ABSTRACT
Foaming agents are a vital and necessary ingredient used in the production of extruded foamed thermoplastics. Foaming agents have several different but similar denitions. Most generally, a foaming agent is a material that, in the vapor phase, expands a thermoplastic melt upon reduction in pressure. Physical foaming agents are materials that are injected into the process as either liquids or gases. Chemical foaming agents (CFAs) are materials that decompose to generate gases during the processing. Some physical foaming agents are low-boiling-point liquids, such as pentane or isopropyl alcohol, that remain liquids in the thermoplastic melt while the melt is under pressure. When the pressure is reduced, the foaming agent quickly changes from liquid to vapor and comes out of solution with the polymer to expand the melt. Another type of foaming agent includes the so-called inert gases, such as carbon dioxide or nitrogen. These materials dissolve as vapors in the plastic melt and come out of solution as vapors to expand the plastic melt. CFAs utilize a decomposable material that produces a gas or gases during decomposition. In this case, CFAs must rst decompose, and then the gas produced behaves much like a physical foaming agent but with some effects inuenced by the residual material from the CFA decomposition.
