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-strai n curves were analysed to find the effective cell gas pressure as a fu nction of time under load. Creep is controlled by the polymer linear v iscoelastic response if the applied stress is low, but at stresses abo ve the foam yield stress, the creep is more rapid until compressed cel l 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 c hannels at the bead boundaries. The foam permeability to air can be re lated to the PP permeability and the foam density.