Thermal resistance and thermal conductivity testing of plastics measures how effectively polymeric materials conduct or resist heat flow, providing essential data for thermal management design, material selection, and quality control. Accredited third-party laboratories perform these evaluations in accordance with internationally recognized standards such as ASTM C518, ASTM E1461, ASTM D5470, ISO 22007, and ISO 8302, covering everything from commodity insulating resins to highly filled thermally conductive compounds. This article explains the testing principles, sample requirements, applicable methods, test items, and typical industrial applications of plastic thermal property measurement.
Technical Overview
Thermal conductivity and thermal resistance are the fundamental parameters that describe how a plastic material responds to a temperature gradient. Thermal conductivity, usually denoted by the Greek letter lambda, quantifies the rate at which heat passes through a material and is expressed in W/(m·K). Thermal resistance, denoted R, is the complementary measure of a material's ability to resist heat flow; for a homogeneous slab of thickness d, the relationship is R = d/λ, expressed in m²·K/W, or in K/W when a specific specimen geometry is defined. Together with thermal diffusivity and specific heat capacity, these properties form the core dataset required for thermal simulation, heat transfer calculations, and material qualification.
Unmodified polymers are natural thermal insulators, with thermal conductivity values typically ranging from 0.10 to 0.50 W/(m·K). By compounding resins with ceramic or carbonaceous fillers such as boron nitride, aluminum nitride, alumina, or graphite, formulators can raise thermal conductivity to 1–20 W/(m·K) or higher, making thermally conductive plastics practical alternatives to metal heat sinks, housings, and structural components in weight-sensitive and electrically sensitive applications. Because filler orientation during molding introduces strong anisotropy, accurate measurement of both through-plane and in-plane values is often necessary. Reliable, standards-based testing by accredited third-party laboratories ensures that reported data are traceable, reproducible, and suitable for engineering design and regulatory submissions.
Testing Samples
Suitable test specimens vary with the material category and the selected measurement method. The principal sample types accepted for thermal resistance and thermal conductivity testing include the following.
Thermally Conductive Engineering Plastics
Filled grades of PA6, PA66, PPS, PBT, PC, PP, PPA, and LCP loaded with thermally conductive fillers, supplied as injection-molded plates, compression-molded plaques, or machined discs. Because processing-induced filler orientation creates anisotropy, both through-plane and in-plane conductivity are frequently characterized, and specimens are typically tested in the as-molded orientation that reflects end use.
Insulating and Foamed Polymer Materials
Rigid and flexible foams such as polyurethane, expanded polystyrene, extruded polystyrene, and polyolefin foams used in building, appliance, and cryogenic insulation. These very low conductivity materials, typically between 0.02 and 0.05 W/(m·K), require large-area flat specimens and steady-state methods operated at defined mean temperatures and temperature differences.
Films, Sheets, and Laminates
Thin polymer films, multilayer laminates, coated fabrics, insulating films, and battery components. Because thin specimens are difficult to measure by conventional steady-state apparatus, they are usually characterized by the laser flash method, sometimes using stacked layers, with the resulting diffusivity converted to thermal conductivity using independently measured density and specific heat.
Thermal Interface Materials
Gap pads, thermal greases, phase-change films, and insulating pads used between heat sources and heat sinks. These products are evaluated as installed systems, with total thermal impedance measured as a function of thickness and clamping pressure so that bulk conductivity can be separated from interface contact resistance.
Testing Items
Core test items offered for plastic thermal characterization include:
- Through-plane thermal conductivity, expressed in W/(m·K)
- In-plane thermal conductivity for anisotropic and oriented compounds
- Thermal resistance per unit area (m²·K/W) and total thermal impedance (K/W) at specified pressure
- Bulk thermal resistance and contact (interface) resistance decomposition
- Thermal diffusivity, expressed in mm²/s
- Specific heat capacity, expressed in J/(kg·K)
- Apparent thermal conductivity at multiple mean temperatures across the service range
- Anisotropy ratio of in-plane to through-plane conductivity
Testing Methods
Several complementary techniques are available, each suited to a particular conductivity range, specimen geometry, and data objective. Selection depends on the expected conductivity level, available sample size, required temperature range, and whether steady-state or transient results are preferred.
Guarded Hot Plate Method (ASTM C177, ISO 8302)
The guarded hot plate is an absolute, primary method in which a guarded main heater is sandwiched between two nearly identical specimens, and side and edge losses are suppressed by auxiliary guard heaters. Because the method does not rely on calibration against reference materials, it offers the highest accuracy, typically within one to two percent, and is the reference technique for insulating plastics and foams with conductivity from roughly 0.01 to 2 W/(m·K). It requires large, flat, parallel-faced specimens and longer test cycles.
Heat Flow Meter Method (ASTM C518, ISO 8301, EN 12667, GB/T 3399)
The heat flow meter is a comparative steady-state method in which the specimen is placed between hot and cold plates and the heat flux is measured by calibrated transducers, with the apparatus calibrated using certified reference materials traceable to national metrology institutes. It is faster than the guarded hot plate, accommodates a wide range of specimen thicknesses, and is well suited to routine quality control of plastics and insulation products with conductivity from approximately 0.01 to 5 W/(m·K). EN 12667 and the related EN series define its application to products of high and medium thermal resistance.
