Effect of fiber and polymer variability on the rheological properties of wood polymer composites during processing

We investigated the effects of fiber variability, size and content on the fusion characteristics of wood particle‐reinforced high‐density polyethylene (HDPE). Five types of wood sawdust were used: eastern white cedar, with sapwood, and heartwood treated separately; jack pine divided into wood and bark; and black spruce. Three different fiber length classes were also used. Composite pellets containing wood particles at 25, 35, and 45% by weight with HDPE were made using a co‐rotating twin‐screw extruder. The pellets were melted and mixed during 7 min in a torque rheometer at 180°C. We also investigated the polymer variation using HDPE, polypropylen (PP), polyvinyl chloride (PVC), and a blend of (80% HDPE+ 20% PP) where wood fiber proportion and length were kept constant. We varied the mixing temperature to reach the melting temperature range of each polymer. Mixing and melting times, maximum torque and stabilized torque were obtained. Adding wood fibers to the HDPE increased processing time and torque energy. At constant fiber length and proportion, torque properties varied with fiber origin. Higher fiber length and proportion increased torque energy and time of stabilization. Thus, using wood fibers with different properties will lead to significant variations in processing, such as in extrusion or injection. The fusion characteristics of wood polymer composite vary among polymers. The PVC showed the highest steady‐state torque. The formulation containing a polymer blend of HDPE and PP showed the highest torque energy. This higher torque energy is explained by the interaction of the incompatible polymeric chains of the two thermoplastic polymers. POLYM. COMPOS., 40:E609–E616, 2019. © 2018 Society of Plastics Engineers