Given the list of options and considerations, how can brand owners and converters decide which packaging material will fulfill the processing, performance and business needs for their food product? In this blog series we will explore the attributes of common rigid plastic food packaging materials, discussing the merits – and drawbacks – of a common material used in food packaging – polystyrene (PS). Continue reading to learn 7 key attributes of PS to consider when selecting a rigid plastic food packaging material for your next project:
Polystyrene, a naturally transparent thermoplastic material, comes in various forms to suit different packaging needs. The most common types include general purpose PS (GPPS), also known as crystal PS, which is created through styrene monomer polymerization, resulting in a clear product.
Another popular variation is expandable PS (EPS), often called "styrofoam," trademarked by Dow Chemical Company in 1941. EPS is commonly used in packaging like foam meat trays and egg cartons, offering versatility and protection for various food products.
High Impact Polystyrene (HIPS) stands out as a specialized form of PS known for its exceptional impact strength. While traditional Homopolymer PS may exhibit brittleness, the addition of 5-10% rubber or butadiene copolymer transforms HIPS into a robust and rigid material, ideal for various packaging applications. This unique blend enhances the toughness and impact resistance of the polymer, ensuring a durable and reliable product for packaging needs.
Polymers come in two distinct morphologies when in a solid state - amorphous and semi-crystalline. While semi-crystalline polymers boast organized and tightly packed molecular chains, amorphous plastic materials showcase a more random and intertwined structure. Picture a plate of cooked spaghetti to visualize the complexity of the polymer chains in amorphous materials like PS. These polymers flow isotropically, meaning they shrink uniformly in all directions, resulting in minimal shrinkage and warping compared to their semi-crystalline counterparts.
Polystyrene (PS) is known as a very easy material to process in thermoforming and form-fill-seal processes. The inherent rigidity of this material offers key functionality such as the snap-ability and score-ability required for form fill seal packaging applications like yogurt multi-packs. In fact, dairy form-fill-seal applications have historically been the largest market for extruded PS.
Furthermore, advancements in certain grades of PS have enhanced its environmental stress crack resistance (ESCR), making it a reliable option for packaging fatty foods like butter. While it performs well in low-heat food packaging scenarios, caution is advised for applications involving temperatures exceeding approximately 185°F, such as hot-fill packaging or microwave use.
PS offers versatility when it comes to decoration, allowing for customization through various printing methods such as screen printing, offset lithography, and flexography. Additionally, compatibility with corona treatment processes ensures excellent adhesion to the material, providing brand owners with the opportunity to create eye-catching and unique designs for their food packaging.
While there are various favorable attributes of PS, which have led to its popularity in a variety of food and medical applications, this material has been the subject of much scrutiny as the packaging industry has placed more and more emphasis on sustainable packaging design and design for recyclability.
Although PS containers are technically recyclable, the material is not considered recyclable in practice or at scale, leading organizations like How2Recycle to classify the material as “Not Yet Recyclable” and “Problematic and Unnecessary” for recycling per the US Plastics Pact.
In addition, the Association of Plastic Recyclers has identified that a limited number of curbside PS collection systems exist in North America, therefore the material does not currently meet the criteria for consumer access to collection established in “APR’s definition of recyclable” or by the U.S. Federal Trade Commission’s Green Guides.