Plastic containers offer a lightweight and durable solution for storing, transporting, and preserving food and beverages. Understanding their composition is relevant for recycling and for making informed decisions about food safety and potential health implications. The composition determines characteristics like heat resistance and flexibility, and dictates how the container should be used. Plastics are a diverse family of synthetic materials, each with a unique chemical makeup tailored for specific applications.
The Resin Identification Codes
Consumers identify the type of plastic resin used in a container by the Resin Identification Code (RIC), a number from one to seven enclosed within a chasing arrows symbol. This standardized numbering system, administered by ASTM International, was designed to help recycling facilities sort plastics by their polymer type. The number corresponds to the specific family of polymer resin constituting the container’s main body.
The RIC codes are:
- Code 1: PET (Polyethylene Terephthalate) is commonly used for single-use beverage bottles, known for its clarity and strength.
- Code 2: HDPE (High-Density Polyethylene) is a stiffer plastic frequently found in milk jugs, detergent bottles, and some food storage tubs.
- Code 3: PVC (Polyvinyl Chloride) is used in items like cling wrap and piping, but its use in food packaging has decreased.
- Code 4: LDPE (Low-Density Polyethylene) is a flexible plastic used in plastic bags, squeezable bottles, and some food storage lids.
- Code 5: PP (Polypropylene) is a heat-resistant material often used for yogurt containers, straws, and microwavable food containers.
- Code 6: PS (Polystyrene) is used to make foam cups and clam-shell takeout containers.
- Code 7: “Other” includes all other plastic resins, such as polycarbonates, acrylics, and multi-layer plastics.
Defining the Core Building Blocks
The synthetic origin of plastic containers begins with small molecules called monomers, the fundamental building blocks of all plastics. These monomers are primarily derived from fossil fuels, specifically petroleum and natural gas. The process of chemically linking these individual monomers together is known as polymerization, which creates very large molecules, or polymers. The resulting polymer is a long chain made up of repeating monomer units, which gives the final plastic material its distinct physical properties. For example, the monomer ethylene is polymerized to create the polymer polyethylene, the basis for both HDPE and LDPE plastic containers.
Essential Additives and Fillers
Plastic containers require the addition of various chemical compounds, known as additives, to achieve the desired performance characteristics. These substances are mixed into the resin to enhance flexibility, stability, color, and resistance to degradation. Plasticizers, such as phthalates, are incorporated to make rigid plastics, like PVC, much more pliable and soft. Stabilizers, including UV stabilizers and thermal stabilizers, prevent the plastic from breaking down when exposed to sunlight or high heat. Bisphenol A (BPA) was historically used in polycarbonate plastics (Code 7) to make them clear and shatterproof. Due to concerns about BPA mimicking hormones, manufacturers introduced substitutes like Bisphenol S (BPS) and Bisphenol F (BPF), though these replacements have also faced scrutiny.
Specific Applications and Safety Considerations
The Resin Identification Codes offer a guide to the suitability of a container for specific uses, especially concerning food and beverage contact. PET (Code 1) is selected for single-serving drink bottles because of its gas barrier properties, which help keep carbonated drinks fizzy. PP (Code 5) is frequently used for containers meant for hot foods and microwaving due to its superior heat resistance. Safety concerns predominantly center on the risk of additives and plasticizers leaching out of the container and into the stored food. Heat significantly accelerates this leaching process, which is why experts recommend avoiding microwaving any plastic not specifically labeled as microwave-safe. Containers made from HDPE (Code 2), LDPE (Code 4), and PP (Code 5) are generally considered safer options for food storage, as they have a low tendency to leach harmful chemicals. Storing acidic or fatty foods in plastic, or physical damage like scratches, can also promote the migration of these substances.