Physical Aging Process in 20-Year-Old Polylactide Bottles

The time-dependent properties of polymer materials for packaging applications, whether for recycling or durability purposes, are the subject of several experimental and predictive studies. Amorphous polymers in the glassy state are known to undergo physical aging by structural relaxation, and it is proposed that this phenomenon be investigated in manufactured pieces. This study examines an injection blow molded polylactide bottle stored for 20 years under ambient conditions. Fast scanning calorimetry is used to determine the impact of the manufacturing process on the relaxation kinetics. By comparing the physical aging kinetics of samples that retain their thermomechanical history with those that have it erased, it is observed that the manufacturing process decelerates the structural relaxation by about 2 orders of magnitude. Therefore, the average relaxation advancement, calculated from standard calorimetric measurements, is almost negligible after 20 years of aging. Furthermore, the material demonstrates a diverse array of responses that vary depending on the specific spatial scale under scrutiny. This finding underscores that the overall signal from the PLA bottle can be dissected into the cumulative contributions of discrete regions, each possessing its own relaxation rate. It is hypothesized that the amorphous phase comprises microdomains with high mobility embedded within a slow matrix. In the fast regions, structural relaxation proceeds entirely, suggesting that these areas should continue their thermodynamic equilibration by crystallization.