How expanded polystyrene foam is made

How expanded polystyrene foam is made

Expanded polystyrene foam (EPF) is a plastic material that has special properties due to its structure. Composed of individual cells of low density polystyrene, EPF is extraordinarily light and can support many times its own weight in water.

Because its cells are not interconnected, heat cannot travel through EPF easily, so it is a great insulator. EPF is used in flotation devices, insulation, egg cartons, flats for meat and produce, sandwich and hamburger boxes, coffee cups, plates, peanut packaging, and picnic coolers. It is generally called Styrofoam, Styrofoam is a trademark of Dow Chemical Company and refers specifically to a type of hard, blue EPF used mainly in boating.

Raw Materials:
EPF's main component is styrene (C 8 H 8 ), which is derived from petroleum or natural gas and formed by a reaction between ethylene (C 2 H 4 ) and benzene (C 6 H 6 ); benzene is produced from coal or synthesized from petroleum. Styrene is polymerized either by heat or by an initiator such as benzoyl peroxide. Stopping the polymerization is difficult; inhibitors such as oxygen, sulfur, or quinol can be used. To form the low-density, loosely attached cells EPF is noted for, polystyrene must first be suspended in water to form droplets.

The Manufacturing Process:
Styrene is made by combining ethylene and benzene. The styrene is subjected to suspension polymerization and treated with a polymerization initiator, which together convert it into polystyrene. Once a polymer chain of the desired length has formed, technicians stop the reaction with terminating agents. The resulting polystyrene beads are then cleaned, and anomalous beads filtered out.

Making styrene:
The basic unit of polystyrene is styrene, which is the product of a two-fold reaction. Ethylene and benzene, in the presence of a catalyst such as aluminum chloride, form ethylbenzene (C 8 H 8 ), which is then dehydrogenated (hydrogen is removed) at 1,112-1,202 degrees Fahrenheit (600-650 degrees Celsius) to form styrene (C 8 H 8 ).

Making polystyrene:
Polystyrene is formed from styrene through suspension polymerization, a process by which tiny drops of the monomer are completely surrounded by water and a mucilaginous substance. Supporting and surrounding the styrene globules, the suspension agent produces uniform droplets of polystyrene.

Preparing the beads:
After polymerization is complete, the mixture—consisting of beads made up of polystyrene chains—is cooled. These beads are then washed out and dried. Uniform bead size is achieved by sorting the beads through meshes which filter out over- and undersized beads.

Making expanded polystyrene foam:
The beads of polystyrene must be expanded to achieve the proper density. This process is known as pre-expansion, and involves heating the polystyrene either with steam or hot air, the heating is carried out in a vessel holding anywhere from 50 to 500 gallons. During pre-expansion, an agitator is used to keep the beads from fusing together.

Molding: The beads are fed into a mold of the desired shape. Low-pressure steam is then injected into and between the beads, expanding them once more and fusing them together.

The mold is then cooled, either by circulating water through it or by spraying water on the outside. EPF is such a good insulator that it is hard to cool the mold down. Using small molds can reduce both the heating and cooling time and thereby speed up the process.

Making extruded, expanded polystyrene foam:
This process yields EPF with small cell size that can be used to manufacture boards used for insulation. The beads are melted, and a blowing agent is added. The molten polystyrene is then extruded into the proper shape under conditions of high temperature and pressure.

Quality Control:
EPF is subject to the numerous tests and standards formulated by the American Society for Testing and Materials for plastics in general. Some of these standards concern EPF in particular because of its unique properties, yet some of the tests applied to EPF were developed to measure the properties of other plastics.

The polystyrene melt is tested to determine whether it is sufficiently viscous to produce EPF with the desired properties. The subsequent polystyrene beads must be of uniform size. The standard for perfectly spherical beads is based on those formed in space shuttle experiments under conditions of zero-gravity.

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