Development of bio-derived and biodegradable polyhydroxyalkanoate (PHA)-based wood plastic composites (WPCs)

Abstract

In response to the concerns over the accumulation of plastic wastes, the wood plastic composites (WPCs) industry has been in particular interest in replacing the petroleum-based polymer matrices with biopolymers. Polyhydroxyalkanoates (PHAs) are potential candidates as these biopolymers can be degraded readily under ambient conditions and can be synthesised intracellularly by bacteria from renewable/waste resources. Their low melt viscosity can also be an advantage in processing. While there is a considerable body of literature on PHA-based WPCs, the majority of the studies have focused on mechanical property optimisation but their mechanical stability and biodegradability were poorly understood. Therefore, this thesis explored the fundamental characteristics of PHA/wood flour (WF) composites with focus on the micro-structure-property relationship and end-of-life behaviour. The extrusion processing setup for composites of a commercially available poly(3-hydroxybutyrateco-3-hydroxyvalerate) (PHBV) and WF was first optimised. A logical next step involved the optimisation of mechanical properties through the use of carefully selected chemical compatibilisers and additives. The expected benefits of chemical compatibilisers were outweighed when compared to the alteration of micro-structure by the non-reactive micro-sized talc filler. An innovative approach for improving processabilty and toughness was then investigated through the inclusion of tougher PHA copolymers including an in-house mixed culture PHBV, for further cost reduction. The potential of this strategy was demonstrated by the improvement, albeit marginal, in composite strain at break. The real-time studies of mechanical stability and biodegradability brought new insights to this biocomposite. Uncoated PHBV/WF composites were stable under indoor ambient conditions for at least 1 year. Exposure to outdoor environment, however, led to reduced mechanical stability. It was determined to be solely associated with the mould growth on the exposed hygroscopic wood particles. The unique advantage of PHBV/WF composites was demonstrated with respect to their significantly higher biodegradation rate in soil than PLA/WF and PE/WF composites. The presence of wood accelerated the biodegradation and reduced the mechanical stability of the composites in soil. A micro-mechanical schematic model was used to illustrate the phenomena. In addition, it was shown that PHA-based WPCs are physically and mechanically stable unless they are exposed to colonies of microorganisms such as moisture-induced mould or those found in soil communities. Overall, this thesis provided better insights into the strong correlations between the micro-structure and the properties of PHA-based WPCs, both initially and in the long-term. Wood particles played an important role in both their performance and biodegradation. To conclude, this thesis has paved the way for development of novel bio-derived and biodegradable composites which provide utility what would otherwise be a low-value stream from the f...