What are the differences in thermal aging between polypropylene and polyethylene?

Thermal aging is a critical factor determining the long-term performance and application suitability of polypropylene (PP) and polyethylene (PE). While both are polyolefins, they exhibit significant differences in their thermal oxidative degradation behavior due to their distinct molecular structures.

Polypropylene contains tertiary carbon atoms in its backbone, making it more susceptible to oxidation compared to polyethylene. The hydrogen atoms attached to these tertiary carbons are more easily abstracted, initiating faster degradation through free radical mechanisms. This structural vulnerability causes PP to generally exhibit lower thermal stability than PE, particularly at elevated temperatures exceeding 100°C.

Polyethylene's simpler structure, with only primary and secondary carbon atoms, provides greater inherent stability. High-density polyethylene (HDPE) typically demonstrates better resistance to thermal aging than low-density varieties (LDPE), though both generally outperform PP in long-term thermal endurance. The degradation kinetics differ substantially, with PP following more complex oxidation pathways that can lead to faster molecular weight reduction and embrittlement.

Stabilization approaches also vary between these polymers. PP requires more robust antioxidant packages, often including hindered phenols and phosphites, to achieve comparable thermal stability to PE. The effectiveness of these stabilizers differs due to the polymers' morphological differences and oxidation mechanisms.

Understanding these thermal aging characteristics is essential for selecting the appropriate material for applications involving prolonged heat exposure, such as automotive components, piping systems, and consumer products requiring extended service life at elevated temperatures.