Experimental study of the influences of the pellet shape on the bulk density and the frictional behavior of polypropylene

Abstract

Bulk 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 investigates the influence of pressure and temperature on the bulk density of two thermoplastic polypropylene pellets of different shapes. Furthermore, the external friction dependent on temperature and pressure of those materials is examined at conditions usually occurring in the solids conveying zone of smooth barrel plasticating units. The experiments are carried out using a tribometer for polymer pellets which was adapted for these tests by making the sample chamber, the piston, and the cylindrical roll heatable. The tests show that long cylindrical pellets exhibit low bulk densities at low pressure and temperature, which can be increased dramatically-even above the values of spheroidal pellets-as those parameters increase. Moreover, the external coefficient of friction is always higher for the long cylinders and strongly dependent on the temperature. Those facts add up and can cause a higher output of single-screw extruders.

Evaluation of the melting behavior and the melt penetration into the solid bed using a model experiment

“…It has been figured out that cylindrical pellets caused a much higher throughput than spheroidal ones. A further work indicates that possible reasons for that are the higher bulk density which can be explained by a higher packing density and a higher coefficient of friction [15]. In this studied case the solids conveying zone mainly determines the throughput of the extruder.…”

“…The measurements of the bulk density at increased pressure and temperature [15] have shown that the bulk density of cylindrical pellets strongly increases with higher pressure, especially at higher temperature. Figure 2 (a) shows that the measured bulk density of the cylindrical bulk reaches or exceeds that of the spheroidal bulk at 120 °C and pressures above 20 bar (2 MPa).…”

“…The self-arrangement of the latter seems not to work well as the theoretically reachable apparent bulk density without applying pressure to the bulk ρ bulk_max_p0 is higher due to a higher possible packing density ξ packing_max . The bulk density (at increased pressure and temperature) and the coefficient of friction [15] of cylindrical bulk solids are higher. This can enforce the solids conveying, resulting in a higher throughput and higher reachable counterpressure, at least in small scale single screw extruders.…”

Melting Behavior of Heterogeneous Polymer Bulk Solids Related to Flood Fed Single Screw Extruders

“…The modelling of these extruders is commonly subdivided into functional zones, which are the hopper, the flow-in zone, the solids conveying zone, the delay zone, the melting zone, and the metering zone. In some cases, additional mixing and degassing zones are used [1][2][3][4][5][6][7][8][9]. The throughput, the quality, and the homogeneity of the processed material are strongly influenced by the melting performance of SSE.…”

“…During melting more polymer melt penetrates into the free space between spheroidal pellets compared to cylindrical ones (Kneidinger et al [42]). In addition, cylindrical shaped bulk solids exhibit a higher external coefficient of friction compared to spheroidal ones, which boosts the solid conveying behavior and increases the extruder output (Längauer et al [8,9]). The coefficient of friction of mixtures of different polymers of investigated by Shim [43].…”