Transient Plane Source Method (ISO 22007-2)
The transient plane source technique, often known as the hot disc method, uses a double-spiral nickel sensor that functions simultaneously as a heater and a resistance thermometer, sandwiched between two pieces of the specimen. A short electrical pulse raises the sensor temperature, and the recorded resistance transient yields thermal conductivity, diffusivity, and volumetric heat capacity in a single measurement. The method covers an exceptionally wide range from about 0.01 to several hundred W/(m·K), requires only small specimens, completes in minutes, and can be configured to determine in-plane and through-plane properties of anisotropic molded plastics.
Laser Flash Method (ASTM E1461, ISO 22007-4, GB/T 22588)
In the laser flash method, a short high-energy pulse irradiates the front face of a small disc specimen while an infrared detector records the temperature rise of the rear face. The resulting curve gives thermal diffusivity directly; thermal conductivity is then calculated as the product of diffusivity, density, and specific heat, with density measured per ISO 1183 and specific heat measured by differential scanning calorimetry per ISO 11357. The technique is ideal for thin specimens and highly conductive filled plastics, covers temperatures from cryogenic conditions to several hundred degrees Celsius, and requires only a small amount of material.
Steady-State Method for Thermal Interface Materials (ASTM D5470)
ASTM D5470 measures thermal transmission properties using metered metal bars that generate and measure one-dimensional heat flow through the specimen. By testing materials at several thicknesses and plotting total thermal impedance versus thickness, laboratories can separate contact resistance at the interfaces from bulk resistance and report intrinsic thermal conductivity. Pressure and temperature are controlled variables, making the method the industry benchmark for gap pads, greases, and phase-change interface products.
Applications
Thermal resistance and thermal conductivity data for plastics directly support design decisions and regulatory compliance across many industries:
- LED lighting: thermally conductive housings, lamp holders, and plastic heat sinks that replace aluminum while providing electrical insulation
- New energy vehicles: battery module housings, cell holders, busbar insulation, and flame-retardant thermal pads in lithium-ion packs
- 5G and telecommunications: high-frequency antenna radomes, base-station enclosures, and housings combining thermal dissipation with dimensional stability
- Consumer electronics: notebook and smartphone chassis, fanless device enclosures, and thermal spreader components
- Home appliances: cooker handles, motor and blower housings, hair dryer components, and heat-resistant internal parts
- Power electronics and electrical equipment: relay and connector housings, insulating sheets, busbar coatings, and drive enclosures
- Building and HVAC: insulation boards, pipe lagging, and energy-code compliance verification for polymeric insulation products
FAQ
What is the difference between thermal conductivity and thermal resistance?
Thermal conductivity is an intrinsic material property that describes how readily heat passes through the material itself, expressed in W/(m·K). Thermal resistance is a derived quantity that combines material behavior with geometry: for a homogeneous layer, it equals thickness divided by thermal conductivity. In practice, conductivity is used to compare and select materials, while resistance describes the actual thermal performance of a component with defined thickness, area, and interface conditions.
How do I choose the right test method for my plastic?
Insulating plastics and foams below about 2 W/(m·K) with large flat specimens are best served by the guarded hot plate or heat flow meter methods. Filled, moderately to highly conductive plastics are efficiently measured by the transient plane source method, which also captures anisotropy. Thin specimens, high conductivity levels, or elevated-temperature requirements favor the laser flash method. Interface materials such as pads and greases are tested by ASTM D5470 to obtain pressure-dependent resistance data. Laboratory engineers can recommend the optimal method based on the expected conductivity range and available specimen geometry.
What specimen size and quantity are required?
Requirements depend on the method. Heat flow meter and guarded hot plate testing typically call for flat panels of 100 to 300 mm across with thicknesses reflecting the end product. Transient plane source testing requires two matching pieces a few tens of millimeters across with polished faces. Laser flash testing uses small discs, commonly 10 to 12.7 mm in diameter and 1 to 3 mm thick. Clients should consult the laboratory before molding specimens to confirm dimensions, flatness, and conditioning requirements.
Over what temperature range can testing be performed?
Steady-state methods typically cover mean temperatures from about -20 °C to 150 °C for polymeric specimens. Transient plane source instruments commonly operate from roughly -30 °C to 200 °C, and laser flash systems can extend from cryogenic temperatures to 500 °C or higher with appropriate furnaces. Testing at several mean temperatures is recommended whenever the material will experience a wide service temperature range.
How is thermal diffusivity converted to thermal conductivity?
Thermal conductivity equals thermal diffusivity multiplied by density and specific heat capacity. Diffusivity is measured directly, for example by laser flash, while density is determined per ISO 1183 and specific heat capacity is measured by differential scanning calorimetry per ISO 11357. The laboratory reports all three input values together with the calculated conductivity so the result is fully traceable.
What is included in a typical test report?
A professional report from an accredited third-party laboratory identifies the test standard, specimen description and dimensions, conditioning history, mean test temperature and applied temperature difference or pressure, measured thermal conductivity or resistance with uncertainty, and, where applicable, diffusivity, specific heat, and density. The report supports supplier qualification, incoming inspection, thermal simulation input, and submission to customers or regulatory bodies.