Chen, X., Feng, J. J. & Bertelo, C. A.
Polym. Eng. Sci. 46, 91-107 (2006)
Abstract - During polymer foaming with
physical blowing agents, plasticization affects the melt viscosity, gas
diffusivity in the melt, and the gas–melt interfacial tension. In this
paper, we propose a model for plasticization during bubble growth, and
estimate its effects under typical foaming conditions. The theoretical
model incorporates well established mixture theories into a recent
model for diffusion-induced bubble growth. These include the
free-volume theories for the viscosity and diffusivity in
polymer-blowing agent mixtures and the density gradient theory for the
interfacial tension. The viscoelasticity of the melt is represented by
an Oldroyd-B constitutive equation. We study the radial growth of a
single bubble in an infinite expanse of melt, using parameter values
based on experiments on polystyrene–CO2 systems. Our results show that
even at relatively low gas concentrations, plasticization increases the
blowing-agent diffusivity markedly and thus boosts the rate of bubble
growth. In contrast, the reduction in melt viscosity and interfacial
tension has little effect on bubble growth. Though not intended as
quantitative guidelines for process design, these results are expected
to apply qualitatively to typical foaming conditions and common
polymer-blowing agent combinations.