In – line vs. off – line rheological characterization of wood polymer composites

In a previous study [1], a commercial PP - based Wood Polymer Composite (WPC) filled with various percentage of white fir fibers (30% – 70% wt.) has been investigated with a parallel plate rheometer in oscillation mode at 170 °C. This temperature has been imposed by the requirement of performing the rheological test within the linear viscoelasticity region (LVR), but the data that are measured are not directly useful for processing, as the required temperature lies between 180 – 195 °C. In order to obtain the WPC viscosity at such temperatures, a model that uses the WPC viscosity measured at 170 °C at different fibers quantity and of neat polypropylene viscosity measured at various temperatures has been proposed. These measurements allowed to obtain shift factors useful to estimate the WPC viscosity at any temperature and percentage of fibers. The aim of this work has been to validate such a model using an in – line rheometer directly connected to a single screw extruder. With this apparatus, 30% and 70% wt. WPC flow curves at 195°C have been measured in a setting that is similar to processing conditions. The results show that the viscosity curve for the 30% wt. WPC validates the model with a reasonably good agreement, while the agreement for the 70% wt. is less evident. Several reasons are proposed to explain this deviation, in particular the presence of an yield stress.