Influence of Particle Concentration and Type on Flow, Thermal, and Mechanical Properties of Wood-Polypropylene Composites

Two kinds of wood particles with different sizes and properties were compounded with polypropylene (PP) in highly concentrated levels (by 50% and 60% weight concentration). Their flow abilities were estimated by viscosity-shear rheological test and spiral flow mold respectively; the results show that higher concentrations result in poor flow ability. However, to estimate the relation between particle size and flow ability, spiral flow-mold testing results are not consistent with those of the rheological test. The thermal melting, crystallization, and stability of the highly filled wood/PP composites were measured by differential scanning calorimetry (DSC). The results clearly showed that the melting temperature, thermal stability, and crystalline degree of composites decrease when filler contents and size increase. The mechanical properties of these materials were assessed by tensile test machine and testing results also show that the filler concentration and size dramatically affect the mechanical properties, such as E module, tensile strength, and breaking strength of wood-particle-filled PP composites. The weld-line strength of all wood composites with the contents was also tested in the same way. By testing, one found that the usual defect of weld line in injection-molding process had a very significant negative impact on the mechanical properties of wood composites and the relation of wood-particle-filling fraction and size to weld-line strength was also analyzed and clarified.