The melting point of plastic is not a single number, but rather a temperature range determined by the polymer’s molecular structure and thermal behavior. Unlike simple crystalline solids, which have a precise melting temperature, plastics are synthetic polymers that exhibit a more complex transition when heated. The temperature at which a plastic begins to soften or flow dictates its suitability for applications ranging from microwave containers to plumbing pipes.
Defining Thermal Behavior: Melting Point vs. Glass Transition
The thermal behavior of plastics is defined by two primary temperatures: the melting point (Tm) and the glass transition temperature (Tg). The melting point applies to the crystalline, or ordered, regions within a polymer structure. True melting is a first-order phase change where the ordered chains break apart rapidly, requiring a significant amount of heat.
The glass transition temperature, in contrast, applies to the amorphous, or disordered, regions of the plastic. This is not a true phase change but a transition from a hard, glassy, and brittle state to a softer, more flexible, and rubbery state. When a polymer reaches its Tg, the molecular chains gain enough energy to begin moving past one another, leading to a noticeable change in mechanical properties.
Polymers are categorized based on these structural features. Amorphous polymers, which are entirely disordered, only exhibit a Tg and soften gradually over a range. Semi-crystalline polymers contain both ordered and disordered regions, displaying both a lower Tg for the amorphous parts and a higher Tm for the crystalline parts. This dual thermal behavior explains why many plastics become rubbery before they finally melt into a liquid.
Structural Factors Governing Thermal Resistance
A polymer’s melting point or glass transition temperature is a direct consequence of its molecular architecture. Longer polymer chains require more energy to overcome the intermolecular forces holding them together, resulting in a higher Tg. The rigidity of the polymer backbone also plays a significant role; chains with bulky or stiff structures, such as those containing aromatic rings, have less flexibility and higher thermal resistance.
Chain branching and packing efficiency are particularly influential. Polymers with significant side-chain branches, like low-density polyethylene (LDPE), are prevented from aligning neatly into tight crystalline structures. This poor packing leads to a reduced melting point because less energy is needed to separate the chains. Conversely, polymers with minimal branching, such as high-density polyethylene (HDPE), pack tightly, resulting in higher crystallinity and a higher Tm.
Intermolecular forces among the chains also contribute substantially to thermal resistance. Strong secondary forces, such as hydrogen bonding found in materials like nylons, dramatically increase the energy required for the polymer to transition. This strong cohesive energy translates directly into higher melting points and greater overall thermal stability.
Thermal Data for Common Household Plastics
The thermal properties of the six most common household plastics demonstrate a wide range of thermal responses:
- Polyethylene Terephthalate (PET, #1), used for beverage bottles, is semi-crystalline. It has a glass transition temperature around 80°C and a melting point (Tm) ranging from 235°C to 260°C. This high Tm allows PET to withstand hot-filling processes, but its Tg explains why a water bottle can soften if left in a hot car.
- High-Density Polyethylene (HDPE, #2), found in milk jugs and detergent bottles, is highly crystalline, with a Tm between 125°C and 137°C.
- Low-Density Polyethylene (LDPE, #4), used for plastic bags and shrink wrap, has more branching, which lowers its Tm to approximately 105°C.
- Polyvinyl Chloride (PVC, #3), used in plumbing pipes and window frames, is primarily an amorphous polymer with a glass transition temperature between 70°C and 90°C. When heated for processing, PVC becomes molten between 160°C and 210°C, but it is prone to thermal degradation if temperatures exceed 170°C, releasing hydrogen chloride gas.
- Polypropylene (PP, #5), frequently chosen for microwave-safe containers and automotive parts, offers good heat resistance due to its melting range between 160°C and 170°C.
- Polystyrene (PS, #6), known for disposable cutlery and foam cups, is an amorphous plastic with a Tg around 100°C. This means it softens drastically near the temperature of boiling water, making foam cups unsuitable for very hot liquids.