A review on semi-crystalline polymer bead foams from stirring autoclave: Processing and properties

Bead foams have been widely used for more than 70 years, with expandable polystyrene and expanded polypropylene (EPP) being the main materials. There are different processes for producing bead foams, depending heavily on the material used (e.g., their thermal behavior). EPP is usually produced by a discontinuous stirring autoclave process, which is the main subject of this study. In this process, thermal treatment during the saturation phase leads to the formation of a second melt peak, which is considered an important prerequisite for the subsequent welding process, in which the individual foamed beads are welded into complex shaped parts by applying saturated steam to the beads in a cavity (so-called steam chest molding). To date, EPP is one of the main bead foams used industrially, but other materials such as polylactide and thermoplastic polyurethane can also be processed using the same technique. This review focuses on the important thermal and physical mechanisms during saturation and expansion and the parameters (material and process) that affect them. The process itself provides multiple possibilities to influence thermal behavior (i.e., crystallization) and expansion. Typical properties of resulting bead foams, which are found in many applications, are also considered.