Extrusion of Low-Density Microcellular HIPS Foams Using CO2

Abstract A continuous extrusion process for the manufacture of low-density microcellular polymer is presented. Microcellular polymers are foamed plastics characterized by a cell density greater than 109 cells/cm3 and a fully grown cell size on the order of 10 μm. The basic approach to the production of microcellular structures is to continuously form a polymer/gas solution, nucleate a large number of bubbles using rapid pressure drop, shape a nucleated polymer/gas solution under pressure, and induce a volume expansion to a desired expansion ratio. Successful completion of these steps in extrusion will enable the manufacture of microcellular foamed plastics with a high cell-population density. Previous research on the continuous processing of microcellular polymers has focused on control of microcell nucleation in extrusion. This paper presents an effective means for the control of cell growth to achieve a desired expansion ratio with CO2 as a blowing agent in microcellular foam processing. Also, a strategy to prevent the deterioration of the cell-population density via cell coalescence during expansion is presented. The promotion of a desired volume expansion ratio and the prevention of the cell coalescence in microcellular foam processing were experimentally verified. By tailoring the extrusion processing parameters, microcellular HIPS foams with a cell density of 1010 cells/cm3 and a controlled expansion ratio in the range of 1.5 to 23 were obtained.