Heterogeneous Nucleation in Foams as Assessed by In-line Ultrasonic Measurements

Abstract

The conditions prevailing during the nucleation stage of the thermoplastic foam extrusion process in which a physical foaming agent is involved are one of the key steps in the design and optimization of the process. The conditions that induce phase separation and bubble nucleation are obviously linked to the solubility parameters, i.e., temperature and pressure at a given foaming agent content, conditions that can be probed during degassing experiments. It has been reported that adding a nucleating agent can significantly modify these degassing conditions. An in-line detection method based on ultrasonic sensors has proven to be sensitive to the onset of the phase separation. This method was used to investigate the influence of both talc and foaming agent concentrations on the nucleation cell density and degassing conditions for polystyrene-HFC-134a mixtures. A qualitative model based on foaming agent concentration gradient near the nucleating agent particles is proposed.