Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions produce free radicals, these radicals start chain reactions. Antioxidants terminate these chain reactions by removing free radicals, and delay other oxidation reactions. They do this by being oxidized themselves.
Phenols
- Function as primary antioxidants – Radical scavengers or chain terminators which trap Alkoxy (RO*) and Peroxy radicals (ROO*).
- Reaction results in Hydroperoxides formation (ROOH) – can lead to further degradation unless secondary AO is present.
- Effective during both – processing and long term heat aging.
Phosphites
- Act as secondary antioxidants – Hydroperoxides decomposer.
- Often used in combination with a primary antioxidant.
- Provide stabilization only at melt processing temperatures.
AO effects
Gels and contaminants in extrusion can cause disruption in the extrusion process and quality problems.
Main reasons for gels:
- Cross-linked Polymer Gels
- Unmelted resin
- Undispersed additives
- Moisture
- Recycled material – degraded or cross-linked polymer residues
OIT
The Oxidation Induction Time (OIT) test, as carried out in a Differential Scanning Calorimetry (DSC), is used to predict thermo-oxidative performance of a material.
Samples are heated up under a nitrogen atmosphere, typically to 200°C. Oxygen is then introduced to the sample cell, and the length of time before the onset of degradation, as seen by the initiation of an endothermic process in the DSC trace, is measured.
OIT is a sensitive measure of the level of anti-oxidative additives within the polymer.
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PE and PP are degraded by heat and shear during processing. Most imminent are changes in melt viscosity (MFR) and die deposits (die build-up). Typical quality failures, especially during machine cleaning and shut-down/start-up cycles, are discolorations, black specs and gels.
Depending on processing parameters the rate of degradation is worse at higher melt temperatures, longer residence times and higher shear rates along with higher output rates. The degradation is further accelerated by air (oxygen, powder) and humidity (hygroscopy, recycle).
However, the plastic part continues to disintegrate during end usage by elevated temperatures (>50°C) or radiation (UV-light, sterilization): discolorations and loss in physical properties are the most common failures.
