The triangular recycling symbol found on plastic containers is intended to identify the type of plastic resin for recycling purposes, but it is often mistakenly used as a guide for microwave safety. When it comes to plastic number 6, or Polystyrene, this confusion is particularly common, as it is frequently used for disposable food packaging. Understanding how this material reacts to heat is crucial for food safety. This article explains why plastic number 6 is not recommended for microwave use and outlines safer alternatives.
Understanding Polystyrene Plastic
Plastic number 6 is the designation for Polystyrene (PS), a thermoplastic polymer derived from the monomer styrene. Polystyrene exists in two main forms: a rigid, clear plastic or a lightweight, foamed material often referred to by the brand name Styrofoam. Its properties, such as being inexpensive, lightweight, and offering good thermal insulation, make it popular for single-use items. Common examples include foam coffee cups, clamshell take-out containers, disposable plates, and rigid clear plastic cutlery. The Resin Identification Code (RIC) primarily serves to help material recovery facilities sort plastics for recycling.
Why Plastic Number 6 is Not Microwave Safe
Polystyrene is considered unsafe for use in a microwave oven due to its poor heat resistance. This material has a low heat deflection temperature, meaning it begins to soften and lose structural integrity quickly when exposed to heat. Structural failure can start around 100°C (212°F), which is the boiling point of water. Microwaves heat food unevenly and rapidly, often creating hot spots that quickly exceed this temperature threshold. When Polystyrene is heated, it will visibly warp, melt, or shrink, compromising the container’s ability to hold the food.
The Risk of Styrene Leaching
Beyond the physical failure of the container, microwaving Polystyrene introduces a chemical risk. When the plastic is heated, the polymer chains that make up the material begin to break down. This process can cause trace amounts of the chemical building block, Styrene, to leach or migrate into the food or liquid. Styrene is recognized as a substance that is reasonably anticipated to be a human carcinogen. Exposure to this chemical has been linked to adverse health effects, including nervous system damage, especially with repeated or long-term exposure. Foods that are high in fat or oil increase the risk of chemical migration, as Styrene is more soluble in fatty substances. The high heat and uneven distribution of energy in a microwave accelerate the breakdown of the polymer bonds, facilitating the transfer of this compound into the meal.
Safe Containers for Microwave Use
The safest options for reheating food are non-plastic materials such as glass and ceramic. These materials do not contain chemical additives and are engineered to withstand high temperatures without structural or chemical degradation. Always ensure that glass and ceramic items are explicitly labeled as “heatproof” or “microwave-safe” to prevent cracking.
If you prefer using plastic, look for the explicit “Microwave Safe” symbol, which often appears as three wavy lines. In terms of recycling codes, plastic number 5, or Polypropylene (PP), is the most generally accepted microwave-safe plastic due to its higher melting point. Plastics numbered 2 (HDPE) and 4 (LDPE) are sometimes used in microwave-safe products but should only be microwaved if they carry the specific wavy-line symbol.