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Posted by David Sherman [24.32.202.83] on Tuesday, March 02, 2010 at 13:31:40 :
In Reply to: Watch this tire safety video! posted by Eric B. [75.42.234.151] on Tuesday, March 02, 2010 at 12:13:56 :
Pretty interesting. I'm not sure about the explanation, though. He says the heat initiates "pyrolysis" which builds up pressure in a runaway reaction. However, pyrolysis means something breaking down chemically due to heat. Tires are built hot and the rubber holds up just fine in the tire molds. I know he's the tire guy and I'm not, but I'm pretty sure what's happening is combustion, not pyrolysis.
Most chemical reactions, including burning, run faster at higher pressures and higher temperatures. The tire essentially starts burning inside. It's reacting with the oxygen in the air in the tire, rather than just the rubber decomposing by itself. It appears that if the rubber gets hot enough to begin to smolder even a little bit inside, the combustion increases the temperature and pressure enough to increase the reaction rate and the reaction runs away until something gives. You notice when the tire blows up, a lot of smoke comes out.
This is basically the same sort of thing that happens with gunpowder. Put a pile of it on the ground in open air and light it, and it just burns quickly, but if it's confined in something, the initial little bit of burning increases the pressure which increases the reaction rate which further increases the pressure, and it explodes.
In a tire at 90 psi, there's 6 times as much oxygen per cubic foot as there is in regular air. In pure oxygen at atmospheric pressure, there's only 5 times as much oxygen as there is in regular air. You know how quickly things burn in pure oxygen. It might be interesting to get a junk tire smoldering and then apply pure oxygen to it from a torch tank. Imagine that happening inside a tire. The only difference is that in the tire, there's nowhere for the heat and pressure to go.
My theory that it's combustion rather than pyroloysis that's going on would be easy to test by repeating the experiment using an inert gas such as nitrogen, rather than air, to inflate the tire.
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