Effect of shear factor on bubble nucleation of polystyrene using supercritical fluid as a foaming agent

Abstract

Abstract The effect of shear factor on bubble nucleation during the foaming extrusion process with polystyrene (PS) using the supercritical fluid (SCF) carbon dioxide (CO2) as a foaming agent was studied based on the energy transformation in this article. The influence of shear factor (which is caused by the velocity gradient in a conical die) on the dynamics of bubble nucleation was investigated and determined. The critical radius for bubble nucleation is presented through the Taylor deformation theory by using a nuclear embryo to represent the state in the bud of the appearance of the nuclear bubble. Theoretical analysis shows that a spherical nuclear bubble can split into two new nuclear bubbles after the deformation in the shear flow field; the radius of the new nuclear bubble is greater than the critical radius, so it can finally grow into a larger nuclear bubble. The shear factor increased the number of the nuclear bubbles in the polymer melt and so increased the density of the bubbles and decreased the diameters of the bubbles. This article proposes a qualitative judgment which indicates that the shear factor in a conical die has a beneficial effect on bubble nucleation, and helps to produce foamed plastics with fine bubbles.