Determination and Analysis of Thermal Conductivity of Plastics

Thermal conductivity determination is a critical characterization step for plastics used in electronics, automotive, energy, and insulation applications, where heat transfer performance directly governs product reliability and safety. This article presents a systematic analysis of plastic thermal conductivity measurement, covering sample types, key test parameters such as thermal conductivity, diffusivity, and specific heat capacity, and internationally recognized methods based on ISO 22007, ISO 8302, ASTM E1530, ASTM D5470, and related standards. It also outlines typical applications, method selection guidance, and answers to frequently asked questions to support material development, quality control, and design verification.

Technical Overview

Thermal conductivity, denoted by the symbol λ (lambda) and expressed in watts per meter-kelvin, W/(m·K), quantifies the ability of a material to conduct heat under steady-state or transient conditions. Plastics are intrinsically poor thermal conductors: most unfilled polymers exhibit thermal conductivity values between 0.1 and 0.5 W/(m·K), which is roughly two to three orders of magnitude lower than metals such as aluminum or copper. This low conductivity is advantageous for thermal insulation, but it becomes a significant limitation in applications where heat must be dissipated, such as LED lighting, power electronics, electric vehicle battery systems, and high-density electronic enclosures.

To satisfy heat dissipation requirements, compounders incorporate thermally conductive fillers such as aluminum oxide, aluminum nitride, boron nitride, graphite, or carbon fibers into polymer matrices, producing commercial grades with conductivity values of approximately 1 to 20 W/(m·K). Accurate determination of these values is essential for thermal simulation, material selection, quality control, and design verification. Thermal conductivity is frequently derived from three independently measurable quantities through the relationship λ = α × ρ × cp, where α is the thermal diffusivity (mm²/s), ρ is the density (kg/m³), and cp is the specific heat capacity (J/(kg·K)).

Because filler particles orient during injection molding, extrusion, and calendering, filled plastics are usually anisotropic, with in-plane conductivity that can be several times higher than through-plane conductivity. A complete and defensible characterization therefore specifies the measurement direction, the test method, the reference standard, the specimen thickness, and the temperature at which the value is reported. All measurements should be performed by accredited third-party laboratories or qualified in-house facilities under controlled environmental conditions, typically 23 ± 2 °C and 50 ± 10 % relative humidity in accordance with ISO 291.

Testing Samples

Unfilled Thermoplastics and Thermosets

Baseline characterization is performed on neat resins such as PE, PP, PS, ABS, PVC, PMMA, PC, PA6, PA66, POM, PPS, and PEEK, as well as thermosetting systems including epoxy, polyurethane, phenolic, and unsaturated polyester formulations. Specimens are usually prepared as flat discs or square plates with parallel faces, free of voids, sink marks, and surface contamination, because trapped air gaps can significantly distort measured values.

Filled and Thermally Conductive Compounds

Ceramic-filled, graphite-filled, and metal-filled compounds represent the fastest growing segment of thermal conductivity testing. Because these materials are strongly anisotropic, specimens should be taken from molded plaques or finished parts in defined orientations so that through-plane and in-plane values can be reported separately. Filler settling and flow-induced orientation must be documented, as both influence the measured result.

Foamed and Insulating Plastics

Rigid polyurethane foam, expanded and extruded polystyrene, polyolefin foams, and other cellular plastics exhibit very low conductivity, typically 0.020 to 0.045 W/(m·K). These materials require large, thick specimens and steady-state methods, since thin-specimen transient techniques may not adequately capture the combined solid, gaseous, and radiative heat transfer mechanisms within the cellular structure.

Films, Sheets, and Thermal Interface Materials

Polymer films, thermally conductive sheets, gap pads, phase-change materials, and thermal greases are tested as supplied or at defined thicknesses. These products are usually evaluated with pressure-controlled steady-state methods that report thermal resistance and impedance, reflecting the real contact conditions found in electronic assemblies.

Molded Components and Finished Parts

Finished components such as LED housings, heat sinks, coil bobbins, battery trays, and connector shields can be sectioned into testable specimens. Testing actual parts, rather than pellets or plaques, captures the true effects of processing history, filler orientation, and weld lines on heat transfer performance.

