Omar Khalid
Thermal conductivity is a property indicating how easily heat can move through a solid material. Plastics, such as TORELINA™, are insulating materials with lower thermal conductivity than metals. The thermal conductivity of plastics varies depending on their composition and structure; for example, plastics with highly ordered crystalline structures, like polyethylene terephthalate (PET), exhibit higher thermal conductivities due to efficient heat transfer along ordered chains. The thermal conductivity of plastics can be increased by incorporating thermally conductive fillers such as graphite, boron nitride, or metal oxides. Accurate measurement of thermal conductivity in plastics is essential for predicting temperature distribution and cooling behaviour during processing, as well as for selecting suitable materials for specific applications. Various methods and instruments are available for measuring the thermal conductivity of plastics, including the C-Therm Trident Thermal Conductivity Instrument, which provides a fast and easy way to measure thermal conductivity across different materials.
| Characteristics | Values |
|---|---|
| Plastics with highly ordered crystalline structures | Higher thermal conductivities due to efficient heat transfer along ordered chains |
| Amorphous plastics | Lower conductivity |
| Incorporation of thermally conductive fillers | Graphite, boron nitride, or metal oxides can enhance a polymer's thermal conductivity |
| High-density polyethylene (HDPE) filled with aluminum particles | Can achieve values exceeding 1 W/m·K |
| Thermal history and processing parameters | Cooling rate and annealing influence crystallinity and void content, which directly affect heat conduction |
| Factors influencing thermal conductivity | Temperature, humidity, molecular mobility |
| Thermal conductivity applications | LED lighting, battery casings, electronic cooling, housings for lighting or battery casings |
| Polymers with moderate thermal conductivity | PI and PEEK (polyether ether ketone) |
| Thermal conductivity measurement tools | C-Therm Trident Thermal Conductivity Instrument, FLEX Transient Plane Source (TPS) hot disc method, Transient Line Source (TLS) |
| Thermal conductivity measurement methods | Steady methods, unsteady methods, laser flash method, hot disk method |
| Thermal conductivity equation inputs | Specimen density, specific heat at the measurement temperature, thermal diffusivity |
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What You'll Learn
Steady vs unsteady methods
Steady-state methods involve measurements taken after the sample has reached a steady state, with zero heat gradient or constant heat over the entire sample. This usually takes about 30 minutes or more. For good heat conductors, Searle's bar method can be employed, whereas for poor conductors, Lee's disc method is used. The major sources of error in such measurements include radiative and convective heat losses in the setup, as well as errors in the thickness of the sample.
Non-steady-state or transient methods, on the other hand, do not require a constant signal value. Instead, the signal is studied as a function of time, which means that measurements can be performed more quickly. These methods are generally carried out by needle probes. However, the disadvantage is that the mathematical analysis of the data is more complex.
One example of a steady-state method is the ASTM Standard C518-10, which determines the steady-state thermal transmission properties using a heat flow meter apparatus. Transient methods include the ASTM Standard D5930-01, which uses a transient line-source technique, and the ISO 22007-2:2008 standard, which employs a transient plane heat source (hot disc) method.
The choice between steady and unsteady methods depends on the material being tested and the specific requirements of the application. Steady-state methods provide more accurate results, especially for materials with high thermal conductivity, but they are time-consuming. Transient methods offer faster measurements but may require more complex data analysis.
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Thermal conduction, convection, radiation
Heat, or thermal energy, can be transferred in three ways: through conduction, convection, and radiation. These processes are essential to understand when studying the thermal conductivity of plastics.
Thermal conduction refers to the transfer of heat within solids or stationary fluids (like water). In the context of plastics, crystalline structures such as polyethylene terephthalate (PET) exhibit higher thermal conductivities due to efficient heat transfer along their ordered chains. Conversely, amorphous plastics like polystyrene have lower conductivity due to their irregular atomic structures. The addition of thermally conductive fillers, such as graphite or boron nitride, can significantly enhance a polymer's thermal conductivity. For instance, high-density polyethylene (HDPE) infused with aluminum particles can achieve values exceeding 1 W/m·K.
Thermal convection, on the other hand, involves the transfer of heat in liquids or gases that are in motion. This combines conduction with fluid flow. Convection plays a role when plastics are exposed to liquids or gases in motion, influencing the overall heat transfer within the material.
Radiation is the third mode of heat transfer, and it occurs without the need for a material carrier. This means that heat is transferred through radiation even in a vacuum. All objects emit thermal radiation, and this radiation can be absorbed by other objects, leading to a transfer of thermal energy.
When considering the thermal conductivity of plastics, it is important to account for factors that can influence heat transfer through these three mechanisms. For example, temperature and humidity can impact thermal conductivity. Elevated temperatures can increase molecular mobility, leading to enhanced conductivity in some cases. Additionally, the thermal history and processing parameters, such as cooling rates and annealing, can affect crystallinity and void content, which in turn influence heat conduction.
Understanding the thermal conduction, convection, and radiation properties of plastics is crucial for their effective use in various applications. For instance, in LED lighting or battery casings, specialty thermally conductive plastics are used for efficient thermal dissipation. Polymers like PI and PEEK (polyether ether ketone) are chosen for their balance between insulation and heat resistance, making them suitable for high-performance applications.
