The Use of the Viscoelasticity in Polymer Foaming to Obtain a Fully Opened Cell Structure

In this work a design tool to control cell-opening in gas foaming of thermoplastic semicrystalline polymers is developed. The sequence of events following bubble nucleation, namely, bubble growth and impingement, are modeled to gain a comprehensive, perspective view on the mechanisms of bubble wall rupture and on the conditions for achieving a fully opened cell morphology by flow induced crystallization. In particular, unlike the previously published literature, the polymer elastic recovery is recognized as an important factor for wall retraction, often dominant with respect to surface tension, which is typically considered as the only driving force determining the retraction of ruptured bubble walls. The new approach is experimentally validated on poly(e-caprolactone) (PCL), foamed with CO₂, as a model polymer/gas system.