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Physical & Chemical properties

Auto flammability

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Description of key information

The auto flammability study is considered to be scientifically unnecessary.

Key value for chemical safety assessment

Additional information

In accordance with section 1 of REACH Annex XI, the auto-ignition temperature study does not need to be conducted for Tripotassium propylsilanetriolate because:

(1) Only a very small proportion of the substance (<<0.1%) is present in a form that could volatilise from the solution;

(2) The substance is only stable as an aqueous solution at high pH;

(3) The ethanol impurity (present at <0.5%) will not self-ignite below 369°C.

The substance is only stable as an aqueous solution at high pH. Experience in handling and use indicates that the evaporation of the water and/or reducing of the pH causes immediate polymerisation of the substances, leaving an amorphous solid.

An aqueous solution will not be capable of autoignition. However, the possibility that components could volatilize from the solution and generate vapours capable of autoignition must be considered.

Tripotassium propylsilanetriolate is completely dissociated into potassium and propylsilanetriolate ions in aqueous solution. Excess potassium hydroxide is also present in the solution. The ionic nature of these substances mean that they are non-volatile.

A very small amount of non-ionised propylsilanetriol will be present in the solution (<<0.1%; see below). As the substance is manufactured and used only as an aqueous solution, the volatility of the substance is appropriately described by the Henry’s Law Constant. This applies until the water boils at around 100°C. Therefore, a realistic assessment of the maximum vapour concentration of propylsilanetriol can be estimated based on its Henry’s Law Constant (HLC =6.3E-05Pa m3mol-1, 2.0E-08 unitless at 100°C) as follows:

HLC (dimensionless) = Cv/Caq

Where Cv = concentration in the vapour phase and Caq = concentration in the aqueous phase.

For a 35% solution of tripotassium propylsilanetriolate in water, the concentration of tripotassium propylsilanetriolate is 350 g/L. Correcting for the molecular weights of propylsilanetriolate (119.2) and tripotassium propylsilanetriolate (239.5), the concentration of propyl silanetriolate is:

Caq     =        119.2 / 239.5 * 350 g/L

=        170 g/L

=        1.7E+08 mg/m3

The first pKa of propylsilanetriol is approximately 10. The pH of the substance as sold is >14.

The concentration of propylsilanetriol can be calculated using the ionisation factor
≈ 10(pKa-pH)= 10^(10-14) = 0.0001. Hence, at pH 14:

Caq     =        1.7E+04 mg/m3propylsilanetriol

=         17 mg/l propylsilanetriol

Therefore:

Cv      =        1.7E+04 mg/m3x 2.0E-08

=         3.4E-04 mg/m3propylsilanetriol

This is a very low concentration, indicating that the presence of propylsilanetriol in the vapour phase is unlikely to result in self-ignition.

The substance is only stable in aqueous solution. As the water content of the substance is evaporated at around 100°C, the propylsilanetriolate/propysilanetriol will immediately condense forming an amorphous polymeric solid.

The substance may also contain ethanol as an impurity at <0.5%. Ethanol may volatilise from solution and may also be present in the vapour phase after evaporation of the water content. Ethanol has a measured self-ignition temperature of 369°C (C-C Chen et al, 2010). Therefore, the ethanol present in the Registration substance will not lead to self-ignition below 369°C.

REFERENCE:

C-C Chen et al (2010), “Autoignition Temperature Data for Methanol, Ethanol, Propanol, 2-Butanol, 1-Butanol, and 2-Methyl-2,4-pentanediol,”J. Chem. Eng. Data, 2010, 55(11), pp 5059–5064.