Polystyrene (PS) is a widely used thermoplastic polymer found in protective packaging, disposable food containers, and building insulation. Its versatility comes from its low cost and ability to form both rigid products and lightweight foams. Recycling PS presents significant logistical and technical challenges due to its high volume and susceptibility to contamination. Specialized processes have been developed to ensure this polymer can be effectively re-entered into the manufacturing stream.
Identifying Recyclable Polystyrene Forms
Polystyrene is identified by the Resin Identification Code #6, often seen on the bottom of containers. This category includes two distinct forms of the plastic, each requiring different handling before processing. Rigid polystyrene is the dense plastic used to make products like plastic cutlery, CD cases, and yogurt cups.
Expanded Polystyrene (EPS) is the foam material, often known as Styrofoam, which is approximately 98% trapped air. While beneficial for insulation and cushioning, this high air content creates a volume hurdle for recycling. Transporting bulky EPS foam is often uneconomical because a truckload contains very little actual plastic material. This problem is solved by densification, a pre-processing step that uses heat or mechanical compaction to reduce the volume by up to 90:1, creating dense blocks that are cost-effective to ship.
The Mechanical Recycling Process
Mechanical recycling is the most common method for handling clean, rigid polystyrene and pre-densified EPS. The process begins with the collection of segregated PS waste, which is sent to a materials recovery facility for sorting. Sophisticated optical sorting equipment, often utilizing near-infrared technology, separates PS from other plastic types and contaminants. This initial sorting aims to achieve a high purity level of the incoming material.
The clean polystyrene is then ground into small flakes and subjected to a mandatory cleaning and washing stage. For food-contact items, hot washing with a caustic soda solution removes surface residues and chemical impurities. Following the wash, the flakes enter a specialized “super-cleaning” process designed to meet strict safety standards. Here, the plastic is melted at temperatures around 240 degrees Celsius, and a high vacuum removes volatile contaminants and impurities, pushing the purity level to over 99 percent. Finally, the melted plastic is cut into uniform pellets, a process known as regranulation, creating a material ready for manufacturing new products.
Advanced Chemical Recycling Methods
For mixed, contaminated, or difficult-to-process polystyrene waste, advanced chemical recycling offers a molecular-level solution. These methods break the polymer down into its original chemical building blocks, effectively resetting the material. The most prominent technique is pyrolysis, which uses extreme heat in the absence of oxygen to thermally crack the polymer chains.
Pyrolysis of polystyrene yields pyrolysis oil, which is rich in the original styrene monomer. Up to 67 percent or more of the output can be the styrene monomer, along with other valuable hydrocarbons like ethylbenzene and toluene. Another approach is depolymerization, which chemically converts the waste back into the styrene monomer. These advanced processes neutralize difficult contaminants, such as flame retardants, allowing them to process waste streams otherwise destined for landfill.
Applications of Recycled Polystyrene
The end-use applications for recycled polystyrene depend on the recycling method employed and the resulting material quality. Pellets produced from mechanical recycling are typically used in non-food contact, durable goods. Common applications include the manufacturing of new EPS insulation boards for construction, plastic lumber, picture frames, and coat hangers.
The high-purity styrene monomer recovered through chemical recycling allows for the creation of new polymers that meet the specifications of virgin plastic. This monomer can be repolymerized to produce new polystyrene suitable for direct food contact, such as yogurt pots and food trays. The recycled monomer also serves as a feedstock for other styrene-based products, including acrylonitrile butadiene styrene (ABS) plastic and synthetic rubbers.