Transient Measurement of Thermal Conductivity of Plastics: Methods, Standards and Applications

Transient thermal conductivity measurement is the industry-preferred technique for characterizing how plastics transport heat, delivering accurate values for thermal conductivity, diffusivity and specific heat capacity in seconds rather than hours. This technical article explains the transient plane source, transient line source and laser flash methods used for plastic materials, the applicable ISO 22007 and ASTM standards, specimen requirements, and typical applications in electronics, automotive and polymer R&D.

Technical Overview

Thermal conductivity (λ, expressed in W/(m·K)) describes a material's ability to conduct heat and is one of the most important thermophysical properties for plastic materials used in thermal management, insulation and structural applications. Unfilled thermoplastics such as polyethylene, polypropylene, polystyrene and polycarbonate typically exhibit thermal conductivity values between 0.10 and 0.50 W/(m·K), while thermally conductive compounds filled with alumina, aluminum nitride, boron nitride or graphite can reach 1 to 20 W/(m·K), and polymer foams can be as low as 0.02 W/(m·K).

Transient measurement techniques determine thermal conductivity by applying a short, controlled heat pulse or a periodically varying heat input to a sensor in contact with the specimen, then recording the temperature response as a function of time. Because the heat pulse lasts only seconds to minutes, the temperature gradients and total temperature rise in the specimen remain small, which minimizes errors from convection, radiation, moisture migration and contact resistance that often affect classical steady-state methods. A single transient experiment can simultaneously yield thermal conductivity, thermal diffusivity and volumetric heat capacity.

For plastics, the ISO 22007 series of standards defines the principal transient methods, complemented by ASTM standards for specific sensor configurations. These methods cover a very wide conductivity range, require only small specimens, and allow efficient temperature-dependent measurements, making them ideal for research, product development and quality control in accredited third-party laboratories.

Testing Samples

Molded Plates and Compression-Molded Discs

For the transient plane source (hot disk) method, specimens are supplied as two pieces with flat, parallel faces that sandwich the sensor, for example squares of at least 20 mm × 20 mm or discs of at least 20 mm diameter. Recommended thickness is typically 4 to 10 mm so that the thermal penetration depth during the measurement remains well inside the specimen. Surfaces should be smooth and free of warpage, sink marks and voids.

Thermally Conductive Compounds

Highly filled engineering plastics used for heat sinks, LED components and battery housings are tested in plate or disc form. Because flake or fiber fillers create anisotropic heat paths, both through-plane (thickness direction) and in-plane conductivities are frequently requested, which requires specimens of adequate lateral dimensions and appropriate anisotropic measurement modules.

Foams and Insulating Plastics

Rigid and flexible polymer foams, EPS/XPS boards and low-density insulating materials are measured with low-power sensor settings. Their very low conductivity (0.02 to 0.05 W/(m·K)) and low volumetric heat capacity demand longer measurement times and careful sensor selection.

Polymer Melts

The transient line-source probe technique (ASTM D5930) allows measurement directly in polymer melts at processing temperatures up to approximately 300 °C. Specimens may be granules or powder packed around the probe in a crucible, or solid blocks with a drilled guide hole for the needle sensor.

Films, Sheets, Coatings and Pastes

Thin films and sheets can be evaluated using modified transient plane source sensors or by the laser flash method on stacked or free-standing specimens. Adhesives, potting compounds, thermal greases and pastes are tested in dedicated sample cells that eliminate air gaps and define a reproducible thickness.

Testing Items

Transient testing of plastics typically reports the following measurands, either at a single reference temperature or as a curve across a temperature range:

• Thermal conductivity λ, W/(m·K), covering approximately 0.005 to 500 W/(m·K) depending on the method
• Thermal diffusivity a, mm²/s, derived from the same transient record or measured directly by laser flash
• Specific heat capacity cp, J/(g·K), or volumetric heat capacity ρcp, J/(m³·K)
• Temperature-dependent property profiles, commonly from -50 °C to 200 °C for standard plastics, with extended ranges available for high-temperature polymers
• Through-plane and in-plane (directional) thermal conductivity for anisotropic filled compounds
• Melt thermal conductivity as a function of temperature for extrusion and injection molding simulation
• Thermal effusivity, relevant for surface touch-temperature applications

Testing Methods

Transient Plane Source (Hot Disk) Method — ISO 22007-2

A double-spiral nickel foil sensor protected by a thin polyimide or mica insulating layer is clamped between two specimen halves. The sensor acts simultaneously as a heat source and a resistance thermometer: a constant electrical power pulse raises the sensor temperature, and the recorded temperature increase versus time is fitted to the theoretical heat conduction model to calculate thermal conductivity, diffusivity and volumetric heat capacity in one measurement. The method requires no contact fluids or heat sink compounds, works from cryogenic to high temperatures, and supports isotropic and anisotropic analysis.

Transient Line-Source Probe Method — ASTM D5930

A thin needle probe containing a heater wire and a thermocouple is inserted into the specimen or immersed in a polymer melt. After a short equilibration, constant power is applied and the temperature rise is recorded. For a line source in an infinite medium, temperature rise is linear versus the logarithm of time, and thermal conductivity is obtained from the slope. This robust technique is well suited to melts, pastes, soft elastomers and granular materials and is widely used for process simulation data.

