Morphological and Kinetic Study of Expandable Polystyrene Pre-expansion and Effects on Foam Properties

Optimized physico-mechanical properties and good pre-expansion and moulding performance ( short cycle time) are required for expandable polystyrene ( EPS). These characteristics are greatly dependent on expanded bead density, fusion and cell structure. On the other hand the morphological parameters of pre-expanded beads are determined by the intrinsic molecular and morphological properties of the expandable particles. The pre-expansion mechanism and its complex morphological and kinetic aspects have been studied; in particular, bead density, cell homogeneity, cell dimensions and number, cell wall thickness, amount of broken cell walls and pentane concentration during different pre-expansion steps have been evaluated. Determinations of bead surface integrity, pentane diffusivity at different densities and critical surface tension of compact and prefoamed EPS have also been made. The results of this investigation give some indications of the moulding behaviour and of the physical properties of the foamed samples. It has been found that very short moulding times (with negative effects on fusion and block shrinkage) are related to an appreciable amount of broken cell walls, probably due to too small a cell diameter.