Rheology of HDPE-wood composites. I. Steady state shear and extensional flow

Steady state flow of wood/high density polyethylene (HDPE) composites was studied using capillary rheometry to approach a fundamental understanding of the rheology of wood-polymer composite melts. Mooney slip analysis was applied to examine the nature of shear flow, indicating that the flow of wood/HDPE melts consist of contributions from both wall slip and viscous flow. Both simple viscous flow and yield stress behavior were observed, depending on wood species and content. It was observed that the dependence of wall slip velocity on shear stress in maple (Acer spp.) formulations resembles that of neat HDPE. In pine (Pinus spp.) formulations, however, wall slip rate is influenced by wood content. A converging flow technique was used to study the extensional flow. In contrast to shear flow properties, the extensional viscosity was found to depend much less on wood species. The effects of both wood content and species were observed through Trouton ratio. Strong inter-particle interaction was recorded for some filled melts as yield stress in shear flow and apparent strain hardening in extensional flow.