Thermoxidation

Thermoxidation is the oxidative degradation of materials caused by heat in the presence of oxygen. It is a form of autooxidation that occurs in the absence of light, distinguishing it from photooxidation. The process is driven by radical chain reactions that can initiate at elevated temperatures, often aided by trace metals or other catalysts. It proceeds through initiation, propagation with peroxy radicals, and termination, producing hydroperoxides that can decompose into aldehydes, ketones, acids, and volatile fragments.

In polymers, thermoxidation causes chain scission, crosslinking, embrittlement, discoloration, and loss of mechanical properties. Polyolefins such as polyethylene and polypropylene are particularly susceptible. In fuels and lubricants, autooxidation leads to gum formation, viscosity changes, and deposits. In fats and oils, thermoxidation is a major pathway for rancidity and flavors. The rate and outcome depend on temperature, oxygen availability, catalysts, moisture, and the material’s intrinsic susceptibility.

Prevention and control rely on stabilization strategies, including antioxidants and radical scavengers (such as hindered phenols and phosphites), metal deactivators, and processing under inert atmospheres or at lower temperatures. Material design and processing conditions can also mitigate thermoxidation. Monitoring methods include oxidative induction time measurements, differential scanning calorimetry, thermogravimetric analysis, and spectroscopic assessment of oxidation products (for example, carbonyl formation).