Polystyrene, commonly abbreviated as PS, is a widely used synthetic aromatic polymer. This thermoplastic material is popular due to its low cost, ease of processing, and diverse applications. PS is a ubiquitous material, appearing in everything from food packaging to insulation.
The Chemical Definition of Polystyrene
Polystyrene is chemically defined as a long-chain polymer created from the monomer styrene. The styrene monomer is a liquid hydrocarbon with a distinctive aromatic ring structure attached to a vinyl group. This starting material is typically derived from petroleum or natural gas sources.
Polymerization links thousands of these individual styrene molecules end-to-end to form the long polymer chains of polystyrene. The resulting molecular structure features a carbon backbone with a phenyl, or benzene, ring attached to every other carbon atom.
The presence of these large phenyl groups prevents the polymer chains from packing closely together in a uniform, crystalline manner. This irregular arrangement makes the base material transparent and rigid at room temperature. The restricted rotation of the chains due to the bulky phenyl rings also contributes to the material’s stiffness.
Different Forms of PS Material
Polystyrene is manufactured in various forms, each tailored for specific functions. The three main structural variations are General Purpose Polystyrene (GPPS), High Impact Polystyrene (HIPS), and Expanded Polystyrene (EPS). GPPS, also known as crystal polystyrene, is the clearest and most brittle form of the plastic. It is characterized by its high transparency and rigidity, making it prone to cracking or shattering upon impact.
High Impact Polystyrene (HIPS) is a modified version created by incorporating rubber into the polymerization process. The rubber particles form a separate phase within the plastic matrix, which effectively absorbs energy from impacts. This addition trades the natural transparency of GPPS for superior durability and an opaque, matte finish.
Expanded Polystyrene (EPS) is a foamed version. EPS is produced by introducing a blowing agent, such as pentane, to polystyrene beads. This foaming process traps air within the structure, giving EPS its excellent thermal insulation and shock absorption capabilities.
Common Uses and Applications
General Purpose Polystyrene is often selected for applications where clarity and rigidity are important. Products like disposable clear cutlery, CD and DVD jewel cases, and transparent lids often utilize GPPS. Its high transparency allows for visibility of contents, while its stiffness provides structural integrity for these items.
High Impact Polystyrene is a common choice for items requiring greater resistance to breaking. HIPS is used for durable components such as refrigerator liners and interior parts, appliance housings, and yogurt and dairy containers. Its ability to be easily painted and glued also makes it suitable for signage and toys.
Expanded Polystyrene is best known for its use in protective packaging and insulation. Its lightweight, shock-absorbing nature makes it ideal for protecting fragile goods during shipping, commonly seen as molded protective foam or packing peanuts. The material’s thermal resistance is employed in disposable hot beverage cups and as insulation boards in the construction industry.
Recycling and Environmental Considerations
Polystyrene is identified by the resin identification code number 6. Recycling PS presents considerable challenges, particularly for foamed Expanded Polystyrene (EPS). EPS is difficult to recycle because it consists of up to 98% air, meaning large volumes yield very little actual plastic, making collection and transport economically inefficient.
Contamination, especially from food service applications, further complicates the recycling process, often leading to rejection at sorting facilities. Since PS does not biodegrade, it breaks down into smaller microplastic fragments. These fragments persist indefinitely in marine and terrestrial ecosystems.
A concern relates to the potential for residual styrene monomer to leach from PS products into food and beverages. Migration of styrene is significantly increased when the plastic is exposed to high temperatures or when it is in contact with fatty or acidic foods. Styrene has been classified as a probable human carcinogen, prompting closer scrutiny of PS in food contact applications.