Thermal Oxidation

Chemical Mechanism

The reaction starts with the removal of a hydrogen atom from the polymer backbone, creating a highly reactive site. This site quickly reacts with oxygen to form a peroxy radical, which then attacks other parts of the polymer chain. This chain reaction continues until the free radicals are neutralized by antioxidants or until the material is cooled.

Thermal oxidation is accelerated by high melt temperatures and long residence times in the processing equipment. The presence of metal ions, such as those from pigments or residues from the polymerization catalyst, can also catalyze the reaction. Understanding this mechanism allows engineers to design better additive packages and process settings.

The goal is to minimize the exposure of the hot melt to oxygen.

Production Defect

The physical signs of thermal oxidation often appear as a change in the colour of the resin, moving from clear to yellow or brown. This discolouration is caused by the formation of conjugated double bonds in the polymer structure. In severe cases, the material may become brittle, leading to parts that crack or fail under low stress.

For recyclers, this is a major concern because every heat cycle increases the cumulative damage to the polymer. The presence of oxidized material can also lead to the formation of gels or unmelts that block filters and gates. This reduces the efficiency of the production line and increases the scrap rate.

Moulders must monitor the melt temperature and screw speed to keep the oxidation levels as low as possible.

Stabilization Method

To prevent the negative effects of thermal oxidation, manufacturers add chemical stabilizers known as antioxidants to the resin. Primary antioxidants, such as hindered phenols, work by neutralizing the free radicals before they can attack the polymer chain. Secondary antioxidants, such as phosphites, react with the peroxides to prevent them from breaking down into more radicals.

Using a combination of these additives provides the best protection during both processing and the service life of the part. Moulders can also reduce oxidation by ensuring that the injection unit is properly purged and by minimizing the amount of air that enters the barrel. For high performance applications, nitrogen blanketing may be used to exclude oxygen from the melting process.

These steps are necessary for producing high quality parts with consistent properties.