Time Dependence of the Compressive Response of Polypropylene Bead Foam

Polypropylene bead foams of a range of densities were compressed using impact and creep loading in an Instron test machine. The stress-strain curves were analysed to find the effective cell gas pressure as a function of time under load. Creep is controlled by the polymer linear viscoelastic response if the applied stress is low, but at stresses above the foam yield stress, the creep is more rapid until compressed cell gas takes the majority of of the load. Air is lost from the cells by diffusion through the cell faces; this creep mechanism is more rapid than in extruded foams, because of the small bead size, and the open channels at the bead boundaries. The foam permeability to air can be related to the PP permeability and the foam density.