The effect of material factors on the density and cell morphology of chemically foamed polypropylene

This paper assesses the effect of various material factors on the cell morphology and density of extruded expanded polypropylene (EPP), rod samples produced using a range of commercial chemical blowing agents (CBAs). This work is part of a research project into the optimisation of the extrusion and subsequent thermoforming of EPP sheet for food packaging applications. In each case the agents were added to PP and extruded under identical processing conditions, and the resulting variations in foam density and cell morphology are reported and discussed. Trials were also carried out to quantify the effects on the foam of additives, such as pigment, sodium benzoate, co-polymer and regrind, and the grade of polymer used (nucleated and un-nucleated). A design of experiment (DOE) methodology was adopted for all of these latter tests. The chemical blowing agents tested, produced foams with markedly different cellular structures with cell densities in the range 4.77x104 to 1.45x106 cells/ cm3. Of the other material factors tested the change in polymer grade and the addition of sodium benzoate were found to most significantly affect the density and cell morphology.