Mechanical properties of wood flake-polyethylene composites. Part I: Effects of processing methods and matrix melt flow behaviour

The structure-property relationship of wood flake-high-density polyethylene (HDPE) composites was studied in relation to the matrix agent melt flow behaviour and processing technique. The flake distribution and flake wetting were optimized to obtain acceptable mechanical properties in these composites using two processing techniques, namely twin-screw compounding and mechanical blending. The microstructure of the composites revealed that the twin-screw compounded composites based on medium melt flow index (MMFI) HDPE always achieved better flake wetting and distribution, and therefore had higher mechanical properties, than those mechanically blended composites or twin-screw compounded composites with low MFI (LMFI) HDPE. For 50:50 wt% composites the overall flake wetting, depending on processing technique and matrix flow behaviour, is ranked as compounded MMFI>compounded LMFI>blended MMFI>blended LMFI. However, the uniformity of flake distribution of the composites follows a somewhat different pattern, i.e. compounded MMFI>blended MMFI>compounded LMFI>blended LMFI. Evidence shows that the medium MFI HDPE penetrates into lumens of wood fibres in wood flakes. This phenomenon combined with flake wetting and flake distribution had a profound effect on the mechanical properties, in particular the impact strength.