(215h) Confined Batch Foaming of Rubbery Elastomers with Carbon Dioxide Using a Mold

Confined batch foaming of polymers with physical blowing agents such as carbon dioxide has been employed in numerous applications to promote or alter foaming outcomes and control the dimensions of foam expansion. In confined pressure-induced foaming the expansions of the polymer are restricted to the inner geometries of the mold. Confined or mold-type foaming is of significance in simulating the conditions of injection molding foaming operations. Both permeable and impermeable type molds have been discussed in the literature. Despite the increasing use of molds reported in the literature, the details of mold design and their impacts on foaming conditions are often not discussed.

In this presentation, the foaming of poly(ethylene-co-vinyl acetate-co-carbon monoxide), which has previously been reported to show large initial expansions and then subsequent relaxations¹, with permeable and impermeable molds will be discussed. Permeable boundaries enhance the escape of gas from the mold while impermeable boundaries restrict the diffusion of carbon dioxide to edges of the polymer film. This study serves as a description of the expected phenomenological observations, in terms of foaming outcomes, that occur when foaming in confinement.

Confined foaming experiments were conducted with permeable and impermeable boundaries. Foaming experiments were also conducted under free-foaming conditions where the polymer is allowed to expand without restriction for comparison. Foaming experiments were carried out from carbon dioxide saturation conditions of 30 and 40 ℃ and 100, 200, and 300 bar followed by rapid depressurization of the high-pressure cell. Foam morphologies in the central and skin layer region have been characterized to understand how confined foaming alters pore formation.

Foaming in confinement leads to foams with greater dimensional integrity. Pore sizes in foaming with permeable and impermeable boundaries are smaller and with more uniform pore distributions, but the trade-off is the degree to which bulk foam densities may be lowered. Outer surface texture of the polymer foam is distinctly influenced by the surface of the mold. This presentation will highlight the consequences of foaming in a mold on altering inner pore morphology and the extent to which gradients are formed along the foam cross-section.

1. Joseph A. Sarver, Jenna L. Sumey, Michael L. Williams, John P. Bishop, David M. Dean, Erdogan Kiran, Foaming of poly(ethylene-co-vinyl acetate) and poly(ethylene-co-vinyl acetate-co-carbon monoxide) and their blends with carbon dioxide, Journal of Applied Polymer Science, 136, 2018, 48841; doi: 10.1002/app.45841
2. Joseph A. Sarver, Jenna L. Sumey, Richard M. Whitfield, Erdogan Kiran, Confined Foaming of Semi-Crystalline Rubbery Elastomers with Carbon Dioxide in a Mold, Journal of Applied Polymer Science, 138, 2021, 50698; doi: 10.1002/app.50698