The Supercritical State Paradigm in Thermoplastic Foaming

"Supercritical" has become automatically and intrinsically associated with "carbon dioxide foaming", with this combination generally resulting into claimed benefits such as microcellular structure, high cell density and enhanced mechanical properties. However the supercritical state remains accessible to any blowing agents. The newly-introduced hydrofluorocarbons (HFCs), following the ban put on chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) in the polystyrene foam industry, are other examples of foaming agents that are processed under temperature and pressure conditions that belong to the supercritical region. Unfortunately, such unusual conditions have resulted in the past into typical processing difficulties that make the resulting benefits associated to the supercritical state questionable. The supercritical state may not be apparently the panacea usually claimed ... and sought for! In this work, density characteristics of supercritical fluids were reviewed and linked to the foam nucleation stage. Foaming experiments using a specific polymer-carbon dioxide system were conducted at the vicinity of the critical locus and the sequence leading to phase separation monitored using an ultrasonic technique. The results suggested a phase separation mechanism similar to that of spinodal decomposition. Density fluctuations reported for supercritical fluids close to their critical locus would translate for the foaming process into concentration fluctuations typical of the spinodal decomposition.