Effect of Different Nucleating Agents on the Degassing Conditions as Measured by Ultrasonic Sensors

Ultrasonic sensors have proven to provide valuable information on the thermoplastic foaming process in polymers. Measurement of the attenuation of the ultrasonic signal can be easily related to the nucleation process, i.e., the onset of bubble formation in the foam matrix (degassing). The ultrasonic sensors can be installed in-line, on the extrusion line, and thus allow direct access to the prevailing processing conditions. In this work, the degassing conditions (pressure and temperature) of a mixture of polystyrene and a physical blowing agent (PBA), HCFC 142b, are determined for two different nucleating agents as well as in absence of any nucleating agent. The resulting cellular structure of the extruded foams is correlated to the degassing conditions determined by the ultrasounds. The results are discussed in light of other observations on the nucleation process.