The Solid-State Batch Process
The basic solid-state microcellular process is a three-stage batch process shown schematically below.
In the first stage, the polymer is placed in a pressure vessel with a high-pressure and a non-reacting gas.  This step is usually conducted at room temperature.  Over time, the gas diffuses into the polymer, and attains a uniform concentration throughout the polymer specimen.
The second stage occurs when the specimen is removed from the pressure vessel and brought to atmospheric pressure.  The specimen is in a ‘supersaturated’ state meaning that it is thermodynamically unstable due to the excessive gas dissolved into the polymer.
In the third stage, the supersaturated specimen is heated to a temperature above the glass transition temperature (Tg) of the polymer-gas mixture, termed the foaming temperature.  This step is typically carried out in a heated bath with temperature control.  Since the polymer is still in a solid state, the foams thus produced are called ‘solid-state foams’ to distinguish them from conventional foams that are typically produced in extrusion from a polymer melt.  A number of polymers have been studied in this process such as, for example, PC, PET, ABS, and PVC.  Some examples of solid-state microcellular structures in several polymers are shown below.  The microcellular structure is remarkably uniform compared to the structure in extruded foams or structural foams.