Antioxidants — Primary, Secondary and Why You Need Both
Direct answer
Polymer degradation during processing is a radical chain reaction, and it takes two different molecules to stop it. A primary antioxidant — usually a hindered phenolic — donates hydrogen to terminate radicals and protects the polymer in service over long heat ageing. A secondary antioxidant — a phosphite or a thioester — decomposes the hydroperoxides formed in the melt before they split into new radicals, and does most of the work during extrusion and moulding. Neither substitutes for the other. A typical melt-processing package uses both, commonly in the range of 500 to 2000 ppm total, with the ratio set by how many heat histories the material will see.
Symptoms that point to an antioxidant deficit
- Melt flow index drifting between passes. Chain scission in PP raises MFI; crosslinking in PE lowers it. Either drift means unprotected processing.
- Yellowing or pink discolouration. Often phenolic transformation products or gas fading; sometimes the antioxidant choice itself, which is why a low-discolouration package exists for white and light-coloured parts.
- Brittleness after service heat exposure. Long-term thermal ageing failure — a primary antioxidant and, for higher temperatures, an additional heat stabilizer.
- Gels or black specks in film. Localised crosslinked or degraded polymer, frequently in recycled or long-residence streams.
Recycled content changes the calculation
Recyclate arrives with its original stabilizer package largely consumed and with a measurable heat history. Restabilising is not optional if the compound is going into a durable part: the practical approach is to raise the secondary antioxidant for the additional processing pass and add primary antioxidant against service life. Where the stream is mixed or of uncertain origin, an oxidation induction time measurement on incoming material is worth more than any datasheet, because it tells you how much protection is actually left.
Selection constraints
Volatility must suit the process temperature — a low molecular weight antioxidant will simply evaporate at 280 °C. Food contact, potable water and medical applications each restrict the permitted list, so confirm the regulatory frame before sampling. Interaction matters too: phenolics can react with certain pigments and with residual acid from flame retardants or catalyst residues, and hindered amine light stabilizers used alongside them may need an acid scavenger to remain effective.
What to specify on your enquiry
- Polymer and grade.
- Process, melt temperature and number of heat histories.
- Recycled content share.
- Service condition — continuous use temperature and expected life.
- Colour requirement, especially white or transparent.
- Regulatory frame — food contact, potable water, medical.
- Annual volume.
Related knowledge
- Compatibilizers for the same recycled streams.
- Lubricants and processing aids where the symptom is friction, not oxidation.
- Purging compound selection when degraded polymer is already in the machine.
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