FRICTIONAL BEHAVIOR OF BIOPLASTICS AT SCREW SURFACES UNDER PROCESSING CONDITIONS

Bioplastics and Wood Plastic Composites (WPC) are quite new Materials. The optimization of processing is a critical factor for the products´ functional properties and also for the energy efficiency of the process. In this work the external coefficient of friction (ECOF) of Polylactic acid (PLA) pellets and two different WPC pellet types, a Polypropylen (PP) based and a Polyvinylchloride (PVC) based WPC, is presented. The ECOF is investigated at a pressure between 4 and 200 bar and a velocity from 0.1 to 1.2 m/s. A polished and a grinded stainless steel surfaces are used for the experiments. The ECOF of PP based WPC is in the range from 0.15 to 0.34 at both surfaces. For the PVC based WPC the ECOF decreases from 0.38 at low pressure to 0.16 at 200 bar. The ECOF of PLA is about 0.1. Only a slight dependence on pressure and velocity can be observed. Apparently significant differences occur for the frictional properties of PP and PVC based WPC. Furthermore, the frictional behavior of WPC deviates significantly from the matrix homopolymers.