Investigating the mechanical properties of expanded polypropylene bead products

Expanded polypropylene (EPP) bead foam products are prepared using the steam-chest molding method. It was shown that 3.9 bar is the most promising steam pressure to get better mechanical properties. Also, the density and tensile property profile across the molded sample were measured. It was observed that the high pressure and temperature steam softens the beads on the surface causing them to squeeze and compact together. This kind of structure leads to increase in the density and tensile property at the outer surface of the molded part as compared to the central part. To better understand the tensile property of the molded part, the sintering between the beads was characterized by observing the original and the fracture surface using scanning electron microscopy (SEM). Both inter-bead and intra-bead failure mechanisms exists in the fracture of the sample due to the tensile load. The uniform percentage of intra-bead failure across the product suggests that the beads are sintered effectively and the lower tensile property in the center is attributed to low temperature of the penetrating steam causing lower density with higher expansion. Therefore, in the center the beads are not affected by high temperature that leads to their higher expansion and ultimately lower density. Also, the lower crystallinity at the center could contribute to their lower tensile strength.