Experiments and Modelling of the Expansion of Crosslinked Polyethylene Foams

The density of chemically-blown low density polyethylene foam was altered by varying the amount of blowing agent, the degree of cross linking of the polymer, and the foam expansion temperature. A theory is proposed for the equilibrium density, based on the gas pressures in a Kelvin foam structure, and a rubber-elastic analysis of the biaxial stretching of the cell faces. The predicted densities are within 3 kg m -3of the experimental values. For commercial types of LDPE foams, the biaxial stresses in the cell faces are significant, but they only cause a minor increase in the foam density above the free-expansion limit. The molecular orientation in the cell faces must affect the foam properties. The diffusion of the nitrogen gas from the foam is insignificant.