Understanding the Links between Structure and Properties in Die Drawing Wood Powder Polymer Composites

Wood powder polypropylene composites (WPC) have been produced with 40wt% of a soft or a hard wood as filler. The die drawing process developed at the University of Leeds [1] has been used to produce oriented composites. Aspect ratios (Ar) for both wood powders were calculated by using a Sysmex image analyser. Prediction of the Young’s modulus of the oriented WCP was calculated using the Cox-Krenchel model [2]. It was found that theoretical predictions agree with the experiment data very well. Both isotropic and anisotropic composites were examined for tensile and flexural properties, and for thermal expansion. The results show an increase in all mechanical properties as the degree of orientation was increased; in particular, the tensile Young’s modulus (E) was improved from 1.9 to 8.2GPa for a softwood composites (C120PP), and from 2.9 to 5.4GPa for a hardwood composites (HB120PP). The linear thermal expansion coefficient (α) of the WPC decreased with increasing nominal draw ratio (RN) and depended on the testing direction; e.g. for C120PP tested parallel to the orientation direction it fell from 70x10 -6 / o C for the isotropic material to about 22x10 -6 / o C for an oriented composite. In addition, the morphology (ESEM) of the composites was monitored at various stage of processing.