The material commonly referred to as “Styrofoam” is a lightweight, foam-like plastic derived from petroleum. This foam is a specialized form of polystyrene, a synthetic polymer used in many everyday products from CD cases to plastic cutlery. Styrofoam, however, is a trademarked brand name belonging to the Dow Chemical Company. It specifically denotes a type of blue or pink extruded polystyrene foam (XPS) used primarily for building insulation. This article explains the transformation steps that convert the raw liquid into the generic foam known as expanded polystyrene (EPS) and XPS.
Converting Styrene into Polystyrene
The manufacturing journey begins with styrene, a liquid hydrocarbon obtained through the refining of crude oil. Styrene is a small molecule, or monomer, that serves as the fundamental building block for the final material. The conversion into plastic involves polymerization, a chemical process where thousands of these individual styrene molecules are linked into long, repeating chains.
This chain-building reaction is typically initiated using heat, often above 60 degrees Celsius, combined with chemical initiators like peroxides. The heat causes the initiator to create free radicals, which react with the styrene monomers, forcing them to bond. This process transforms the clear liquid monomer into solid polystyrene, a hard, colorless plastic resin.
The resulting solid polystyrene is processed into small, uniform pellets, known as “virgin beads.” These beads are the foundational material for foam production. To prepare them for expansion, a blowing agent is introduced into the beads during or immediately after the polymerization stage.
The Expansion Process
The transformation of solid polystyrene beads into lightweight foam relies on a physical mechanism centered around a volatile chemical called a blowing agent. This agent, most commonly pentane, is infused under pressure into the virgin polystyrene beads. Pentane is a hydrocarbon that is liquid inside the bead but has a very low boiling point.
The foaming process begins when these impregnated beads are exposed to intense heat, usually delivered by steam. The steam softens the plastic matrix and simultaneously causes the liquid pentane to vaporize. This rapid vaporization forces the softened plastic to expand dramatically, acting like millions of tiny balloons within the bead.
This steam-induced expansion can increase the volume of the original bead by 20 to 50 times, creating a structure that is roughly 98% trapped air. This initial expansion is known as pre-expansion, producing light, fluffy “pre-puffs.” These pre-puffs are then allowed to stabilize, or “age,” in silos for several hours to allow the internal pressure to equalize before the final molding stage.
Distinguishing Expanded and Extruded Polystyrene
Once the pre-puffs are stable, they are processed into Expanded Polystyrene (EPS) or Extruded Polystyrene (XPS), differing based on their manufacturing technique. EPS, used for packaging peanuts, disposable cups, and coolers, is created using a batch process. The pre-expanded beads are poured into a large mold and subjected to a final blast of steam, causing them to expand and fuse together into the final shape.
The resulting EPS product has a characteristic granular appearance because the individual, fused beads are still visible. This structure is excellent for cushioning and general insulation, but it makes EPS slightly more permeable to water vapor over time. Its finished form is often easily crumbled, reflecting its beaded construction.
XPS, the material sold under the Styrofoam brand, is manufactured through a continuous extrusion process that yields a stronger, more uniform product. Instead of pre-expanded beads, the solid polystyrene resin, blowing agent, and other additives are melted together into a viscous fluid. This molten mixture is continuously forced through a specialized die, where the pressure drop causes the blowing agent to foam the mixture.
The extrusion method produces a foam with a completely closed-cell structure and a smooth, continuous surface, without the visible beads of EPS. This uniformity results in a material that is denser, more rigid, and significantly more resistant to moisture absorption. This makes XPS the preferred choice for continuous insulation boards in construction.