Rheology of Wood Polymer Composites

Wood plastic composite (WPC) is a material composed of a thermoplastic matrix filled with wood fibers at different concentrations. This material is widely used for decking and automotive applications. With respect to wood it requires less maintenance, shows better durability in wet environment and can be obtained from recycled materials. With respects to plastics the main advantage is its lower cost. The aim of this thesis has been to investigate the mechanical and rheological properties of PP based Wood Polymer Composites. The mechanical characterization has shown that wood fibers make the composite stiffer without lowering strength values too much, while thermal properties have confirmed that the processing window for this material is rather narrow, being limited upwards by wood fiber degradation and downwards by the melting temperature, which is around 165°C. Commercial PP-based WPCs filled with different filler concentration have been investigated with an off – line rheometer in oscillation mode at 170°C. Complex viscosity increases with the percentage of fibers. All materials show a shear-thinning behaviour with similar slopes of the flow curves and neat PP also displays a Newtonian plateau at low shear rates. The test temperature is imposed by the requirement of performing the test within the linear viscoelasticity region, but the data that are measured are not directly useful for processing, as a convenient processing temperature should be around 195°C. In order to obtain the WPC viscosity at such temperatures, a model that uses the WPC viscosity measured at 170°C at various wood quantities and of neat PP viscosity measured at various temperatures is proposed. The main hypothesis of this model is that the effects of temperature and filler content on the composite viscosity are disjoint. These measurements permit to create shift factors that allow to estimate the WPC viscosity on the basis of neat PP viscosity, temperature and fibers content. In order to validate the model, flow curve of 30%wt. and 70%wt. WPC at 195°C have been measured with an instrumented extruder slit die that allows the determination of the flow characteristics of the material in a condition that is very similar to the actual processing conditions. The slit die, connected with a single screw extruder, has a channel that is 50 mm wide and 105 mm long. Three heights are used for the Mooney procedure and three pressure transducers are flush mounted along the slit to eliminate the need for the Bagley correction. The results show that the viscosity curve for the 30% wt. WPC validates the model presented with a reasonably good agreement. Since the agreement for the 70% wt. is less justified and such a material displays a marked yield stress, we can conclude that it is the yield stress that is probably the single issue that makes the present model questionable. In this regard, a future work that we propose is to modify the model by selecting a Carreau-Yasuda model with an yield stress to account for the increase in the viscosity at low shear rates