Omar Khalid
The thermal conductivity of a substance is its ability to conduct heat and is measured in watts per meter per kelvin [W·m−1·K−1]. Metals are considered excellent conductors of heat due to their free-moving electrons, which allow them to rapidly transfer heat. Plastics, on the other hand, are poor heat conductors due to their atoms' inability to vibrate quickly. The thermal conductivity of plastics varies depending on their composition and structure, with values ranging from 0.02-0.05 W/(m/K) for most plastics to 0.33 W/(m/K) for Styrofoam. This article will explore the factors influencing the thermal conductivity of plastics and how these materials are applied in various contexts.
| Characteristics | Values |
|---|---|
| Thermal conductivity of most plastics | 0.02-0.05 W/(m/K) |
| Thermal conductivity of Styrofoam | 0.33 W/(m/K) |
| Thermal conductivity of aluminium | 205 W/(m/K) |
| Thermal conductivity of galvanized steel | 52 W/(m/K) |
| Thermal conductivity of teflon, PVC, and ABS | N/A |
| Thermal conductivity of polyethylene terephthalate (PET) | Higher than average due to efficient heat transfer along ordered chains |
| Thermal conductivity of amorphous plastics like polystyrene | Lower than average |
| Thermal conductivity of high-density polyethylene (HDPE) filled with aluminium particles | Exceeds 1 W/m·K |
| Thermal conductivity of polycarbonate with added fillers | N/A |
| Thermal conductivity of PI and PEEK (polyether ether ketone) | Moderate |
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What You'll Learn
How is the thermal conductivity of plastic measured?
The thermal conductivity of plastics is measured to understand the processing of the material and establish its appropriate applications. Plastics with highly ordered crystalline structures, such as polyethylene terephthalate (PET), exhibit higher thermal conductivities due to efficient heat transfer along ordered chains. Conversely, amorphous plastics like polystyrene have lower conductivity. The thermal conductivity of plastics can be increased by the incorporation of thermally conductive fillers such as graphite, boron nitride, or metal oxides. For instance, high-density polyethylene (HDPE) filled with aluminum particles can achieve values exceeding 1 W/m·K.
There are various methods available to measure the thermal conductivity of plastics. One common method is the use of the C-Therm Trident Thermal Conductivity Instrument, which provides a fast and easy way to measure the thermal conductivity of plastics and polymers. Both the Modified Transient Plane Source (MTPS) and Transient Plane Source (TPS) can rapidly and accurately measure the thermal conductivity of polymers under different environmental conditions. The Transient Line Source (TLS) is used to measure the thermal conductivity of polymer melts.
Another method is the guarded hot plate apparatus, which is used to measure the thermal transmission properties of homogeneous insulation materials. This is done by placing a solid sample of the material between two plates, one heated and the other cooled or heated to a lesser extent. The temperature of the plates is monitored until they reach a constant temperature. This method is outlined in ASTM C177, which measures steady-state heat flux, and ISO 8302, which determines steady-state thermal resistance and related properties.
Additionally, there are steady and unsteady methods for measuring thermal conductivity. In steady methods, the thermal conductivity is determined from the amount of heat flux (W/m2) that moves through a unit cross-sectional area per unit time. Unsteady methods, which are more commonly used today due to quicker measurements, determine thermal conductivity from the rate at which heat diffuses through the specimen (thermal diffusivity, m2･S-1). These methods include the laser flash method and the hot disk method.
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Why is plastic a poor heat conductor?
Plastic is a poor conductor of heat due to the nature of its atomic structure. The atoms in plastic are tightly bound together, which restricts their ability to vibrate and move quickly. This is important because the transfer of thermal energy occurs through vibrating molecules colliding with one another. Therefore, since the atoms in plastic cannot vibrate quickly, thermal conduction does not occur efficiently.
The thermal conductivity of plastic is extremely low, ranging from 0.02-0.05 W/(m/K). In comparison, aluminium has a thermal conductivity of 205 W/(m/K), which is substantially higher and allows for a much faster transfer of heat. Metals, in general, have high thermal conductivity due to the loosely bound electrons on their atoms, which readily vibrate and move under a heat source, easily distributing thermal energy.
Some plastics have higher levels of thermal conductivity than others. Synthetic polymers, for example, can exhibit high conductivity traits and act as electrical conductors. On the other hand, polyurethane and polystyrene, two common types of plastic used in household items, have lower levels of thermal conductivity. Styrofoam, a type of polystyrene, has a thermal conductivity of approximately 0.33 W/(m/K) due to the abundance of trapped air bubbles that restrict energy flow.
The low thermal conductivity of plastic has practical implications in everyday life. Plastic cookware and dishes, for instance, remain safe to touch even after being heated in a microwave. Plastic is also used as an insulator to protect electrical components and systems from heat and electricity. These properties make plastic a useful material for specific applications where heat conduction needs to be minimised.
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How does plastic thermal conductivity compare to metal?
Plastic has a low thermal conductivity compared to metal. Thermal conductivity (k) is the measure of a material's ability to conduct heat. The thermal conductivity of most common plastics ranges between 0.1 to 0.5 W/m·K, with an average of 0.02-0.05 W/(m/K). Metals, on the other hand, are excellent conductors of heat due to their loosely bound electrons, which readily vibrate and move under a heat source, transferring thermal energy efficiently. For example, aluminum, a common metal used for beverage containers, has a thermal conductivity of 205 W/(m/K), which is substantially higher than any plastic. This means that aluminum transfers heat 100,000 times more efficiently than plastic per unit of distance under the same environmental temperature.
