Determination of the bulk density of different plastic granules as a function of pressure and temperature

This work deals with the bulk density of differently shaped raw materials, considering recycled, shredded film, spheroidal shaped virgin bulk material and two different cylindrical shaped polypropylene pellets. The mass flow rate and the pressure buildup behavior of single screw extruders, which are frequently used for the recycling of polymers, are strongly influenced by the shape of the raw material. The differences mainly affect the solids conveying zone, where the external coefficient of friction and the bulk density in the screw channel largely influence the behavior. In single screw extrusion applications, higher bulk density usually leads to a higher mass flow rate and a better pressure build-up behavior. In this paper, the density of differently shaped bulk materials is analyzed in detail at high pressures and high temperatures comparable to the conditions existing during extrusion. Especially the density of shredded film was not investigated in detail till now. It is shown that the density of virgin material and the two different cylindrical shaped pellets behave in a similar way, while the shredded film differs from the others.