Testing Items

A comprehensive thermal characterization program for plastics typically includes the following measurable parameters:

• Thermal conductivity λ in W/(m·K), reported for both through-plane and in-plane directions where anisotropy is expected
• Thermal diffusivity α in mm²/s, determined directly by flash and transient methods
• Specific heat capacity cp in J/(g·K), measured by differential scanning calorimetry per ISO 11357
• Density ρ in kg/m³, measured per ISO 1183 for use in diffusivity-to-conductivity conversion
• Thermal resistance in m²·K/W and thermal impedance, particularly for interface materials and stacked assemblies
• Contact resistance at material interfaces under defined clamping pressure
• Thermal conductivity as a function of temperature, from sub-ambient conditions to several hundred degrees Celsius

Testing Methods

Both steady-state and transient techniques are applied to plastics, and the appropriate method depends on the expected conductivity range, specimen geometry, anisotropy, and test temperature. The principal internationally recognized methods are summarized below.

Guarded Hot Plate Method (ISO 8302, ASTM C177, GB/T 3399)

The guarded hot plate is the primary absolute method for low-conductivity materials. A heated metering section surrounded by guard heaters establishes one-dimensional heat flow through the specimen stack, and conductivity is calculated directly from measured power, area, thickness, and temperature difference. Accuracy on the order of 1 to 2 % is achievable, but the method requires large, thick specimens and long stabilization times, making it best suited to insulating foams and reference measurements.

Heat Flow Meter Method (ISO 8301, ASTM C518, ASTM E1530, EN 12667)

The heat flow meter is a comparative steady-state technique in which a calibrated transducer measures heat flux through a disc-shaped specimen, commonly about 50 mm in diameter. ASTM E1530 is widely applied to engineering plastics over a broad temperature range, typically up to about 300 °C, with reproducibility on the order of 3 to 5 %. Regular calibration with certified reference materials is essential for maintaining traceability.

Guarded Heat Flow Method for Conductive Plastics (ASTM D5470)

ASTM D5470 was developed specifically for materials and interfaces in the moderate to high conductivity range, including thermally conductive compounds and thermal interface materials. The specimen is clamped between metered bars under controlled pressure, and thermal resistance is measured as a function of thickness. Extrapolating resistance to zero thickness separates contact resistance from bulk resistance, yielding the true bulk conductivity of the plastic. This method is particularly valuable for materials in the 0.5 to 10 W/(m·K) range used in electronic thermal management.

Transient Plane Source Method (ISO 22007-2)

The transient plane source, or hot disc, technique uses a double-spiral nickel sensor that acts as both heat source and temperature monitor. It allows rapid measurement, typically within seconds to minutes, on slab-shaped specimens, covers temperatures from approximately -50 °C to several hundred degrees Celsius, and can evaluate both isotropic and anisotropic conductivity from a single experiment. Reproducibility of approximately 3 to 5 % makes it a practical tool for research and incoming inspection of conductive compounds.

Laser Flash Method (ISO 22007-4, ASTM E1461, GB/T 22588)

The laser flash method determines thermal diffusivity by applying a short energy pulse to the front face of a small disc specimen, typically 10 to 12.7 mm in diameter and 1 to 3 mm thick, and recording the temperature rise on the rear face. When combined with independently measured specific heat capacity and density, conductivity is calculated through the relation λ = α × ρ × cp. The method covers temperatures from room temperature to well above 500 °C, requires only small specimens, and is the preferred technique for high-temperature characterization and for highly filled grades. Specimens are usually coated with a thin graphite layer to ensure uniform absorption of the flash energy.

Transient Line Source Method (ASTM D5930)

The transient line source, or needle probe, method measures conductivity from the temperature response of a heated wire embedded in or contacted by the specimen. It is well suited to viscous melts, uncured resins, gels, elastomers, and soft materials that cannot be machined into precise steady-state specimens, and it supports in-process characterization during compounding studies.

