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

Abstract

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 evaluation of the kinetics of bubble growth will also be explored.

Steady flow simulation of a polymer‐diluent solution through an abrupt axisymmetric contraction using internally consistent rheological scaling

“…Recently, researchers from the Industrial Materials Institute, NRC, Canada,31, 33, 91 used an inline detection method based on ultrasonic sensors to investigate the influence of the flow on the foaming behavior of various PS‐gas mixtures in an extrusion slit die. The authors observed that under flow conditions, the degassing pressure was consistently higher than the solubility pressure.…”

Foaming Polystyrene with a Mixture of CO2 and Ethanol

“…Since the ultrasonic waves are scattered in the presence of bubbles, this technique enables the detection of the early cell formation. This technique was described in detail in [6].…”

The influence of specific mechanical energy on the ultrasonic characteristics of extruded dough

“…Consequently, a vital part of quality control is the development of on-line monitoring tech- niques to measure and monitor structural changes that affect product properties during the extrusion process. Currently, measurements of ultrasonic properties are considered as potential tools to describe and monitor these changes (Luan, 1991;Ross, Pyrak-Nolte, & Campanella, 2004), and ultrasound evaluation has been successfully applied to on-line assessment of the processing of polymer melts (Franca, Jen, Nguyen, & Gendron, 2000) and foamed polymer melts (Sahnoune, Tatibouet, Gendron, Hamel, & Piche, 2001), systems that have obvious similarities to extruded food systems. Ultrasound has the advantage of relatively low cost, probes and associated electronics that are relatively robust, and the ability to probe the properties of opaque materials (Povey & McClements, 1988).…”