Correlation of Two Biodegradability Indices of PLA-Based Polymers under Thermophilic Aerobic Laboratory Conditions

The biodegradation of bioplastics is a topic of interest worldwide. This work aims to measure the biodegradability of five polylactic acid (PLA)-based bioplastics under aerobic, thermophilic laboratory conditions and correlate their weight loss with their CO2 generation over a 3-month period, as both are considered indexes of biodegradation. The experimental design was based on the simulation of composting conditions by placing the bioplastic samples mixed with compost in sealed glass vessels that were regularly opened. The results showed significant variability in biodegradation, as dry weight losses ranged from 15.1–99.7%, while CO2 generation ranged from 9.2–14.9 g C–CO2/kg dry mixture (bioplastic + inoculum) depending on the sample. Moreover, no significant correlation between the weight losses and the gross CO2 production was calculated (p = 0.656), indicating the importance of carefully selecting the methods to assess biodegradation potential. This lack of correlation also reveals that different pathways are likely involved during the biodegradation of bioplastics and that the weight loss alone cannot indicate the conversion of solid C to CO2. This work proposes the need to develop an optimal degradation index for bioplastics that would provide a better understanding of their biodegradability in composting reactors. This index should combine dry weight loss and CO2 generation to assess the biodegradation of bioplastics with high confidence.