Supercritical fluids in thermoplastics foaming : Facts or fallacies?

The past two decades have seen extensive interests and efforts for developing processes based on supercritical fluids (SCF). Microcellular foaming is one of these processes that take advantage of the unique properties of supercritical fluids when they are used as physical foaming agents (PFA). In this technology, the emphasis has been mostly focused on inert gases such as carbon dioxide and nitrogen that both inherently provides very high cell densities and very small cell sizes. Incidentally the benign carbon dioxide is frequently considered as the panacea of PFA, in response notably to the environmental pressures related to the destruction of the ozone layer. Hydrofluorocarbons (HFCs) have also been identified as potential alternative agents for extruded polystyrene foam. Unfortunately, HFCs remain difficult to process at the high concentrations required to yield low-density foams. Surprisingly, the processing difficulties occur when the pressures required for dissolving high HFCs concentrations reach the range located immediately above the critical pressure of the PFA used. PS/HFC systems have been well documented in terms of abnormal behaviors occurring as the foaming agent gets into the supercritical conditions, and similar observations have also been made for other SCF used for thermoplastic foaming. These observations are reported here, and attempts are made to link the supercritical nature of the fluid to the PFA heterogeneities suspected under these conditions.