Expandable polystyrene (EPS) are small polystyrene pellets or beads containing some dissolved liquid, in most cases pentane, sometime butane. The dissolved liquid serves in the later processing stages as a foaming agent.

The processing of the EPS is made in three basic stages, namely: pre-foaming, storage, final expansion.The EPS pellets are produced in most cases by the same companies producing also the conventional polystyrene, as e.g. general purpose polystyrene (GPPS) or high impact polystyrene (HIPS).

The polymer producers supply the EPS pellets to the EPS processors.

The Sulzer process to produce EPS beads

The Sulzer process is a fully continuous process.

Pentane is added in liquid form to a polystyrene melt. Pentane is injected at high pressure to the polymer melt, where it is firstly dispersed and in a later stage dissolved in the polymer melt. The melt with the blowing agent undergoes a thermal treatment phase and is in a further stage cooled down with simultaneous mixing and additional homogenizing. The dispersion, dissolving, thermal treatment, cooling and homogenizing stages are all carried out in static mixing equipment of special design.

The cooled and highly viscous mixture is pressed through fine holes of the suspension granulator. At the outlet of the holes the polymer is solidified to beads by cutting action with rotating knifes in cooled water environment. The same water that solidified the polymer melt is used to transport the beads to a centrifuging and drying system where the polymer beads and the water are separated. The water is cooled and sent back to the pelletizing system and the EPS beads to silos or containers.

The whole process is carried out under conditions that supress any foaming.