Heat transfer in solids is measured by thermal conductivity, a property that determines how quickly a material can transmit heat from a warmer area to a cooler one. This measurement is expressed in Watts per meter Kelvin (W/m·K). Materials that allow rapid heat transfer, such as metals, have high thermal conductivity values, while materials that resist this flow have low values. Understanding this difference is key to knowing how plastics perform when exposed to temperature changes.
Plastic is a Thermal Insulator
Plastic is not a good conductor of heat; it is classified as a thermal insulator. The thermal conductivity of most common plastic resins, like polyethylene or polypropylene, falls between 0.1 and 0.5 W/m·K. This value is extremely low compared to highly conductive materials such as aluminum, which has a thermal conductivity around 205 W/m·K.
This vast difference means that plastic resists the transfer of thermal energy far more effectively than metals. Insulators actively impede the flow of heat, making them feel warm to the touch even when cold because they restrict the rapid transfer of heat away from your hand. Conversely, a good conductor feels cold because it quickly pulls heat away from your skin.
How Heat Moves Through Polymer Chains
Plastic’s poor performance as a heat conductor lies in its molecular structure as a polymer. Heat transfer in solids occurs mainly through two microscopic mechanisms: the movement of delocalized free electrons and the vibration of the atomic lattice, known as phonons. Plastics are non-metallic materials, meaning their electrons are tightly bound in covalent bonds and are not free to move. The absence of these mobile free electrons eliminates the primary mechanism responsible for the rapid heat conduction seen in metals.
Consequently, heat must travel through the plastic exclusively via phonon vibrations, which are waves of atomic displacement. The structure of most plastics is often amorphous, resembling a mass of long, entangled chains rather than a neat, crystalline lattice. This highly disordered arrangement significantly disrupts the smooth propagation of phonons, causing them to scatter. This scattering slows the transfer of thermal energy, resulting in the material’s low thermal conductivity.
Heat moves more effectively along the strong covalent bonds of an individual polymer chain than it does across the chains, where only weak van der Waals forces exist. This difference creates a high resistance to heat flow throughout the bulk material. Even in plastics with some crystalline structure, the amorphous regions and the interfaces between areas act as boundaries that further impede the movement of thermal vibrations.
Practical Applications of Poor Conductivity
The low thermal conductivity of plastic is a desirable property leveraged in many applications. One common use is creating handles for cooking utensils, pots, and appliances. The plastic handles remain cool to the touch, protecting the user from the high heat of the metal by preventing rapid thermal energy transfer.
In construction and refrigeration, plastic foams like expanded polystyrene (EPS) and polyurethane foam are widely used as insulation materials. These materials combine the low conductivity of the polymer with trapped pockets of air or gas, which are also poor conductors, minimizing heat transfer. This insulation is effective for keeping buildings warm and refrigerators cold.
Plastic also plays a role in electrical systems, where its poor thermal conduction pairs with its electrical insulating properties. The plastic sheathing around electrical wiring prevents both heat and electricity from transferring to the surroundings. Insulated beverage containers, such as travel mugs and coolers, rely on plastic components to slow the exchange of heat, maintaining the temperature of drinks for longer periods.