolysis products (HP) of HD5
Predicted vapour pressure / Pa at 25°C
Predicted water solubility / mg/l
Hydrolysis at pH 7 and 20-25°C / h
Intermediate HP1, H5L5-diol
Intermediate HP2, H4L4-diol
Intermediate HP3, H3L3-diol
Intermediate HP4, H2L2-dio
1At concentrations above approximately 1000-2000 mg/l condensation reactions can occur, forming dimers and higher oligomers, and limiting the concentration of methylsilanetriol or methylsilanediol dissolved in water.
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