DescriptionBulk solids are the raw material for almost every polymeric thermoplastic product. Their properties determine the quality of solids conveying and also influence the melting behavior of the material in processing units. This study exhibits the influence of temperature and pressure on the bulk density and the external coefficient of friction of polypropylene dependent on pellet shape. The experiments were done using a previously developed screw tribometer, that was adapted to make the shaft, the sample chamber (cross section 21 x 27 mm²) and the piston heatable. The used polymeric material was a polypropylene homopolymer with standard spheroidal and long cylindrical pellet shape. The bulk density tests were conducted at a temperature of 20, 60, 80 and 120 °C and either at constant pressure of 4, 20 or 35 bar or with dynamically applied pressure from 4 to 35 bar. All experiments were repeated four times for both pellet geometries. In addition, the external coefficient of friction was determined at the same temperature and pressure and at 0.1 m·s-1 velocity of the shaft. Results and Discussion It is shown that different pellet shapes of the same grade exhibit diverging bulk properties which are strongly dependent on the ambient conditions of the system. Long cylindrical pellets appear non beneficial for the extrusion process at standard conditions. This changes dramatically when the pressure and the temperature are rising. The bulk density then exceeds the one of spheroidal virgin pellets. Furthermore, the coefficient of friction is always higher for long cylindrical pellets. This divergence increases with the temperature. Through the higher detected external coefficient of friction and the high compressibility, fast heating and melting of the long cylindrical polypropylene pellets can be achieved in an extruder which can then lead to a significant rise in the mass flow rate compared to virgin pellets.
|Period||12 May 2015|
|Event title||Danube Vltava Sava Polymer Meeting|