Plasticization effects on bubble growth during polymer foaming

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 wellestablished 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-CO 2 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. POLYM. ENG. SCI., 46: 97-107, 2006. POLYMER ENGINEERING AND SCIENCE-2006 FIG. 3. Radial profiles of (a) gas concentration c(r) and (b) viscosity (r) at different times during bubble growth. The initial concentration is c 0 ϭ 0.2%.

Experimental and numerical studies on the effects of pressure release rate on number density of bubbles and bubble growth in a polymeric foaming process

“…The zero shear viscosity was used as the value of viscosity. Although CO 2 -induced viscosity depression has been reported by many authors (Elkovitch et al, 2000(Elkovitch et al, , 2001Han et al, 2002;Kwag et al, 1999Kwag et al, , 2001Lee et al, 2000;Royer et al, 2000;Utracki and Simha, 2001;Xue and Tzoganakis, 2003), its effect on bubble growth may be limited (Chen et al, 2006). The zero-shear viscosity used in this study was not measured under a high-pressure CO 2 condition (Table 1).…”

The nucleation and growth of gas bubbles in a Newtonian fluid: an energetic variational phase field approach

“…Nucleation is modeled at best phenomenologically, with much uncertainty regarding the proper initial conditions for the growth stage [15]. Considerable work has been done on bubble growth, albeit limited to the growth of a single bubble in an infinite sea of melt or in a shell meant to simulate the effects of nearby bubbles [46,7]. Coarsening is often ignored completely, since bubble coalescence involves a topological change of the interfaces that cannot be readily handled by a sharp-interface model.…”

Improving Bio-oil Quality through Co-Pyrolysis of Corn Cobs and Polypropylene in a Stirred Tank Reactor

“…The reduction of char yield with increasing plastic composition in the feedstock seems to correspond to the reduction of biomass content therein, in addition to the synergetic effect provided by sufficient hydrogen donation as generated from the pyrolysis of large amounts of plastic melt in this region. The use of nitrogen to generate bubbles in the plastic melt appears to be effective in reducing the bulk viscosity of the plastic melt, easing the convective heat transfer (Chen et al, 2006). Additionally, the use of a stirrer, which increases the shearing of the plastic melt on the reactor wall, apparently contributes to the augmentation of the convective heat transfer (Vlachopoulos & Strutt, 2002).…”

“…2, and the reactor diameter is 96 mm, but which is equipped with nitrogen flow to enhance the convective heat transfer from the reactor wall, on which a heater is installed, to the plastic feedstock to overcome the low thermal conductivity of plastic PP. Nitrogen bubbles in the plastic melt can reduce its viscosity and, as a result, the convective heat transfer would be upgraded (Chen et al, 2006). This study is aimed to produce bio-oil by co-pyrolysis of corn cobs and PP plastics in a stirred tank reactor equipped with nitrogen flow at a low heating rate.…”