Meaning
Chemical breakdown of a polymer caused by the combined effects of heat and oxygen leads to a loss of mechanical properties and a change in appearance. Undergoing thermal oxidative degradation involves the formation of free radicals that attack the carbon backbone of the plastic, causing chain scission or cross linking. This process is most active during high temperature processing or when the part is exposed to hot environments for long periods.
The degradation stops only when the oxygen is removed or the temperature drops below the activation energy.
Heat Exposure
Maintaining high temperatures in an extruder for too long can trigger the breakdown of the resin. Since thermal oxidative degradation is a time dependent process, the residence time of the material in the machine must be kept as short as possible. Yellowing of the plastic is often the first sign that the chains are breaking down.
This color shift indicates a change in the chemical structure of the polymer.
Additive Protection
Inclusion of antioxidants and stabilizers is necessary to prevent the material from failing during its lifecycle. Because thermal oxidative degradation happens at the molecular level, these chemicals work by scavenging free radicals before they can damage the polymer. Multiple types of stabilizers are often used together to provide protection during processing and long term use.
These additives are essential for parts that will be used outdoors.
Longterm Stability
Performance of a plastic part over several years depends on its ability to resist the effects of the environment. If thermal oxidative degradation is not controlled, the part will eventually become brittle and crack. Testing in an oven can simulate years of use in a few weeks.
Accurate predictions of a part life depend on understanding these degradation rates. Measurement of the oxygen induction time provides a way to quantify this stability.
