Friction between WPC Bulk Polymers and Metal Surfaces at high pressures AND HIGH Velocity

Wood Plastic Composites (WPC) are a recent development in material sciences that seem to offer considerable potential. The optimization of the material processing is the subject of present research in order to increase the production energy efficiency and to improve the product’s functional properties. In addition to other parameters, the external coefficient of friction (ECOF) between the WPC-pellets and the surfaces of the inner barrel and the screw are decisive factors for solids conveying in plasticizing units. This work deals with the ECOF of Polypropylen (PP) based WPC. The ECOF is investigated at pressures between 8 and 200 bar and velocities from 0.1 to 1.2 m/s. Two different stainless steel surfaces (polished Ra = 0.12 µm and grinded Ra = 0.12 to 0.57 µm) are considered. The ECOF of PP based WPC ranges from 0.12 to 0.34 at the polished surface. Below a pressure of 20 bar the ECOF is increasing with increasing pressure, above 50 bar no pressure dependence was observed for this surface, whereas at the grinded surface at 20 and 200 bar ECOF values of 0.3 to 0.5 and 0.3 were determined respectively. Apparently significant differences occur for the frictional properties of PP-H and PP based WPC. Furthermore, the frictional behaviour deviates significantly from the matrix homopolymers. In the context of the available literature a behaviour similar to these results was observed by Aurrekoetxea et al.