Ageing performance of biodegradable PLA for durable applications

The durability of 2 commercially available injection mold grades and one extrusion grade of polylactic acid (PLA) was assessed. Here, the materials were exposed to elevated temperatures/humidity for a period of several weeks. Moisture absorption, molecular weight change, mechanical properties, crystallinity changes, and microscopic imaging were all monitored at regular intervals throughout the course of the combined heat and humidity ageing study. The conditions for the ageing study were set at 60°C and 45% relative humidity and all three PLA grades showed a significant increase in crystallinity and conversely reduced mechanical performance over the 68-day ageing period. Microscopic images of the materials suggested that surface degradation was not dominant. It is known that the moisture absorption of PLA which is revealed by weight gain is usually low for PLA, but the amount absorbed is sufficient to induce bulk degradation, and the mechanical performance was reduced over the ageing period. All materials exhibited a loss in molecular weight over time. The molecular weight at any given time was reflective of the starting molecular weight and thus the extrusion grade PLA showed better mechanical performance than the injection mold grades at any given time. This behavior was satisfactorily modeled using an acid-catalyzed hydrolysis model. Interestingly the higher molecular weight PLA exhibited slower degradation kinetics as compared to the lower molecular weight injection molding grades. This study showed that commercially available PLA resins are not suitable for use in applications that require long sustained durability in environmental conditions such as these where high temperature and humidity are encountered for any appreciable amount of time.