A comparative study of degradation mechanisms of PHBV and PBSA under laboratory-scale composting conditions

Biodegradable plastics appear as one promising means to help solving the increasing issue of environmental pollution by plastics. The present study aims at comparing the biodegradation mechanisms of two promising biodegradable plastics, PHBV Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and PBSA Poly(butylene succinate-co-adipate) with the objective to provide a better understanding of the mechanisms involved and identify the most relevant indicators to follow biodegradation. For this purpose, the progress of the biodegradation process was monitored under controlled composting conditions at the laboratory scale at 58 °C using several methodological approaches for evaluating polymer degradation. Indicators of the extent of material disappearance based on respirometry and mass loss measurements were combined to other indicators evidencing the morphological, structural and chemical modifications induced at the surface or in the bulk of the material as surface erosion by SEM, decrease of molecular weight by GPC, crystallinity changes by DSC and chemical changes by ATR-FTIR. As expected, both polymers were rapidly biodegraded in less than 80 days. However, in spite of its higher molecular weight and degree of crystallinity, PHBV degraded faster than PBSA, which led to suggest that different biodegradation mechanisms would be involved. At this regard, a two-phase scenario was proposed for each polymer on the strength of all the degradation-induced changes observed at the polymer surface and in its bulk. Based on these two scenarios, the discrepancy in biodegradation rate between PHBV and PBSA would be essentially attributed to significant differences in crystals morphology and spatial organization of both polymers.Regarding the relevance of the different indicators studied, mass loss stood out as the most relevant and accurate indicator to assess the disappearance of material especially when combined with respirometry and mineralization kinetics assessment. Besides, indicators focusing on the surface changes as SEM, AFM and POM were emphasized since seen as powerful tools to evidence morphological changes at different scales. At last, changes in thermal properties as crystallinity rate and melting temperature, even if complex to interpret due to the wide range of interdependent mechanisms they bring into play appeared as inescapable tools for improving the understanding of the underlying mechanisms involved in polymer biodegradation.