(25f) Evolution of Polystyrene Cell Structure during Extrusion Foaming Process By Population Balance Simulation

Polystyrene foams are widely used lightweight materials, employed in many different applications. The final characteristics (i.e., thermal and sound absorption properties) of the manufactured objects are highly dependent on the final bubble (or cell) size distribution (BSD) of the foam. This is determined not only by the initial chemical recipe of the components, but also the flow dynamics of the composites system during manufacture. However, the relationship between the process conditions and the final foam characteristics is highly empirical. Hence, accurate prediction of the cell structure of polymer materials based on mathematical models is identified as the most efficient way to optimize the process and control product performance. By introducing the population balance equation (PBE), coupled with the flow equation of polystyrene melt and the mass transfer mechanism of CO₂ between melt phase and bubble phase, the numerical model for the foaming process of polystyrene/supercritical CO₂ (PS/scCO₂) system through an extrusion die has been established to simulate the nucleation and growth behavior of bubble population.

Based on the simulation results, it has been shown that the evolution of the cell size and the cell number density in the die can be divided into four stages, which can be attributed to distinctive transport behavior of the multiphase flow. And the evolution behavior of the cell structures has close relationship with the physical properties of the polymer/CO₂ such as surface tension, Henry’s law constant, as well as the adequate processing conditions, including CO₂ content and extrusion speed. The relevant results can guide controlling the bubble structures according to industrial desire.