Monitoring deformation mechanism of foam cells in polyethylene foams via optical microscopy: Effect of density and microstructure

Abstract

The effect of cellular microstructure and density of low-density polyethylene (LDPE) foam during the compressive deformation were evaluated. Progressive foam cell structure deformation at crosshead speed of 10 mm/min was closely monitored and captured using a portable digital microscope with the assistance of a proposed coating technique to enhance contrast of the foam cells. Results revealed that higher density foam, LDPE1.0, experienced higher compressive stress than the LDPE0.5 foam. It was believed that thicker cell walls in the LDPE1.0 foam contributed to the foam stiffness and higher stress was required to impose cell bending, buckling, and collapse. It was also discovered that the existence of larger cells within a group or cluster of small cells in the LDPE1.0 foam acted as stress concentration points and initiated the formation of shear bands in the foam cells deformation mechanism. The extent of compressive strain on the foam cells were also investigated, and results showed that cells undergo lower compressive strain compared to the actual deformation level given directly by the machine crosshead. Cells located in the middle section of the foam recorded a strain of about 13% when the bulk foam was deformed at 20% strain. The success in capturing the actual cell strain using the proposed coating technique proved that with the right improvisation, the micromechanical deformation of optically challenged white polyethylene foam samples can be monitored effectively using optical microscopy.