David G. Bland

Structure-property relationships in coextruded foam/film microlayers

In this work, we have shown for the first time that assemblies of up to 64 alternating foam and film layers can be successfully made via the microlayer coextrusion technology. We have investigated the effect of some important processing variables on the foam-film structure, and have demonstrated how they can be optimized. With the optimum concentration of the right type of chemical blowing agent, the cell size can be significantly reduced for the polypropylene foam-film systems by increasing the number of layers. The foam layer density does not change significantly with increasing number of layers. Measurements of the cell density and cell volume indicate that enhanced nucleation and/or reduced coalescence is responsible for the reduction in cell size with an increasing number of layers. Constrained cell growth comes into play when a single cell/layer morphology is observed. Different types of polymers can be used as the film layer to alter the flexibility of the foam/film composite. The compressive behavior of the hard/hard polypropylene foam/polypropylene film system is similar to that of cork in terms of the compressive modulus, collapse stress, and densification strain. The tensile and compressive moduli increase with the increasing number of layers and this could be explained on the basis of an increase in the fraction of material in the cell walls, using the conventional Gibson-Ashby model. The soft/soft polyethylene foam/ethylene-styrene copolymer film system exhibits enhanced toughness in tension. The effect of the foam/film composition on the tensile and compressive moduli can be explained using the composite series and parallel models.