Polystyrene is a synthetic polymer created through the linking of many individual styrene molecules. As one of the most widely produced plastics globally, it is derived from petroleum and is known for its versatility and low cost. This material is manufactured in both solid and lightweight foamed varieties, giving it a vast range of uses across numerous industries. Its chemical structure provides unique physical properties that have made it a common material in packaging, construction, and consumer goods, but its widespread use also raises questions about its environmental impact.
The Basic Chemistry of Polystyrene
Polystyrene is formed from the monomer styrene, a liquid aromatic hydrocarbon derived from petrochemicals. The monomer features a vinyl group attached to a bulky benzene ring structure (phenyl group). The polymer is created through addition polymerization, where thousands of styrene monomers link together end-to-end to form long, repeating carbon chains.
The resulting polymer is a homopolymer, composed of only one type of monomer unit. Large, bulky phenyl groups attached along the polymer backbone prevent the chains from packing neatly into a crystalline structure. This results in an amorphous polymer that is transparent and rigid at room temperature. This structure gives solid polystyrene its characteristic hardness and clarity, though it is also relatively brittle.
Different Forms and Common Applications
Polystyrene is engineered into several distinct forms, each tailored for specific commercial and consumer applications based on its physical properties.
General Purpose Polystyrene (GPPS), often called crystal PS, is the pure, unmodified version of the polymer, valued for its high transparency and rigidity. This form is widely used for items where clarity is desired, such as disposable drinking cups, optical disc jewel cases, and clear food containers.
High Impact Polystyrene (HIPS) is created by incorporating a rubber component, typically polybutadiene, into the polymer matrix. The addition of rubber enhances the material’s durability and impact resistance, making it suitable for applications requiring greater toughness. HIPS is opaque and finds use in products like refrigerator liners, yogurt cups, and appliance housings.
The most recognizable forms are the foamed varieties, which include Expanded Polystyrene (EPS) and Extruded Polystyrene (XPS). EPS is produced by expanding tiny polystyrene beads with a blowing agent, creating a lightweight material that is over 95% air. This foam provides excellent thermal insulation and shock absorption, utilized for protective packaging and construction insulation.
XPS is a denser, more rigid foam created through an extrusion process. It offers superior moisture resistance and higher compressive strength, making it effective for foundation and wall insulation in buildings.
Sustainability, Recycling, and Health Concerns
Polystyrene is designated with the resin identification code “6,” but this symbol does not necessarily indicate that the material is accepted by municipal recycling programs.
Recycling Challenges
The foam varieties, especially EPS, present logistical challenges for recycling. Their low density means they take up a large volume while yielding very little plastic material, making the high cost of collecting and transporting them economically unviable compared to other plastic resins.
Most polystyrene products, particularly those used in the food service industry, are prone to contamination. This complicates the sorting and cleaning process for mechanical recycling. The overall recycling rate for polystyrene remains low, contributing to its persistence in the environment where it breaks down into smaller fragments and microplastics.
Chemical Migration and Health
Concerns exist regarding the potential for chemical migration from polystyrene products into food. Styrene, the monomer used to create the polymer, is classified as a probable human carcinogen by some health organizations. Studies show that residual styrene can leach from containers, and this migration increases significantly when the plastic is exposed to elevated temperatures or high-fat or acidic foods. Regulatory bodies monitor these levels to ensure they remain below established safety thresholds, but the material requires careful consideration in food-contact applications.