Effect of molecular weight and pellet shape on the external coefficient of friction of different polypropylene grades

The pressure build-up behaviour, the mass flow rate and the temperature development in the solids conveying zone of single screw extruders are mainly affected by the prevailing frictional conditions. The internal and the external coefficient of friction have to be distinguished. The internal coefficient of friction describes the friction between the polymer pellets themselves. The external coefficient of friction (ECOF) describes the friction between the polymer pellets and the screw or the barrel surface. The ECOF of polymer pellets should be rather low at the screw surface and high at the barrel surface to achieve a high mass flow rate and sufficient pressure in the solids zone of single screw plasticizing units. The effect of molecular weight and pellet shape on the ECOF of different polypropylene (PP) grades is presented in this paper. The tribological measurements were carried out as a function of velocity and normal force, using a previously developed tribometer. The applied normal forces were 500, 720, 1000 and 1800 N, and the velocity was varied between 0.1 and 1.2 m/s. The measurements were performed at room temperature on a polished steel shaft. To investigate the effect of the molecular weight on the ECOF, three PP types with different molecular weight were used. To determine the effect of the pellet shape on the ECOF, two different cylindrical PP-pellets were produced using a laboratory compounder. The measured ECOF of the PP-pellets was in the range between 0.15 and 0.6. The results show that a decrease in the ECOF of all PP types with a higher pressure or a lower velocity can be observed. An increasing molecular weight leads to an increase in the ECOF of the PP-pellets. Also a strong influence of pellet shape and size on the ECOF could be observed.