Effect of Filler Content and Temperature on Steady-State Shear Flow of Wood/High Density Polyethylene Composites Luigi Carrino, 1,2 Stefano Ciliberto, 1 Giuseppe Giorleo, 1 Umberto Prisco 1 1 Department of Materials and Production Engineering, University of Napoli Federico II, Piazzale Tecchio 80, Napoli, Italy 2 CAMPEC, Consorzio per le Applicazioni nei Materiali Polimerici e Compositi, via n. Macello, Portici (NA), Italy Steady state shear flow of wood/high density polyeth- ylene composites is investigated through capillary rhe- ometry to gain better insight into rheology, extrudate distortions, and wall slip phenomena of wood/polymer composite melts. Effects of filler content and tempera- ture on onset and end of stick-slip transition, in terms of shear rate and shear stress, are also studied. Results show that shear rates at stick-slip transition decrease while corresponding shear stresses increase with the addition of filler. Furthermore, temperature raises the shear rate and the shear stress at which the transition occurs. It is observed a log-linear relation- ship in the plots of wall slip versus the shear stress, in particular, increasing the filler content and decreasing the temperature, these plots are shifted to higher shear stress, as a consequence of viscosity increment. Wall slip and filler content play a fundamental role in sur- face morphology; specifically, extrudates become smoother with increasing filler content and shear rate, whose increment always results in a rise of the wall slip. POLYM. COMPOS., 32:796–809, 2011. ª 2011 Society of Plastics Engineers INTRODUCTION Even though inorganic fillers presently dominate the thermoplastic industry, wood-derived fillers have been obtaining much interest lately. Their attractiveness origi- nates from the fact that natural fillers represent renewable and low-cost reinforcements that can improve mechanical properties such as stiffness, strength, and heat deflection temperature under load. Wood-polymer composites (WPC) are nowadays widely used for decking, automotive applica- tions and building material. In particular, WPC, based on wood flour (WF), have attracted increasing attention in the past decade thanks to the reinforcing potential of micro- scopic lignocellulosic fibers [1]. However, optimization of the WPC processing opera- tions, especially injection molding and extrusion, to reduce costs and compete with solid wood and conventional wood composite products is one of the key intent for scientific research about wood filled plastics. To this aim, a reliable characterization of the rheological behavior of WPC melts is central, also considering that these materials show a sig- nificant increase of the melt viscosity, which creates vari- ous processing difficulties such as flow instabilities, stick- slip, and surface defects of the extrudates. Li and Wolcott have conducted comprehensive investi- gations on wall slip in wood-filled high density polyethyl- ene (HDPE). In [2], the authors have shown through the Mooney analysis that the shear flow of wood composites in capillary dies is characterized by varying level of wall slip depending on both wood content and species. However, in [3], they showed that the shear and extensional properties of HDPE/maple melts depend significantly with wood con- tent but not as much by the commercial wood particle size grade and that the additive plays its role as an internal lubricant in both shear and extensional flow. Important research results about WPC rheology are in [4–6]. In these articles, the authors investigated the extru- date surface tearing in capillary extrusion of highly filled HDPE/WF composites in relation to rheological properties, wall slip phenomenon [4], effect of filler particle size and matrix molecular weight [5], and lubricant [6]. Maiti et al. [7, 8] investigated the melt rheological prop- erties of PP/WF composites at filler concentration 3–20 wt%. They reported an increase of the melt viscosity and decrease of melt elasticity depending on the filler concen- tration. The authors argued that the melt viscosity of PP increased due to increased obstruction by the irregular- shaped WF particles. Other researchers [9–11] also reported a large increase of the viscosity of wood filled composites in comparison to the neat polymers. Correspondence to: Umberto Prisco; e-mail: umberto.prisco@unina.it DOI 10.1002/pc.21101 Published online in Wiley Online Library (wileyonlinelibrary.com). V V C 2011 Society of Plastics Engineers POLYMERCOMPOSITES—-2011