Modeling flow and cell formation in foam sheet extrusion of polystyrene with CO2 and co-blowing agents. Part I: Material model

In foam extrusion, process parameters, material properties, and the blowing agent have an influence on the resulting foam properties. For safety and environmental reasons, carbon dioxide (CO2) has gained importance as a physical blowing agent for the production of low-density polystyrene foam sheets. The sole use of CO2 often leads to corrugation, open cell structures, or surface defects on the foam sheet. As an alternative, blowing agent mixtures based on CO2 and organic solvents such as ethanol, acetone, or ethyl acetate can be used, changing solubility and flow behavior of the gas-loaded melt. Modeling of the foaming process in the extrusion die could help to reduce experimental effort and accelerate the development of novel blowing agent mixtures. A model approach to describe the melt behavior of polystyrene loaded with various blowing agent mixtures in the extrusion die is developed. Part I of the article describes the modeling of material properties, that is, rheological behavior by a Carreau-WLF approach with shift factors for temperature, pressure, and blowing agent effects on the glass transition temperature. Solubility behavior is modeled by a combined Henry solubility coefficient approach, showing good agreement with experimental data. Based on the material model, a process model is developed in Part II of this work.