The difference in thermal conductivity between plastic and metal is demonstrated by the ice cube experiment. When ice cubes are placed on metal and plastic blocks, the cube on the metal block melts much faster than the one on the plastic block. This is because metal is a better conductor of heat, so it transfers energy more quickly to the ice cube. However, metals feel cold to the touch because energy conducts away from our fingers into the metal when we touch it, lowering our skin temperature. Plastics, on the other hand, are good insulators, so even if they are at a lower temperature, they feel warm because little energy conducts from our fingers into the plastic.
The thermal conductivity of plastics can vary depending on their composition and structure. For example, plastics with highly ordered crystalline structures like polyethylene terephthalate (PET) have higher thermal conductivities due to efficient heat transfer along their ordered chains. In contrast, amorphous plastics like polystyrene have lower conductivity. The incorporation of thermally conductive fillers such as graphite, boron nitride, or metal oxides can also enhance a polymer's thermal conductivity. For instance, high-density polyethylene (HDPE) filled with aluminum particles can achieve values exceeding 1 W/m·K.
The choice between using plastic or metal for thermal insulation depends on the specific application. For example, in canned beverages, metal cans are excellent for cooler storage because they facilitate faster heat transfer between the cold fridge air and the warmer beverage. However, plastic containers with vacuum-sealed interior walls are designed to keep beverages warmer or colder for extended periods. Similarly, in construction and refrigeration, low-conductivity plastics like expanded polystyrene (EPS) and polyurethane foam are preferred due to their ability to trap air and minimize heat transfer.
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What is the thermal conductivity of different types of plastic?
Thermal conductivity is an intensive property that indicates a substance's ability to conduct heat. It is often denoted by the letter "k" and measured in watts per metre-kelvin (W/m·K). The thermal conductivity of plastics is typically low compared to metals, making them excellent thermal insulators. The thermal conductivity of most common plastics ranges between 0.1 to 0.5 W/m·K, while the average thermal conductivity of plastic is approximately 0.02-0.05 W/(m/K).
The thermal conductivity of plastics varies depending on their type and structure. Plastics with highly ordered crystalline structures, such as polyethylene terephthalate (PET), exhibit higher thermal conductivities due to efficient heat transfer along ordered chains. On the other hand, amorphous plastics like polystyrene have lower conductivity due to their higher density in the solid state. The thermal conductivity of amorphous plastics at 0-200°C ranges between 0.125-0.2 Wm-1K-1.
The incorporation of thermally conductive fillers can significantly enhance a polymer's thermal conductivity. For instance, high-density polyethylene (HDPE) filled with aluminum particles can achieve values exceeding 1 W/m·K. Polymers like PI and PEEK (polyether ether ketone) are utilized in high-performance applications due to their stability and moderate thermal conductivity, balancing insulation and heat resistance.
The thermal history and processing parameters, such as the cooling rate and annealing, also influence the crystallinity and void content, which directly affect heat conduction. Factors such as temperature and humidity can alter thermal conductivity. For example, elevated temperatures can increase molecular mobility, leading to a slight enhancement in conductivity in some cases.
Understanding the thermal conductivity of plastics is crucial for selecting the appropriate material for specific applications, especially in thermal insulation, electronics, and high-temperature environments.
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How does temperature affect the thermal conductivity of plastic?
The thermal conductivity of plastics is a critical factor in determining their suitability for specific applications, especially in thermal insulation, electronics, and high-temperature environments. Plastics are widely used due to their lightweight nature, corrosion resistance, and ease of fabrication. However, their thermal conductivity coefficients vary, with most common plastics ranging between 0.1 to 0.5 W/m·K.
Temperature has a direct impact on the thermal conductivity of plastics. As a general trend, the thermal conductivity of plastics increases slightly with temperature. This relationship holds true in the range of 0-100°C, with only extremely low temperatures (around 40K) causing a clear decrease in thermal conductivity. The increase in thermal conductivity with temperature is influenced by the molecular mobility of plastics. Elevated temperatures enhance molecular mobility, facilitating slightly better heat conduction.
However, the relationship between temperature and thermal conductivity in plastics is not linear and can vary depending on the specific type of plastic. For amorphous plastics, such as polystyrene, the thermal conductivity decreases with increasing temperature above their glass transition temperature (Tg). This decrease in thermal conductivity with temperature is attributed to the increase in molecular motion, which disrupts the ordered structure and reduces the overall conductivity.
Additionally, the thermal history and processing parameters of plastics, such as the cooling rate and annealing, can influence crystallinity and void content, which, in turn, affect heat conduction. For instance, semi-crystalline thermoplastics with ordered crystalline regions exhibit better thermal conductivity due to efficient heat transfer along these ordered chains. On the other hand, amorphous plastics with irregular structures have lower conductivity.
The incorporation of thermally conductive fillers, such as graphite, boron nitride, or metal oxides, can significantly enhance the thermal conductivity of plastics. These fillers increase the packing density of molecules, promoting better heat transfer. However, it's important to note that while temperature plays a role in influencing the thermal conductivity of plastics, the overall conductivity remains relatively low compared to metals, making plastics excellent thermal insulators in various applications.
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Frequently asked questions
The thermal conductivity of plastic varies depending on the type of plastic. Most plastic has an average thermal conductivity of approximately 0.02-0.05 W/(m/K).
Metals typically have a much higher thermal conductivity than plastics due to their loosely bound electrons, which allow for greater molecular vibration and, therefore, more effective heat transfer.
The thermal conductivity of plastics can be influenced by various factors, including temperature, humidity, crystallinity, and the incorporation of fillers such as graphite, boron nitride, or metal oxides.
