Burning of liquid silicones

1. Experiment is contained in the WACKER's Experimental Kit.	No
2. Experimental procedure has been modified	/
3. A separate experimental procedure has been devised	Yes
4. Video clip available	Yes (as wmv or as mov)
5. Flash animation available	No
6. Other materials: Worksheet 4

Burning of Liquid Silicones

1 Materials, Chemicals, Time Needed

Teclu burner
Tripod
Piece of sheet metal to place on tripod
Crucible tongs
Dropping pipet
Various silicones from WACKER's Experimental Kit
Products from the experiments "Hydrolysis of chloromethanes" and "Rate of hydrolysis of chloromethylsilanes"

One ignition and burning operation lasts about 5 minutes because the metal plate has to heat up first.

2 Procedure and Observations

Remove flammable materials from the immediate vicinity because a naked flame is used.

The Teclu burner heats the bottom of the metal plate on the tripod very efficiently. When the plate is hot, pour 1 to 3 drops of the silicone onto the middle of it from above.

Then step back. When the flame has extinguished, the experiment can be repeated. Should the silicone fluid emit vapors instead of igniting spontaneously, light the vapors directly with the flame and then continue heating the plate from beneath. The heated silicone fluid emits some vapor before burning with a glaring white flame (left picture). The fire lasts for 5 to 20 seconds. Whitish gray flocs rise up out of the ashes. Some silicone fluids only burn when ignited directly with the burner flame. A fine white ash is left behind on the metal plate (right picture).

3 Discussion of Results

At elevated temperatures in an adequate supply of air, methylsilicones burn to form a silica with a very large surface area, water and carbon dioxide. In the experiments, the flame sometimes looks yellowish and the ash is not quite white. This indicates that combustion did not quite go to completion. The equation for complete combustion is:

The composition of the silica only approximates to the formula SiO₂. We are dealing here with neither discrete SiO₂ molecules nor quartz, but rather with tiny grains in which each silicon atom is tetrahedrally surrounded by four oxygen atoms and each oxygen atom forms a bridge between two silicon atoms. The oxygen atoms at the edge of a grain each have one bond to a hydrogen atom.

4 Tips and Comments

Combustion of silicones is easy to perform and may be done during preliminary studies of different polymers as it provides information about elemental composition.
For a discussion of combustion, see page 7 of “Silicone fluids, resins and rubbers worksheets”.

5 Supplementary Information

The worksheets mentioned above contain supplementary information on, for example, enthalpies of combustion, which can be useful for comparing combustion of silicones with other polymers.