The temperature plastic can withstand before melting varies significantly because “plastic” encompasses a vast family of synthetic polymers. These materials are manufactured with distinct chemical structures that dictate their thermal properties and performance limitations. The temperature at which a plastic begins to soften, deform, or liquefy varies drastically between polymer types. Understanding these limits is important for safety, especially when plastics are used in food storage, high-heat environments, or industrial applications. Heat resistance is a function of the plastic’s molecular makeup, determining whether it maintains structural integrity or breaks down under thermal stress.
Identifying Plastic Types
The first step in determining a plastic’s temperature limit is identifying its chemical composition, typically communicated through the Resin Identification Code (RIC). This universal coding system features a number, usually inside a chasing-arrows triangle, ranging from 1 to 7.
The six most common plastic types are represented by codes 1 through 6, with category number 7 reserved for all other plastics, including blends and high-performance polymers.
- Code 1: Polyethylene Terephthalate (PET), commonly used for beverage bottles.
- Code 2: High-Density Polyethylene (HDPE).
- Code 5: Polypropylene (PP), often used for microwave-safe containers.
- Code 6: Polystyrene (PS), often found in foam packaging.
Defining Heat Tolerance
A plastic’s thermal performance is measured by several distinct temperature benchmarks, not a single melting point. The Maximum Continuous Use Temperature (MCT) is the most important for long-term applications. This metric defines the highest temperature at which a plastic can function reliably over an extended period without significant degradation of its mechanical or electrical properties.
The Glass Transition Temperature (Tg) applies primarily to amorphous plastics. Below the Tg, the polymer is rigid and glass-like, but above this point, the material transitions to a softer, more rubbery state while remaining solid. For plastics used in structural applications, the Heat Deflection Temperature (HDT) indicates the temperature at which a sample deforms under a specified physical load.
The Melting Point is the temperature at which a crystalline or semi-crystalline plastic fully transitions from a solid to a liquid state, allowing it to flow. Unlike amorphous plastics, which gradually soften, crystalline materials like polyethylene and polypropylene exhibit a relatively sharp melting point.
Specific Temperature Limits for Common Plastics
The specific temperature limits for commodity plastics vary widely based on their molecular structure. Polypropylene (PP) is one of the most heat-tolerant common plastics, with an MCT often around 130°C (266°F). Its melting point is significantly higher, typically ranging between 160°C and 170°C (320°F to 338°F), making it suitable for microwave-safe containers.
Polyethylene Terephthalate (PET) has a high melting point of approximately 255°C (491°F), but its practical heat resistance is much lower. The MCT for PET is only about 60°C (140°F), meaning it rapidly loses structural integrity and deforms when exposed to hot liquids. High-Density Polyethylene (HDPE), commonly used for milk jugs, can handle continuous temperatures up to about 90°C (194°F).
Polycarbonate (PC), often included in the #7 category, is an engineering plastic with better heat tolerance than PET, often possessing an MCT around 125°C (257°F). In contrast, Polystyrene (PS) has a low softening point, often starting to deform between 90°C and 100°C (194°F to 212°F). This demonstrates that a high melting point does not always equate to a high continuous use temperature, which is the more practical measure.
Health Consequences of Overheating
Exposing plastics to heat can trigger a chemical failure known as leaching, which carries potential health consequences. Leaching occurs when heat provides the energy for chemical additives or unreacted monomers to migrate out of the polymer matrix and into contacting food or liquid. This chemical migration can happen at temperatures far below the visible softening or melting point of the material.
A common concern is the leaching of Bisphenol A (BPA) from polycarbonate plastics. When heated, BPA, a chemical with hormone-mimicking properties, can be released into food and beverages. Similarly, Polyvinyl Chloride (PVC) contains plasticizers called phthalates to make it flexible, and these chemicals have also been shown to leach when PVC is heated.
Thermal degradation can also result in the release of Volatile Organic Compounds (VOCs) or toxic fumes when the material is heated significantly above its recommended limits. The chemical risk is tied not only to structural failure but also to the molecular breakdown that occurs with thermal energy exposure.
Real-World Heat Scenarios
Understanding plastic temperature limits is essential for common household scenarios. The concept of “microwave safe” relies on the plastic remaining structurally stable and chemically inert when exposed to high food temperatures, which can exceed 100°C (212°F). Polypropylene (PP) is widely used because it can typically withstand temperatures up to 150°C without deformation or significant leaching.
Dishwasher safety is also a concern, but temperatures are usually lower than in a microwave, peaking between 50°C and 65°C (120°F to 150°F). Most common plastics, including HDPE and PP, can handle these temperatures, though placing them on the top rack is advised to avoid the heating element. The primary risk in a dishwasher is warping from sustained heat exposure.
For simple sterilization with boiling water, the plastic must tolerate 100°C (212°F) without deforming. PP is suitable for this task, while plastics like PET will soften and distort quickly due to their low maximum continuous use limit. Leaving plastic items in a closed car on a hot day can also be risky, as interior temperatures can easily exceed 60°C (140°F), causing deformation in lower-tolerance plastics and triggering chemical leaching.