Plastic is a vast family of synthetic polymers, each with a unique chemical structure that dictates its response to heat. The temperature at which a plastic changes state is complex, defined by a range of thermal behaviors rather than a single point. The temperature required for a plastic to soften or deform can differ by hundreds of degrees, depending on its molecular arrangement and chemical composition. This variability explains why some plastic containers withstand a dishwasher cycle while others warp instantly in hot water.
The Difference Between Softening and Melting
The terms softening and melting describe two distinct phenomena occurring at different temperatures: the glass transition temperature (Tg) and the melting temperature (Tm). Softening occurs at the Tg, where an amorphous material transitions from a hard, glassy state to a softer, rubbery state. At this temperature, the long polymer chains gain enough thermal energy to begin moving past one another, but the material remains structurally solid.
Melting (Tm) is the point where the material undergoes a true phase change from a solid to a viscous liquid. This phase change is specific to semi-crystalline polymers; they only flow completely when they reach the higher Tm, where the ordered crystalline structures break down. Amorphous plastics lack these ordered regions, so they soften gradually over a temperature range instead of having a sharp melting point.
Key Factors Determining a Plastic’s Heat Tolerance
The wide variation in Tg and Tm across different plastics is directly linked to their molecular architecture. Polymers with highly flexible backbones, such as polyethylene, have low Tg values because the chains require minimal energy to begin moving. Conversely, polymers with rigid structures or large, bulky side groups, like polystyrene, have higher Tg values because these groups impede chain rotation and mobility.
Molecular weight is important, as longer chains result in greater entanglement, which restricts movement and generally increases both Tg and Tm. The presence of chemical cross-links between polymer chains significantly limits their ability to move, which can drastically raise the Tg or prevent the material from melting entirely. Heat resistance is a function of how tightly the polymer chains are bound to one another.
Thermal Profiles of Everyday Plastics
The plastics encountered most often in daily life exhibit distinct thermal profiles based on their composition.
Polyethylene Terephthalate (PET)
PET (code 1), commonly used for beverage bottles, has a glass transition temperature (Tg) ranging from 67°C to 81°C, and a distinct melting point (Tm) near 260°C. This high Tg allows it to maintain its shape at room temperature.
High-Density Polyethylene (HDPE) and Polypropylene (PP)
HDPE (code 2), found in milk jugs, has a very low Tg, often between -125°C and -80°C, meaning it remains flexible even in freezing conditions. Its melting temperature ranges from 120°C to 135°C. PP (code 5), used in yogurt containers and microwavable trays, has a Tg around -9°C and a melting range between 145°C and 195°C, making it suitable for items exposed to hot liquids.
Polystyrene (PS)
General purpose Polystyrene (PS) (code 6), used for disposable cutlery and foam cups, is an amorphous plastic that softens around 90°C to 105°C. Since it lacks crystalline structure, it does not have a precise melting point and will simply deform and flow once it exceeds its Tg. These thermal limits explain why a PS foam cup cannot hold boiling water, while a PP container can be safely microwaved.
Safety and Practical Implications of Heating Plastics
Exposing plastic containers to heat, even below the softening point, increases the migration of chemical additives into food and liquids. Heating provides the energy necessary for compounds like bisphenol A (BPA) and phthalates, which are not chemically bonded to the polymer chains, to leach out. This chemical leaching occurs in common scenarios, including microwaving food in plastic or washing plastic dishes in a high-temperature dishwasher cycle.
Heating plastic also risks releasing microplastics and nanoplastics into contents, especially if the material is heated near its Tg. If plastic is heated to the point of thermal degradation or combustion, it releases hazardous volatile organic compounds (VOCs). For example, superheated polystyrene releases styrene and benzene, while chlorine-containing plastics like PVC release highly toxic fumes such as dioxins and furans upon burning.