Polypropylene (PP) is highly susceptible to thermo-oxidative degradation. During compounding, moulding and service life, heat and oxygen generate free radicals that cause chain scission — seen as rising melt flow rate (MFR), loss of impact strength, embrittlement and discolouration. An antioxidant package is essential for almost every PP grade.
Primary vs secondary antioxidants
Primary antioxidants (hindered phenolics) are radical scavengers. They interrupt the oxidation chain and provide long-term thermal stability during service life. The workhorses are Irganox 1010 and Irganox 1076.
Secondary antioxidants (phosphites) decompose hydroperoxides before they form new radicals, protecting the polymer during high-temperature processing and preserving colour. The industry standard is Irgafos 168.
The standard PP package
Most PP grades use a synergistic blend of a hindered phenolic + a phosphite — for example Irganox 1010 (or 1076) together with Irgafos 168. The phenolic protects long-term; the phosphite protects the melt and reduces yellowing. Typical total loading is roughly 0.05–0.3%, adjusted to the grade and process.
How to choose
- Processing temperature & multiple heat histories (e.g. recycled PP) → more phosphite.
- Long service life / high-temperature end use → higher-MW phenolic (Irganox 1010).
- Thin films / low volatility needs → Irganox 1076 or high-MW options.
- Food contact → check the additive's regulatory approvals.
- Also exposed to UV/outdoors? → add a light stabiliser package (see our HALS vs UV absorber guide).