Enhanced wood plastic composites: Wood fibres as reinforcement in PLA composites

In the study, two wood fibre types, bleached softwood kraft pulp (BSKP) and thermomechanical pulp (TMP) with fibre lengths of 2.28 and 1.40 mm, respectively, were melt processed with poly(lactic acid) (PLA) using a twin-screw extruder to fibre content of 30 wt-% and injection moulded. In addition, three wood flour (WF) types especially targeted for wood plastic composites (WPC's) with average particle sizes of 915, 500 and 150 ?m were selected as wood fillers and processed with same parameters. Microscopic analysis as well as macroscopic images showed very uniform fibre dispersion in BSKP and TMP fibre composites, whereas with wood flours clear irregular particle sizes and shapes were visible. Elastic modulus was increased with all the wood component types, but a clear reduction of impact strength and tensile strength were seen in wood flour composites, a phenomenon typical to polymer fillers. The reinforcement capability of wood fibres comes apparent by detecting an increased material ductility and strength when wood in fibre form is used as reinforcement. In addition, water absorption remained lower when fibres were used instead of wood flours. Thus, the use of wood fibres in WPC's instead of conventional wood flours offers improved material properties, uniform appearance with colouring possibilities and all together, an enhanced bio-based material for added-value products.