Application of Ultrasonic Sensors in the Study of Physical Foaming Agents for Foam Extrusion

The extrusion foaming process involves several critical steps, in which the physical foaming agent plays a significant role: plasticization, solubility, nucleation and bubble growth. Although these aspects can be studied by different techniques, a novel method based on ultrasonic sensors has proven to provide valuable information with respect to the thermoplastic foaming process. This technique can be either used off-line as a characterization tool to improve our understanding of the foaming agent characteristics, or it can be installed in-line, on the extrusion line, as a control device. Review of the different applications of this technique will be covered in this paper, with numerous examples given to the mixture of PS with CO2. The degree of plasticization of the polymer as a function of the blowing agent concentration will be addressed first, followed by the detection of the conditions prone to induce nucleation, in terms of pressure, temperature, type of nucleating agents and flow conditions. The ev...