Effects of Formulations and Processing Parameters on Foam Morphologies in the Direct Extrusion Foaming of Polypropylene using a Single-screw Extruder

The direct extrusion foaming process of low-density polypropylene (PP) in a typical single-screw extruder is researched. Effects of five variables on the volume expansion behavior and cellular morphologies are investigated. They are three kinds of polypropylene with different melt flow rates (MFRs), two kinds of chemical blowing agents, cross-linking modification method and blending modification method, two operating parameters (the screw speed and the die temperature), and die shapes. The experimental results prove that: the lower the MFR (from 0.45 to 10 g/10 min), the better the PP foamed; use of azodicarbonamide as the blowing agent helps to achieve PP foam with larger volume expansion, while use of endothermic foaming agent EPIcor 882 results in rigid foams with lower foaming ratios; melt strength can be effectively promoted, and low-density PP foam can be produced by proper addition of cross-linking agents or blending agents properly; low die temperatures (about 155 C) and high screw speeds (from 16 to 48 rpm) are beneficial to foaming; high die pressure in some range is good for foaming, so the die in which it is easy to establish high die pressure such as the filament die, is good for low-density foams.