Melt Flow Instabilities of Wood Polymer Composites

Melt flow instabilities during extrusion of wood polymer composites (WPC) containing 30–60 wt% wood flour (WF) have been investigated. The research emphasized elucidation of the extrudate surface tearing mechanism and its relation to wall slip. This interesting phenomenon has been known in the WPC industry for years; however, it has not received much research interest. It was observed that increasing the wood flour loading up to 50 wt% aggravated the surface tearing; however, addition of 60 wt% wood flour completely eliminated the surface defect due to strong wall slip and plug flow. It was also found that addition of lubricants and increasing the shear rate significantly improved the surface appearance of the filled compounds. Molecular weight and molecular weight distribution of the polymer matrix influence the melt flow properties of the composites. The significance of the entrance pressure measurement and its usefulness for quantitative assessment of filler–matrix interactions in composite materials is also demonstrated in this paper.