Polystyrene, commonly known as styrene plastic, is a widely used synthetic polymer derived from the chemical building block called styrene. Styrene is a liquid hydrocarbon that is transformed into a solid material through polymerization. Polystyrene’s combination of low cost, lightweight nature, and versatility allows it to be molded into countless products, ranging from clear, rigid items to light, shock-absorbing foams. Its prevalence, however, has raised questions regarding its chemical safety and environmental fate.
The Chemical Composition of Styrene Plastic
The solid plastic known as polystyrene (PS) is chemically distinct from its starting material, the styrene monomer. The styrene monomer is a clear, oily liquid, which is an organic compound with a vinyl group attached to a benzene ring. This structure provides the chemical reactivity necessary for the polymerization process.
Polymerization links thousands of these individual styrene molecules together into long, chain-like macromolecules. The resulting polystyrene is a long-chain hydrocarbon polymer that is solid and inert at room temperature. Ring-shaped phenyl groups hanging from the carbon backbone interfere with the chain’s motion, which imparts the plastic’s characteristic rigidity. Polystyrene is a thermoplastic, meaning it softens and melts when subjected to heat, allowing it to be easily molded and reformed during manufacturing.
Common Applications of Polystyrene
Polystyrene is manufactured in several forms to suit a wide variety of applications, each with distinct physical properties. General Purpose Polystyrene (GPPS) is the standard, rigid, and transparent form of the plastic. This clear, brittle material is commonly used for products requiring visual clarity, such as CD cases, disposable cutlery, and transparent food containers.
High Impact Polystyrene (HIPS) is created by incorporating a small percentage of rubber, typically polybutadiene, into the polymer structure. This addition improves the plastic’s durability and impact resistance, making the material opaque. HIPS is utilized for applications requiring greater strength, including yogurt cups, appliance housings, and refrigerator liners.
A third major form is Expanded Polystyrene (EPS), the familiar foam material often mistakenly called Styrofoam. This form is made using a blowing agent, such as pentane, to create a final product that is over 95% air. EPS is lightweight, provides thermal insulation, and offers superior shock absorption, making it ideal for protective packaging and construction insulation.
Health and Disposal Considerations
The safety of polystyrene centers on the potential for residual styrene monomer to migrate from the plastic into foods and beverages. Regulatory bodies generally approve polystyrene for food contact; however, studies indicate that chemical leaching is more likely when the plastic is exposed to heat, or when it holds acidic or fatty contents. Exposure to high temperatures, such as in a microwave, can trigger the release of residual styrene and other compounds like benzene, which are classified as probable human carcinogens.
Polystyrene products are marked with the resin identification code #6, indicating that the plastic is theoretically recyclable. However, polystyrene is difficult and economically challenging to recycle due to its low density and high volume, which makes transportation costly. Expanded Polystyrene (EPS), in particular, is easily fragmented, contributing significantly to litter and marine debris.
The plastic also presents a long-term environmental burden because it does not readily biodegrade, meaning it can persist in landfills or the natural environment for hundreds of years. The manufacturing process itself has been cited as a source of hazardous waste. Consequently, many municipalities and businesses are exploring alternatives to mitigate both the potential health risks and the environmental impact of disposal.