The foam cups commonly used for hot and cold beverages are often incorrectly referred to by the trademarked name “Styrofoam.” These cups are actually made from Expanded Polystyrene (EPS), a foamed plastic. EPS is widely used because its unique properties result directly from its chemical composition and specialized manufacturing process.
The Core Material: Expanded Polystyrene
The underlying substance of the foam cup is polystyrene, a type of plastic polymer. Polystyrene is formed by linking many smaller chemical units called styrene monomers into long chains. The polymer structure consists of a chain of carbon atoms with a phenyl group—a six-carbon ring—attached periodically. This backbone gives the material its basic strength and thermoplastic properties.
The generic name for the material used in foam cups is Expanded Polystyrene (EPS), which is a white, rigid, closed-cell foam structure. This is distinct from the trademarked product, Styrofoam, which is manufactured by Dow Chemical and is typically a blue or pink extruded foam used primarily for building insulation. The polystyrene component makes up only about two percent of the final volume of the foam cup, with the remaining 98 percent being trapped air.
How the Foam is Formed
The process of transforming solid polystyrene into the airy structure of a foam cup involves a physical foaming process. It begins with tiny, hard polystyrene beads impregnated with a blowing agent, most commonly a hydrocarbon gas called pentane. The beads are placed into a pre-expander, where they are exposed to steam heat.
The steam causes the polystyrene to soften while simultaneously heating the pentane blowing agent above its boiling point. This rapid vaporization forces the plastic beads to expand significantly, often reaching 40 to 50 times their original size. These pre-expanded beads, called “prepuffs,” are then allowed to age briefly to stabilize their internal pressure before they are moved to the final molding stage.
In the final step, the prepuffs are placed into a mold shaped like a cup and subjected to another burst of steam. This second exposure causes the beads to soften again and expand further, fusing them together into a solid, lightweight mass that conforms to the shape of the mold. The finished cup is then cooled and ejected, having achieved its characteristic low density and cellular foam structure.
Properties Driving Their Common Use
The foamed structure of EPS provides several performance characteristics that make it highly effective for beverage containers. Its most recognized property is its excellent thermal insulation, which results from the high percentage of air trapped within the closed-cell structure. These thousands of tiny, sealed air pockets create a barrier that slows the transfer of heat, keeping hot drinks warm and cold drinks cool for extended periods.
The material’s lightweight nature is a major factor in its common use. Since the cup is 98% air, it is very light, which minimizes transportation costs and makes the cups easy to handle for both vendors and consumers. The low material density and straightforward molding process also contribute to a low production cost, which is a significant factor for high-volume disposable consumer goods. The material also offers a degree of cushioning, providing some protection against minor impacts.
Environmental and Disposal Considerations
The widespread use of foam cups has led to significant environmental challenges, primarily related to their disposal. Expanded Polystyrene is not biodegradable, meaning that when discarded, it persists in the environment for hundreds of years. Due to its very low density and high volume, it takes up a disproportionate amount of space in landfills compared to other forms of waste.
Recycling EPS presents difficulties because of the material’s composition and structure. The low density makes collection and transport economically inefficient, as a large volume of material is required to make up a useful weight for processing. Furthermore, foam cups are often contaminated with food and drink residue, which makes them unsuitable for many conventional recycling systems.
A public concern involves the potential for residual styrene to leach from the cup material into the beverages it holds. Studies show that trace amounts of styrene monomer can transfer into liquids, with the rate increasing when the liquid is hot, acidic, or contains fat. Regulatory agencies, such as the U.S. Food and Drug Administration, consider the minute exposure levels from food-contact EPS to be safe. However, the issue of chemical migration remains a topic of discussion regarding food safety.