Durability of virgin and recycled polypropylene: color and mechanical changes under accelerated thermal and chemical ageing

Polymers are continually exposed to diverse environmental conditions throughout their service life, which can lead to physical and chemical changes in their structure and a consequent alteration in their aesthetic and functional properties. The ability to preserve these characteristics within acceptable limits is essential for long-lasting and safety-critical applications. Accurately predicting how polymers will perform under specific environmental conditions is fundamental to the commercial success of plastic products, particularly those made from recycled materials. In this study, two types of accelerated aging tests were conducted—thermal and chemical—to evaluate changes in mechanical properties and color. Thermal aging involved subjecting three polypropylene formulations (two virgin and one recycled, each filled with 20% talc) to elevated temperatures of 150 °C, 155 °C, and 160 °C. The goal was to identify the time required to halve mechanical performance compared to initial values and to monitor color shifts. Chemical aging was performed by immersing specimens in 1% solutions of three commercial detergents at 95 °C, significantly harsher than typical washing or dishwasher conditions, to assess chemical durability and visual changes. Upon completion of thermal aging, tensile tests revealed that elongation at break dropped by 50%, while the elastic modulus increased—indicating heightened crystallinity or annealing effects. In contrast, chemical aging did not significantly diminish mechanical properties, confirming polypropylene’s chemical resistance, but resulted in pronounced color changes and blister formation in certain materials. SEM and IR mapping revealed soap residue inside these blisters. Remarkably, blistering also occurred in specimens soaked in water alone, suggesting that moisture exposure—rather than detergents—was the main culprit. Some samples that had yellowed initially partially reverted to their original color over several months, particularly following exposure to a specific rinse aid, possibly due to additive extraction.