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What is Expanded Polystyrene?

Adam Hill
Adam Hill

Expanded polystyrene is a plastic foam material that has certain desirable properties because of its structure. It is extraordinarily light and buoyant, and a good insulator against heat and sound. It can be used as a building material or a design element, and can be molded into many shapes for a number of household uses as well.

In most cases, expanded polystyrene foam is white and is composed of small, interconnected beads. It is made by combining the chemicals ethylene and benzene, to make a compound known as styrene. The styrene is then treated with other chemicals which cause the styrene molecules to polymerize, or to group together in long chains. This reaction is only allowed to proceed to a certain point, and is then stopped. The resulting beads are allowed to cool and are then cleaned.

Expanded polystyrene is a good insulator against heat.
Expanded polystyrene is a good insulator against heat.

After formation and cleaning, the beads must be expanded, which happens in three main stages. First, the beads are heated with hot air or steam until their density is three percent of its original value. The beads are then cooled for 24 hours and molded. Once inside the mold, they are injected with low-pressure steam, which further expands the beads and fuses them together. When the mold is cooled, the expanded polystyrene is finished and ready for use or shipment.

Expanded polystyrene is technically recyclable.
Expanded polystyrene is technically recyclable.

Expanded polystyrene differs from a similar product, called extruded polystyrene, in important ways. Extruded polystyrene is made using chlorofluorocarbons (CFCs), which are considered by many to be harmful to the balance of ozone in the earth’s atmosphere. Expanded polystyrene is made without these compounds, making it more benign to the environment. Both products, however, can be recycled like all plastics.

Another important advantage of expanded polystyrene, especially for products like disposable cups, is that it is very cost-effective. Manufacturing polystyrene foam requires much less energy than the manufacture of paper-based alternatives. Additionally, it has the potential to create much less waste than paper. For example, when properly burned, one ton (907 kg) of polystyrene cups produces only 0.2 ounces (5.66 g) of ash, whereas the same amount of paper produces 200 pounds (90.7 kg) of ash.

It is also notable that polystyrene foam does not biodegrade. This is considered a disadvantage by some, but the fact that it is chemically inert makes it a stable fill material which helps provide for safe and sanitary landfill reclamation. Despite this fact, the prevailing trend has been to reduce and recycle polystyrene foam wherever possible.

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Discussion Comments


What is the ratio ton of styrene/ton of polystyrene produced? Most polymers is close to 1. Is it the same with PS?


how about mentioning all the pollution in the streams, oceans, marine life due to EPS?


what is expanded polystyrene: thermo or thermo-set? thanks to anybody who answers my question.


thanks a lot for this useful information.


its awesome and it helped me with a project.

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    • Expanded polystyrene is a good insulator against heat.
      By: peuceta
      Expanded polystyrene is a good insulator against heat.
    • Expanded polystyrene is technically recyclable.
      By: RTimages
      Expanded polystyrene is technically recyclable.