12 March 2009
Tire Pyrolysis
Overheating Warning
Translated from its original French by Marc-Alain Nantel
Every year in Canada, numerous people are seriously injured or die due to a tire explosion caused by a pyrolysis phenomenon, which is the chemical breakdown of the tire brought on by excessive heat. Tire pyrolysis begins when the temperature of the rubber that goes into the production of the tire attains ± 250°C.
It then breaks down in a variety of chemical substances, such as methane, hydrogen, carbon black, and styrene. When they make contact with the oxygen inside the tire, these flammable vapours are set ablaze by auto-ignition when the temperature gets to ± 430ºC, bringing on an explosion so powerful that debris can be blown 300 metres away.
Origin of the explosion
A report based on expert examination following a fatal work injury enumerates the possible heat sources of this phenomenon:
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Overheating brakes
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Poor tire condition or over/under tire inflation
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Using a blow torch on a wheel or near the tire
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Welding on or near the wheel
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Automobile fire
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Contact with high-tension power line or lightning
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Presence of contaminants inside the tire
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Absorption of flammable liquids by the tire
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Installing and uninstalling tires
 Explanation of the phenomenon
When the vehicle is in motion, the tires cool down thanks to the air circulating at high speed. If an abnormal condition occurs, causing the tires to overheat, the cooling process is much less efficient when the vehicle is at a standstill, increasing the risks of pyrolysis. The heat causes the air inside the tires to dilate and significantly increase their pressure, bringing about the degradation and softening of the rubber. If the tires are not cooled, the pyrolysis phenomenon begins and an explosion can occur at any moment within 24 hours.
If you suspect unusual tire overheating,
it’s better to remain at a safe distance
until the tire has cooled down.
Extreme caution
Whether you’re a truck driver, automotive technician, or other, if you suspect that a tire is unusually overheated, it’s better to keep a safe distance for a reasonable lapse of time until the tire has cooled. If possible, measure the tire’s temperature with an infrared temperature sensor; it must be below 250°C. As soon as the situation is favourable (temperature around 70°C), you must deflate the tire to a minimum and carry out a careful inspection.
Good practices
The afore-mentioned enumeration of potentially dangerous heat sources resulting in pyrolysis must be a guide to be memorized by you and your staff, and your work processes must reflect this awareness in order to avoid potential dangers.
Preventive training
According to numerous reports of serious accidents involving tire pyrolysis, the dangers linked to a tire blowout are overlooked in the workplace. Truckers, suppliers, and technicians must be made aware of this phenomenon, which has caused the loss of many lives in Canada.
This picture illustrates the brake actuator and the
threaded bar to be screwed to the end in order to
compress the diaphragm spring inside, thus releasing
the pressure on air brakes. Said rod is usually stored
onthe side of the actuator. It’s the only good way of
neutralizing the pressure on brake shoes and
preventing brakes from overheating.
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