EXPERIMENTS AND MODELING OF THE EXPANSION OF CROSS-LINKED POLYETHYLENE FOAMS

The density of chemically-blown low density polyethylene foam was alte red by varying the amount of blowing agent, the degree of cross linkin g of the polymer, and the foam expansion temperature. A theory is prop osed for the equilibrium density, based on the gas pressures in a Kelv in foam structure, and a rubber-elastic analysis of the biaxial stretc hing of the cell faces. The predicted densities are within 3 kg m(-3) of the experimental values. For commercial types of LDPE foams, the bi axial stresses in the cell faces are significant, but they only cause a minor increase in the foam density above the free-expansion limit. T he molecular orientation in the cell faces must affect the foam proper ties. The diffusion of the nitrogen gas from the foam is insignificant .