1. Experiment is contained in the WACKER's Experimental Kit.


 2. Experimental procedure has been modified


 3. A separate experimental procedure has been devised


 4. Video clip available

Yes (as wmv or as mov)

 5. Flash animation available


 6. Other materials: Worksheet 4, Worksheet 5

Flammability of Silicone Rubber Compared With That of Other Polymers

TopDown 1 Materials, Chemicals, Time Needed
  • Bunsen burner
  • Crucible tongs
  • Test tubes
  • pH paper
  • Various plastic specimens, e.g. polyvinyl chloride (PVC), polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET) and polystyrene (PS)
  • Various silicones from WACKER's Experimental Kit HTV(s), HTV(b), HTV(w), ELASTOSIL® M 4601, ELASTOSIL® M 4400, ELASTOSIL® E 43, ELASTOSIL® N 199

Allow 5 to 7 minutes for each plastic specimen.

TopDown 2 Procedure and Observations

Where possible, all experiments should be performed in a fume cupboard. A fume cupboard must be used when PVC is being burned!

a) Using tongs, hold a piece of plastic sample for a short time at the edge of a hot burner flame. Then observe if the plastic continues to burn, note the color of the flame, whether soot or fumes form, whether the flame crackles and whether the material melts.

b) Continue by heating another part of the sample in a test tube and carefully checking the odor. Use damp indicator paper to establish if the decomposition products of the reaction are acidic, alkaline or neutral.

c) Finally, check how thermoplastic the plastic is. Do this by using the tongs to carefully heat another sample of the plastic over a small bunsen burner flame and trying to bend the warm sample.

TopDown Repeat this procedure for all the samples.

The following observations were made for the various plastic samples in several reproducible reference experiments:

TopDown 3 Discussion of Results

The various observations made for the flame tests on the plastic samples may be explained in terms of the elemental composition and molecular structure of the different plastics.
As with most organic compounds, burning of organic polymers mainly yields carbon dioxide and water, as may be seen from the following reaction equation for the combustion of polyethylene:

In addition to carbon dioxide and water, some conventional plastics may yield other, toxic and nontoxic combustion products. This will depend on the composition of the plastics.
An example is Polyvinylchlorid , which has recently hit the headlines as an environmentally polluting plastic because it releases hydrogen chloride gas and other toxic chlorine compounds when it burns. (Note: Special incinerators are able to burn PVC in such a way that the environment is not polluted because the chlorine compounds are removed from the combustion gases before they are discharged into the atmosphere. In the experiment, the hydrogen chloride HCl formed during the combustion of polyvinyl chloride PVC is detected with moist pH paper, which turns red and thus indicates an acidic reaction.

The very smoky flame produced when polystyrene is burned is due to the comparatively high content of carbon (one benzene ring per monomer unit).
The following reaction scheme for the complete combustion of a silicone shows that silicon dioxide is formed in this case as well:

Silicone fluid + oxygen Silicon dioxide + carbon dioxide + water
The observations made for the combustion of the silicones correlate very well with the above equation for the combustion of silicone fluid. The resultant white product is pyrogenic silica, which is formed during the combustion. The white smoke is also finely divided silica. Combustion or strong heating of silicones fails to produce any basic or acidic decomposition products.
The experiments show that silicones are much more difficult to ignite than organic plastics, but will burn well. The activation energy is thus very high, but combustion itself releases a great deal of heat. This behavior is explained by the very stable Si-C and Si-O bonds (hence the high activation energy) and the high enthalpies of formation for SiO2 and CO2 (hence the great deal of combustion heat). Compounds can be added to the silicone rubber (TiO2, platinum or aluminum compounds) to yield especially flame retardent materials which can be modified in such a way that the flame extinguishes again after a short time.
The differences in the thermoplastic behavior (see column “Flexibility” in the table of results) may be explained in terms of the structure of the plastics studied. All the organic polymers studied are thermoplastics. These consist of linear or branched macromolecules (see diagram below) that interact to a greater or lesser extent with each other. On being heated, whole molecules or parts thereof can slide past one another because the motion of the particles as a whole increases and because the molecules are not crosslinked with chemical bonds. The plastic becomes softer and pliable as a result. When it freezes, it remains in that shape.
Unlike the organic polymers studied, the silicones examined are elastomers or thermosets (see diagrams below). Since their macromolecules are chemically bonded to each other, these silicones are not thermoplastic.

Top5 Literatur 4 Tips and Comments

  • Since there are no observed differences among the silicones, the study can be restricted to one type of silicone.
  • To study thermal loading, i.e. how the plastics behave when they are heated, an alternative experimental set-up may be chosen (see experiment "Burning of liquid silicones")
  • The experiment is easy to perform and highly reproducible. It makes a good classroom experiment.
    It teaches the pupils about different plastics and how they behave during combustion. From their observations, the pupils come to understand the relationship between the properties of a macromolecular material, its elemental composition and its molecular structure.
  • This experiment can be used as a gateway to discuss the safety and environmental aspects of different plastics.
TopBottom  5 References
  • M. Tausch, M. von Wachtendonk (editors), CHEMIE S II, STOFF-FORMEL-UMWELT, C.C. Buchner, Bamberg (1993), (1998), S. 337 - 352
  • M. Tausch, M. von Wachtendonk (editors), STOFF-CHEMIE S I, FORMEL-UMWELT, C.C. Buchner, Bamberg (1996), (1997), S. 228 - 233
  • M. Tausch, M. von Wachtendonk (editors), CHEMIE 2000+, C.C. Buchner, Bamberg (2001), S. 60 - 67