Microstructure and micromechanical studies of injection moulded Chemically Modified Wood / Poly(lactic acid) Composites

Conventional wood plastic composites (WPCs) in general combine wood residuals with thermoplastics such as polyethylene and polypropylene. The basic idea in this work is to replace these olefin plastic matrixes in WPCs with a so-called biobased plastic, namely poly(lactic acid) (PLA). To reduce the water sensitivity of such biocomposites, for typical outdoor use, the idea is also to incorporate a modified wood component. The aim of this work was to study the microstructure of injection moulded WPCs based on PLA and modified wood, and to investigate some of their micromechanical behaviour. Four different PLA/wood formulations (weight-% ratio 50/50) were studied: PLA combined with 1) unmodified MDF fibres, 2) acetylated MDF fibers: 3) acetylated wood particles: and 4) thermally modified wood particles. The processing effects on the form and shape of the wood component were studied by a matrix extraction procedure combined with light microscopy. The microstructure of the WPCs were studied by scanning electron microscopy (SEM) using a sample preparation technique based on UV laser ablation, i.e. a surface preparation procedure allowing microscopic observations without microtoming. Microtensile testing was performed on samples prepared from the core and skin layers of the WPC samples. It was observed that the processing of the WPCs resulted in severe damage and fragmentation of the wood fibres and particles. Especially, the processing with a thermally modified wood component yielded a lot of wood cell wall fragments/fines. Good dispersion of the wood reinforcements in the PLA matrix was observed in all cases, even though the distribution and orientation of the fibres varied between different regions in the samples. No principal fibre orientation was outlined in the core, while closer to the surface regions the fibres tend to orient in the flow direction. Supposedly a turbulence effect at the end of the moulded samples also induced disorientation of the fibres. Long and slender shape, i.e. a higher aspect ratio, of the wood components were as expected found to be more efficient as a reinforcement than particulate shaped ones. The composites with acetylated MDF fibers were also observed to be weaker than composites with unmodified ones. On the other hand, SEM observations of fracture surfaces exhibited better reinforcement-matrix interaction for the modified wood than for the unmodified wood. Although the fracture surfaces observations outlined the importance of fibre orientation, with a more brittle type of failure in the core and fibre breakage or debonding closer to the surface, there was no significant difference found between mechanical properties for the core and skin layer specimens. Other factors have therefore to be investigated in order to explain the rather uniform strength measurements within different regions in the composites.Validerat; 20101217 (root