Expandable polystyrene is the raw material or resin used for the molding of expanded polystyrene Products. It is manufactured in the form of very small polystyrene beads with a weight-average Molecular weight between 160,000 and 260,000 and contains blowing agent, usually pentanes or Butane.

The bead diameter can vary between 0.007-inches (0.2 mm) to 0.12-inches (3.0 mm). EPS is primarily used for building insulation, insulating Concrete forms (ICF), Geofoam, flotation, and, because of its unequaled versatility, packaging of Virtually anything requiring cushioning or insulation properties EPS resin is defined as "expandable polystyrene" resin which is produced by adding blowing agents during polymerizing SM.

There are two basic types of expandable polystyrene, regular And modified. The regular grades are designed primarily for packaging and related uses. Usually these grades Comply with FDA requirements for use in direct food contact use. The modified grades contain a Flame retardant to decrease the potential of fire spread from a small flame source.