Plastic cups, ranging from disposable containers to durable tumblers, are made from diverse synthetic materials called polymers. Understanding the specific polymer used is important because the material dictates the cup’s function, safety profile, and disposal method.
Primary Polymers Used in Cup Manufacturing
Most plastic cups are constructed from a few common polymer types, selected for distinct chemical and physical properties. These polymers are identified by a common acronym and a corresponding Resin Identification Code (RIC). The RIC is a number, typically found inside a chasing-arrows triangle, that identifies the resin type for sorting purposes.
Common Plastic Polymers
- Polyethylene Terephthalate (PET, RIC 1) is prized for its glass-like clarity, making it popular for cold drinks like iced coffee and smoothies.
- Polypropylene (PP, RIC 5) is known for its durability, flexibility, and resistance to high temperatures. This heat resistance makes PP a frequent choice for reusable cups that must withstand hot liquids and dishwashers.
- Polystyrene (PS, RIC 6) is used for both brittle, clear cups and its foamed counterpart, which provides thermal insulation for hot beverages.
- Polylactic Acid (PLA) is a bioplastic derived from renewable sources like corn starch, often categorized under RIC 7. PLA is valued for being compostable under industrial conditions, though it lacks the heat stability of traditional plastics.
Matching Material Properties to Function
Manufacturers select specific polymers based on required properties like optical clarity, impact resistance, thermal stability, and manufacturing cost. The high transparency and low cost of PET make it the standard for single-use cold beverage cups.
PP is the preferred material for reusable stadium cups or food-service containers because its higher melting point allows it to be repeatedly washed in high-temperature dishwashers without warping. PS is often chosen for its rigidity and economical production, although its lower temperature threshold means it can deform when exposed to very hot liquids. For durable, high-end reusable cups, a material like Tritan, a copolyester, is frequently used for its superior impact resistance and heat stability. PLA functions as an environmentally conscious alternative for cold drinks, as its low glass transition temperature means it cannot hold hot liquids without losing structural integrity.
Understanding Recycling Codes and Disposal
The Resin Identification Code (RIC) serves as a guide for material sorting, but it does not guarantee that a cup is recyclable in any given community. The chasing-arrows symbol with a number inside is a material identifier, not a universal sign of local recycling feasibility. PET (RIC 1) and PP (RIC 5) are generally the most widely accepted plastics by recycling facilities due to established markets for their recycled material.
The recycling process for other types of cup material is often more complex or non-existent in many areas. Polystyrene (RIC 6), particularly in its foamed state, is rarely collected because of its low density and high transportation cost relative to its material value. PLA, often marked as RIC 7, requires specialized industrial composting facilities, and if mixed with traditional plastics, it can contaminate the recycling stream. Furthermore, even widely recyclable plastics like PET can be rejected if they contain significant food or liquid residue.
Health and Chemical Safety Concerns
A primary concern for consumers revolves around the potential for chemicals to transfer from the plastic into the beverage, a process known as leaching. This leaching is often triggered or accelerated by high temperatures, acidic liquids, or physical wear. Historically, the focus was on Bisphenol A (BPA), an industrial chemical once commonly found in hard polycarbonate plastics used for reusable containers.
Though PET and PP are naturally BPA-free, the shift to “BPA-free” alternatives has led to materials like Tritan, engineered to avoid the chemical altogether. Research indicates that heat exposure can cause other compounds, including chemicals in PS, to migrate into liquids, especially hot ones. Another growing concern is the shedding of microplastic particles, which studies have found can be released from cups when exposed to the heat of hot drinks or physical wear over time.