Insights into the biodegradation of PHA / wood composites: Micro- and macroscopic changes

This study presents a comprehensive analysis of the microscopic and macroscopic changes during the biodegradation of composites of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) and wood flour (WF) in soil, to provide insights into the biodegradability of this emerging material. Composite samples with different wood contents (0, 20 and 50 wt%) were buried in soil at a field trial site in a subtropical region of Australia, alongside polylactic acid (PLA) and polyethylene (PE) based wood composites as reference. The degradation rate of the PHBV/WF composites increased with wood content, with weight loss after 12 months being five times greater for PHBV/50 wt% WF than for neat PHBV plaques under identical conditions. The mechanical properties of neat PHBV were retained after 12 months of soil burial. However, PHBV/50 wt% WF lost its mechanical integrity after 12 months, despite only a 13% loss in weight. PLA and PE composites by contrast showed only slight decreases in mechanical properties, which could be associated with moisture induced degradation at the surface only. It is proposed, with evidence from optical microscopy, that for the PHBV-based samples, localised stress loosened the interface, allowing channels for the bacteria and fungi to access PHBV in the bulk of the matrix for local enzymatic biodegradation. With this network of interconnecting pores and cracks, crack propagation and mechanical failure would readily result from an applied stress. Overall, although the erosion rate for the exposed PHBV surfaces in the different samples could be similar, the accessible surface area in PHBV/50 wt% WF composites is significantly larger following local biodegradation and subsequent void formation than for neat PHBV and thus delivers a higher biodegradation rate.