Mechanical Stability of Polyhydroxyalkanoate (PHA)-Based Wood Plastic Composites (WPCs)

A growing common public interest in a more sustainable society drives efforts into the development of bio-based and biodegradable polymers. Such renewable polymers can replace conventional non-degradable polymers as the matrix for wood plastic composites (WPCs). However, despite ongoing advancements towards the implementation of these biocomposites, experience with biocomposite mechanical durability is limited. This comparative study investigated the mechanical stability of biocomposites based on polyhydroxyalkanoate (PHA) and wood flour at 50 wt% wood content in real-life in-service conditions, including indoor, outdoor and in soil environments. Polylactic acid (PLA) and polyethylene (PE) wood composites served as comparative reference materials. Aging studies over a 12-month period showed that PHA composites were mechanically stable under indoor conditions. A reduction in mechanical stability was observed for outdoor exposure but the composite property deterioration was independent of the polymer type. In soil, the biocomposites steadily degraded towards a complete loss in mechanical properties, in contrast to the PLA and PE based composites. Overall, the rate of water absorption and the rate of polymer biodegradation are principal factors controlling mechanical stability over time. These factors promote the formation of cracks and voids which in turn weaken the material performance under mechanical loading. Without such crack formation, the composites retained mechanical properties over the aging time. The results provide a basis for consideration of strategic applications of PHA-based WPCs in cases where sustained or time-constrained mechanical performance is required.