Investigating the Foaming Process of a Thermoplastic Elastomer (TPE) Using Rheology and Image Analysis

This study introduced a method to simultaneously characterize time-resolved rheological and volumetric changes occurring during the foaming process of a thermoplastic elastomer containing an encapsulated physical blowing agent. For this, a conventional rotational rheometer was equipped with a commercial digital camera to capture video recordings. These video recordings were digitized into volumetric data using custom-written computer-vision-based algorithm. We first investigated rheological and volumetric changes during temperature ramp tests. The calculated cell volume fractions varied quantitatively depending on cell expansion and shrinkage. The viscoelastic moduli followed the characteristic behavior of the cell volume fraction. Furthermore, to analyze the volumetric changes under hypothetical processing conditions, a four-stage protocol comprising time sweep and temperature ramp was designed. The results revealed that the cell shrinkage induced by internal gas permeation at high temperatures was significantly greater than that induced by internal pressure reduction during cooling. Finally, comparison between foam densities computed using our algorithm and those measured using a densimeter revealed good agreement within 3% relative error. This demonstrates the applicability of our algorithm for quantitatively assessing volumetric changes of foam.