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Thermal conductivity instrumentation
Measuring the thermal conductivity of plastics is essential for determining their suitability for specific applications, especially in thermal insulation, electronics, and high-temperature environments. Thermal conductivity (k) represents a material's ability to conduct heat, and plastics generally have low thermal conductivity compared to metals, making them excellent thermal insulators.
Various instruments and methods are available for measuring the thermal conductivity of plastics, including:
- C-Therm Technologies Ltd.: C-Therm offers the Trident, the only commercial tool that can measure the thermal conductivity of polymers across solids, liquids, powders, pastes, and textiles. They also offer the C-Therm TCi Thermal Conductivity Analyzer, which utilizes a patented high-precision MTPS sensor for fast and accurate measurements of polymer composites. The C-Therm TLS Needle is another instrument used for testing polymer melts involving high temperatures and pressures, commonly seen in plastic injection moulding.
- Transient Line Source (TLS) Method: This method is particularly useful for testing polymer melts, as it can handle high temperatures and pressures. It complies with ASTM 5334 and D5930 standards.
- FLEX TPS Sensor: Used in conjunction with the C-Therm TCi Thermal Conductivity Analyzer, this sensor allows for accurate and reliable measurements of polymer composites.
- MTPS Sensor: C-Therm's patented high-precision MTPS sensor enables quick and easy thermal conductivity measurements.
- Polymer Discs and ISO 22007-2: This method involves using polymer discs and adhering to the ISO 22007-2 standard for measurement.
These instruments and methods provide accurate and reliable means to quantify the thermal conductivity of plastics, aiding in the selection of suitable materials for specific applications and contributing to advancements in plastic engineering and technology.
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Polymer thin films
The thermal conductivity of polymer thin films can be measured using the FLEX transient plane source (TPS) hot disc method, which is available on the Trident tool. This method is recommended for advanced users who want to test the thermal conductivity of polymer composites. The Thin Films utility within this method provides a solution for testing the thermal conductivity of polymer thin films. The Trident tool is the only commercial tool that can measure the thermal conductivity of polymers across solids, liquids, powders, pastes, and textiles.
The thermal conductivity of thin films can also be measured using a thermal vac or thermal vacuum. This involves applying electrically powered heat to a component at one end, while the other end is anchored to a thermal vac chamber with a temperature-controlled interface plate. The thermal vac chamber also includes calibrated instrumentation to control and monitor temperatures and pressure, which is necessary for accurate measurements.
A single thick film measurement, followed by double and triple thick film measurements, can also be conducted to determine the thermal conductivity of thin films. By observing the relationship between changes in temperature and film thickness, the thermal conductivity can be determined by relating thermal resistivity to thermal conductivity.
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Thermal conductivity of polymer melts
The thermal conductivity of polymer melts is a critical area of study, as it directly impacts the design of extruders and injection moulding machines. The heat migration mechanism during the melting process and the subsequent cooling process influences the final product's crystallinity, surface roughness, hardness, and morphology.
Several methods are available to measure the thermal conductivity of polymer melts. One common technique is the short-hot-wire method, which is particularly suitable for molten polymers with high viscosities, as natural convection effects can be ignored. This method involves a dc power supply and voltage and current-measuring and control systems, including digital multimeters and a personal computer. The results have shown that the short-hot-wire method can measure thermal conductivity within uncertainties of 3%.
Another method for measuring the thermal conductivity of polymer melts is based on modulated DSC (Differential Scanning Calorimetry). This technique covers a range of 0.1 to 1.5 W °C−1 m−1, which is applicable to polymers, ceramics, and glasses. While this method does not require modifications to the apparatus, it does need one additional calibration step to compensate for heat loss through the inert purge gas surrounding the test specimen.
The Du Pont 910 DSC cell and base have also been modified to measure the steady-state heat flux and temperature gradients in molten polymeric specimens. This method involves measuring thermal quantities and applying a calibration factor obtained from reference materials.
The thermal conductivity of polymer melts is influenced by various factors, including the polymer's structure and composition. For example, 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 addition of thermally conductive fillers, such as graphite or boron nitride, can significantly enhance a polymer's thermal conductivity.
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Frequently asked questions
The C-Therm Trident Thermal Conductivity Instrument is a fast and easy way to measure the thermal conductivity of plastics and polymers.
TLS is a technique used to measure the thermal conductivity of polymer melts. It is useful in situations where high temperature and pressure are required, such as in plastic injection moulding.
Steady methods involve determining thermal conductivity from the amount of heat flux that moves through a unit cross-section area per unit time. Unsteady methods, on the other hand, determine thermal conductivity from the rate of heat diffusion through the specimen. Unsteady methods like the laser flash method and the hot disk method are more commonly used today as they provide quicker measurements.
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.
Factors such as temperature and humidity can alter the thermal conductivity of plastics. For example, elevated temperatures can enhance conductivity slightly by increasing molecular mobility. Additionally, fillers such as graphite, boron nitride, metal oxides, silver, copper, and CNTs can significantly increase the thermal conductivity of polymer composites.
