In-line rheometry of polypropylene based Wood Polymer Composites

A completely satisfactory off-line rheological investigation of Wood Polymer Composites (WPCs) is very challenging at processing temperatures: when using rotational rheometry in oscillatory mode, the linear viscoelastic region is often too small, while in capillary rheometry the sieving effect of the fibres may invalidate the results. Moreover, all off-line tests present the risk of wood degradation. To this aim, an in-line extrusion rheometer (slit die) has been developed. The major advantages are that the measurements are made in real processing conditions and degradation problems are less severe thanks to reduced oxygen level inside the extruder barrel. In order to verify the methodology, tests have been conducted on a commercial polypropylene based WPC with 30 wt.% white fir fibres and compared with off-line measurements performed at a lower temperature on the same material. For comparison, a temperature shift factor has been used, that has been estimated by characterizing the polypropylene matrix alone.