Modified Transient Plane Source Method — ASTM D7984

A one-sided spiral sensor applies a short heat pulse and measures the dynamic temperature response at the surface, from which thermal effusivity and, with known density and heat capacity, thermal conductivity are calculated. The configuration accepts small, singly faced specimens, films and uneven surfaces, making it popular for quality control screening.

Laser Flash Method — ISO 22007-4 and ASTM E1461

A short laser or flash-lamp pulse irradiates the front face of a disc specimen (commonly 10 to 12.7 mm diameter, 1 to 3 mm thick), and an infrared detector records the temperature rise of the rear face. The half-rise time yields thermal diffusivity; combining diffusivity with independently measured specific heat capacity and density gives thermal conductivity via λ = a · ρ · cp. Transparent or translucent plastics are coated with a thin graphite or gold film to ensure absorption and emission. The method covers an extremely wide diffusivity range and elevated temperatures.

Temperature Wave Analysis — ISO 22007-3

A periodic temperature wave is generated in a thin metal film in contact with the specimen, and the phase delay of the wave propagating through the sample is analyzed. The technique is suitable for small specimens and thin layers and provides diffusivity and effusivity information over a range of modulation frequencies.

General Test Procedure and Quality Assurance

Regardless of the specific technique, a reliable transient measurement follows a consistent workflow:

• Condition specimens to equilibrium, typically 23 °C and 50 % relative humidity per ISO 291, unless otherwise specified
• Verify specimen geometry, surface flatness, parallelism and freedom from voids and moisture
• Select sensor size, heating power and measurement time so that the thermal penetration depth remains within the specimen boundaries
• Perform the transient recording, typically lasting a few seconds to a few minutes per temperature point
• Fit the model to the valid time window and calculate thermal conductivity, diffusivity and heat capacity
• Verify instrument performance against certified reference materials such as fused silica, Pyroceram or stainless steel, and repeat measurements to confirm repeatability
• Report results with measurement uncertainty, sensor parameters, temperature and specimen description

Accredited third-party laboratories operating under ISO/IEC 17025 quality systems routinely perform these measurements, providing traceable results with typical measurement uncertainties of 3 to 5 % for thermal conductivity and better than 5 % for diffusivity and heat capacity.

Applications

Transient thermal conductivity data for plastics are essential across many industries:

• Electronics thermal management: thermally conductive housings, LED substrates, heat spreaders and potting compounds for power electronics
• Electric vehicles: battery module housings, cell separators, busbar insulation, charging connectors and thermal interface pads
• 5G and telecommunications: high-frequency antenna enclosures and heat-dissipating structural components
• Wire and cable: insulation and jacket compounds where heat dissipation affects ampacity and aging
• Household appliances: cool-touch handles, kettle bodies, iron housings and cooker components
• Polymer R&D: development of boron nitride, alumina and graphite filled compounds, and validation of filler dispersion and orientation effects
• Injection molding and extrusion simulation: melt thermal property inputs for mold-filling and cooling analysis software
• Building and refrigeration insulation: characterization of polymer foams and low-conductivity insulation products

FAQ

What are the advantages of transient methods over steady-state methods for plastics? Transient tests take seconds to minutes instead of hours, need only small specimens, avoid large temperature gradients that cause moisture migration and radiation losses, and deliver conductivity, diffusivity and heat capacity from a single run. Steady-state methods such as the guarded hot plate remain valuable mainly for thick insulation products.

What specimen size is required? For the hot disk method, two flat pieces of roughly 20 to 40 mm across and 4 to 10 mm thick are typical. Laser flash requires discs of 10 to 12.7 mm diameter and 1 to 3 mm thickness. Line-source probes need blocks large enough to contain the heated needle, typically 40 mm or more in each direction.

What temperature range can be tested? Standard configurations cover approximately -50 °C to 200 °C, suitable for most commodity and engineering plastics. Specialized sensors and furnaces extend the range from cryogenic temperatures to beyond 500 °C for high-performance polymers.

Can filled and anisotropic plastics be measured correctly? Yes. The transient plane source method offers anisotropic modules that separately determine in-plane and through-plane conductivity, and laser flash instruments provide dedicated in-plane fixtures. Correct specimen orientation and adequate lateral dimensions are essential.

How accurate are the results? With proper calibration against certified reference materials and skilled operation, thermal conductivity uncertainty is typically within 3 to 5 %, and thermal diffusivity within about 5 %. Interlaboratory comparisons organized under ISO 22007 demonstrate good reproducibility when specimen preparation and time-window selection are controlled.

Which method should be selected for a thermally conductive plastic of 5 to 20 W/(m·K)? The transient plane source method and the laser flash method are both suitable; laser flash is often preferred at the upper end of the range and at elevated temperatures, while hot disk provides a direct conductivity value without requiring separate density and heat capacity data.

Do results from different transient methods agree? When specimens are well prepared and methods are applied within their validity ranges, agreement within combined uncertainties is normally achieved. Differences usually trace to contact resistance, anisotropy, or moisture effects, which is why reporting full test conditions is critical.

How should samples be submitted for testing? Provide flat, void-free specimens meeting the dimensional requirements, state the material grade and filler content, specify the temperatures of interest and whether directional properties are required, and condition hygroscopic plastics such as polyamide before testing in accordance with ISO 291.