The question of “at what temperature does Styrofoam melt” is complex because the material does not melt like ice or metal. Styrofoam is the common, trademarked name for Expanded Polystyrene (EPS), a material widely used for insulation and protective packaging. Its unique structure complicates its thermal behavior when exposed to heat. Understanding its thermal limits requires focusing on the temperatures at which it begins to physically soften or chemically degrade, rather than a single melting point.
Defining the Material
The material commonly called Styrofoam is a lightweight, cellular plastic made from a polymer called polystyrene. This polymer is a thermoplastic, meaning it can be softened by heating and then hardened by cooling, a process that allows it to be molded into various shapes.
The final product, Expanded Polystyrene, is composed of approximately 95% to 98% trapped air. This air is encased in a rigid framework of the polystyrene polymer. The high percentage of trapped air makes the material an effective thermal barrier, but it also means that the polymer itself is distributed very thinly, making the overall structure highly sensitive to thermal changes.
The Thermal Threshold
Polystyrene does not exhibit a distinct, sharp melting point like many other solids. Instead, its thermal behavior is defined by a critical temperature known as the Glass Transition Temperature (\(T_g\)). This is the temperature at which the rigid, glassy polymer chains gain enough energy to begin moving freely.
For the amorphous polystyrene used in EPS foam, this transition occurs around \(90^\circ\text{C}\) to \(105^\circ\text{C}\) (\(194^\circ\text{F}\) to \(221^\circ\text{F}\)). Once the material reaches this range, it begins to soften, distort, and lose the structural integrity that holds its air-filled cells together. This softening temperature is what consumers perceive as “melting,” as the foam visibly warps and collapses.
The actual, theoretical melting point (\(T_m\)), where the polymer chains transition into a true viscous liquid, is much higher, closer to \(240^\circ\text{C}\) to \(260^\circ\text{C}\) (\(464^\circ\text{F}\) to \(500^\circ\text{F}\)). However, this liquid state is rarely observed outside of industrial processing because, at these extreme temperatures, thermal degradation intervenes.
Thermal Degradation and Safety
When polystyrene is exposed to temperatures significantly exceeding its softening point, especially in the presence of flame or extreme heat, it undergoes a chemical breakdown called thermal degradation. This process involves the polymer chains breaking down into smaller, volatile compounds. Thermal degradation in air begins around \(270^\circ\text{C}\) (\(518^\circ\text{F}\)).
Instead of cleanly melting, the material depolymerizes, releasing a complex mixture of gaseous and aerosolized byproducts. The most significant of these is styrene monomer, the chemical building block of the polymer. Other hazardous substances released include carbon monoxide, carbon dioxide, and various volatile organic compounds (VOCs), such as benzaldehyde.
Inhaling the fumes from burning or severely overheated polystyrene presents a health risk, primarily due to these breakdown products. The styrene monomer itself is classified as a likely human carcinogen.
Practical Applications of Heat Resistance
The difference between the softening temperature and the degradation temperature explains why EPS foam is used for hot beverages. Boiling water reaches a maximum temperature of \(100^\circ\text{C}\) (\(212^\circ\text{F}\)). Since this is near the material’s glass transition temperature, the cup can hold the liquid without immediate structural failure. The foam is an excellent insulator that slows the heat transfer to the entire structure.
In contrast, heating the foam in a microwave is strongly discouraged because the heat transfer is fundamentally different. Microwaves heat the food and liquid inside the container. If the contents contain fats or oils, localized hotspots can quickly exceed the \(100^\circ\text{C}\) softening point and even the \(240^\circ\text{C}\) melting range. This rapid, uneven heating causes the container to warp, melt, and leach styrene monomer into the food. Therefore, while EPS can safely contain pre-heated boiling water, it should not be subjected to the intense, uneven heat generated by a microwave oven.