Effect of Blend Composition on Extrusion Foaming Behaviour of Linear and Branched Polypropylene Ternary Blends

Polypropylene (PP) as a linear resin has low melt strength. The melt strength of polymer is the main feature for the success of low density extrusion foam. Use of a high melt strength such as branched PP (Br-PP), is required to achieve large volume expansion by preventing cell coalescence and gas loss. In this paper, the effects of long chain branched polypropylene on melt elasticity and volume expansion in the extrusion process of PP ternary blends are investigated. The equilibrium creep compliance (Je0) and damping factor (tan δ) of melts were evaluated as melt elasticity index by oscillation rheometry. To setup the extrusion foaming process, the Haake 25 L/D single-screw extruder was improved by design and manufacturing to a special 38 L/D screw with a long mixing zone. Extrusion temperatures were optimized to reach a stable foaming process. The consistency of process pressure as a measure of foam extrusion system stability was evaluated. The cell population density and expansion ratio were determined for foam samples. The effects of the blowing agent (BA) on the amount of pressure build-up were measured at various branched PP levels in the blend. The experiments showed that, by increasing the branched PP, Je0 increases, and tan δ decreases, which is an evidence of improved melt elasticity. The maximum expansion ratio for the blends was achieved at about 55% of branched PP resins and 20% of B.A. The results also showed that, by increasing the branched PP, the cell population density increases to a maximum of 7.5×106 (cell/cm3) and no significant increase was observed at higher branched PP levels.