Yellowing in polypropylene is a common quality problem, especially in white and natural grades. It usually comes from oxidation and from the stabiliser system itself reacting with its environment.
Common causes
- Thermo-oxidation during processing or overheating.
- Gas fading (NOx discolouration) — hindered phenolic antioxidants can form coloured by-products when exposed to combustion gases (e.g. gas heaters, transport).
- Acidic catalyst residues reacting with additives.
- Overuse or the wrong phenolic/phosphite ratio.
How to prevent it
- Use a balanced antioxidant package — a phosphite such as Irgafos 168 markedly reduces colour formation alongside a phenolic like Irganox 1010.
- Add an acid scavenger (e.g. calcium stearate) to neutralise catalyst residues.
- Consider low-discolouration / gas-fade-resistant stabilisers where NOx exposure is a risk.
- Control processing temperature and avoid long residence times.
- Add HALS if the part also sees light exposure.
HexaChem can review your stabiliser package and recommend a low-yellowing solution, with samples available in Vietnam.