Method selection should follow a simple hierarchy: insulating foams and thick boards are best evaluated by guarded hot plate or heat flow meter methods per EN 12667; unfilled and moderately filled engineering plastics by ASTM E1530 or ISO 22007-2; highly conductive compounds by ASTM D5470, ISO 22007-2, or the laser flash method; and thin films and interface materials by ASTM D5470. Where regulatory or customer acceptance is involved, testing should be assigned to accredited third-party laboratories with documented traceability to national standards.

Applications

Thermal conductivity data for plastics directly support design decisions, safety assessments, and regulatory compliance across a wide range of industries:

• Electronics thermal management: LED lamp housings, heat spreaders, fan-free enclosures, and heat sink replacements based on conductive polyamide, PPS, and PEEK compounds
• Electric vehicles and battery systems: module housings, cell holders, cooling components, busbar insulation, and fire-resistant barrier layers requiring validated heat transfer data
• Power electronics and 5G infrastructure: inverter housings, antenna radomes, and high-frequency device substrates where heat dissipation and dimensional stability interact
• Building and construction: insulation boards, window profiles, and pipe systems evaluated per EN and ISO insulation standards for energy performance declarations
• Wire and cable: jacket and insulation compounds whose conductivity influences conductor ampacity calculations and thermal aging behavior
• Appliances and consumer products: kettle bodies, hair dryer components, cookware handles, and motor housings requiring safe surface temperatures
• Research and development: filler selection, formulation optimization, processing studies, and generation of input data for finite element thermal simulation

FAQ

What is the typical thermal conductivity range of common plastics?

Unfilled plastics generally fall between 0.1 and 0.5 W/(m·K). Representative room-temperature values include polystyrene at approximately 0.12, PVC at approximately 0.17, PMMA at approximately 0.19, polycarbonate at approximately 0.20, polypropylene at approximately 0.22, PA66 at approximately 0.25, and HDPE at approximately 0.45 W/(m·K). Thermally filled compounds range from roughly 1 to 20 W/(m·K), while rigid foams fall between 0.020 and 0.045 W/(m·K).

Why do in-plane and through-plane results differ for filled plastics?

Plate-like fillers such as graphite and boron nitride, as well as fibers, align parallel to the flow direction during molding or extrusion. Heat is therefore conducted much more efficiently along the filled plane than across it, and in-plane conductivity can exceed through-plane conductivity by a factor of two to ten. Both directions should be measured and clearly identified in any test report for anisotropic materials.

Which method is most appropriate for a conductive grade around 2 W/(m·K)?

ASTM D5470 under controlled clamping pressure is widely used in this range because it separates contact resistance from bulk resistance. The transient plane source method per ISO 22007-2 and the laser flash method per ISO 22007-4 are equally accepted alternatives, and cross-verification with two independent methods is recommended for qualification-critical applications.

What specimen size and conditioning are required?

Typical specimens are discs of approximately 50 mm diameter for heat flow meter testing, 10 to 12.7 mm diameter and 1 to 3 mm thickness for laser flash testing, and slab specimens sized according to sensor dimensions for transient plane source testing. Specimens must have flat, parallel faces, be free of surface contamination, and be conditioned per ISO 291. Moisture-sensitive polymers such as polyamides should be dried to a defined state before measurement, since absorbed water alters conductivity.

Can testing be performed at elevated or sub-ambient temperatures?

Yes. Laser flash instruments routinely operate from below room temperature to 500 °C and beyond, transient plane source systems typically cover -50 °C to several hundred degrees Celsius, and heat flow meter systems per ASTM E1530 operate to approximately 300 °C. Temperature-dependent curves are recommended for any application involving wide operating ranges or transient thermal loading.

Why do results from different methods sometimes disagree for the same material?

Small discrepancies arise from contact resistance at specimen surfaces, differing specimen thicknesses, sensitivity to anisotropy, uncertainty in specific heat and density values used in conversions, and moisture effects. Differences of 5 to 15 % between methods are not unusual for filled compounds. Specifying the standard, method, direction, thickness, and temperature in every data sheet and test report ensures that results remain comparable and traceable.