Determination of Thermal Conductivity of Plastics: Test Methods, Standards, and Applications

Thermal conductivity testing of plastics is a critical measurement for material selection, product design, and quality control in industries ranging from electronics thermal management to building insulation. This article explains the fundamental principles of thermal conductivity determination for polymer materials, introduces internationally recognized test methods such as ISO 22007 and ASTM E1530, and provides practical guidance on sample preparation, data interpretation, and typical applications.

Technical Overview

Thermal conductivity, denoted by the symbol λ (lambda) and expressed in W/(m·K), quantifies a material's ability to conduct heat. It is defined through Fourier's law of heat conduction, which relates the heat flux through a material to the temperature gradient across it. For plastics, thermal conductivity is a fundamental thermal property that directly influences heat dissipation, insulation performance, processing behavior, and long-term reliability of polymer products.

Unmodified polymers are inherently poor thermal conductors. Typical values range from approximately 0.10 to 0.25 W/(m·K) for amorphous plastics such as PS, PMMA, and PC, and from 0.20 to 0.50 W/(m·K) for semi-crystalline plastics such as PE, PP, PA, and POM. Thermally conductive compounds, modified with fillers such as graphite, boron nitride, aluminum nitride, or aluminum oxide, can reach values of 1 to 20 W/(m·K), while polymer foams used for insulation may fall below 0.040 W/(m·K). Accurate determination of these values requires appropriate instrumentation and standardized procedures.

Thermal conductivity measurement is closely linked to two related properties: thermal diffusivity (a, in mm²/s) and specific heat capacity (cp, in J/(g·K)). These quantities are related by the equation λ = a × ρ × cp, where ρ is density. Depending on the measurement technique, laboratories may directly measure thermal conductivity or derive it from diffusivity and heat capacity data. Factors such as temperature, degree of crystallinity, filler content and orientation, molecular chain orientation, moisture content, and porosity all affect the measured result and must be controlled or documented during testing.

Testing Samples

Molded Plaques and Compression-Molded Sheets

Rigid thermoplastics and thermosets are typically tested as flat plaques produced by injection molding, compression molding, or machined from finished parts. For steady-state methods such as the guarded heat flow meter technique, samples are commonly 50 mm in diameter or 50 × 50 mm square, with a thickness between 2 mm and 25 mm. The two opposing surfaces must be flat, parallel (typically within 0.05 mm), and smooth to ensure intimate contact with the instrument plates.

Films and Thin Sheets

Polymer films below 1 mm in thickness require special attention. Stacking multiple film layers can introduce contact resistance between layers, so methods such as the laser flash technique or dedicated thin-film sensors are often preferred. Samples should be free of wrinkles, bubbles, pinholes, and surface contamination that would distort heat transfer.

Filled, Reinforced, and Thermally Conductive Compounds

Compounds containing glass fiber, carbon fiber, graphite, or ceramic fillers are frequently anisotropic, with in-plane conductivity significantly different from through-thickness conductivity. The test direction must be defined, and multiple specimens from different orientations may be required. Pellets or granules can also be characterized using transient methods with appropriate sensors or by preparing molded specimens.

Foams and Insulation Materials

Rigid and flexible polymer foams are usually tested with the guarded hot plate or heat flow meter method using larger samples, often 100 × 100 mm or greater, to average out cellular structure variations. Specimens must be cut cleanly without crushing the cells, and conditioning to equilibrium is required before measurement.

Sample Conditioning

Prior to testing, specimens should be conditioned in accordance with ISO 291 or ASTM D618, typically at 23 °C and 50% relative humidity, unless otherwise specified. Moisture-sensitive materials such as polyamides may require drying, and the conditioning state must be reported together with the results.

Testing Items

Accredited third-party laboratories offering plastic thermal conductivity determination typically report the following parameters:

• Thermal conductivity (λ), in W/(m·K), measured at one or more specified temperatures
• Thermal diffusivity (a), in mm²/s, obtained from transient or flash methods
• Specific heat capacity (cp), in J/(g·K), often determined by differential scanning calorimetry (ISO 11357 or ASTM E1269) as supporting data
• Thermal resistance (R), in m²·K/W, relevant for insulation products and multi-layer structures
• Apparent (effective) thermal conductivity for heterogeneous, anisotropic, or porous materials
• Temperature-dependent conductivity profiles across a defined temperature range, for example from -40 °C to 300 °C

Testing Methods

Two principal families of techniques are used: steady-state methods, which establish a constant temperature gradient, and transient methods, which analyze the time-dependent temperature response to a heat pulse. The choice depends on the conductivity range, sample geometry, temperature range, and required accuracy.

Guarded Hot Plate Method (ISO 8302, ASTM C177)

The guarded hot plate method is an absolute, primary method featuring the highest accuracy, typically better than 2%. A heated plate is sandwiched between two specimen halves, and guard heaters eliminate lateral heat losses. It is the reference method for insulation materials and low-conductivity plastics, though measurement times are long, often several hours per temperature point.

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

The heat flow meter technique is a comparative steady-state method in which the specimen is placed between two temperature-controlled plates and one or more calibrated heat flux transducers measure the heat flow. ASTM E1530 is widely applied to plastics, rubbers, and composites in the approximate range of 0.1 to 40 W/(m·K). It offers a good balance of accuracy (typically 3% to 5%), speed, and convenience for routine quality control.

Transient Plane Source Method (ISO 22007-2, GB/T 32064)

The transient plane source, or hot disc, method uses a double-spiral nickel sensor that acts as both heat source and temperature recorder. The sensor is sandwiched between two pieces of the specimen, and a constant electrical power pulse is applied. The method covers a broad conductivity range from approximately 0.005 to 500 W/(m·K) and yields thermal conductivity, diffusivity, and volumetric heat capacity simultaneously from a single measurement lasting seconds to minutes.

Modified Transient Plane Source Method (ASTM D7984)

This variant employs a one-sided, planar sensor with a protective backing, allowing testing of a single specimen surface. It is well suited to inhomogeneous materials, coatings, and applications where only one finished surface is accessible, with a typical measurement time of a few seconds.

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

The laser flash method determines thermal diffusivity by applying a short laser or light pulse to the front face of a small disc-shaped specimen, typically 12.7 mm in diameter and 1 to 3 mm thick, and recording the temperature rise on the rear face with an infrared detector. When combined with independently measured density and specific heat capacity, it calculates thermal conductivity. It is the preferred method for high-conductivity filled plastics and for measurements over wide temperature ranges, including cryogenic and elevated-temperature conditions.

Transient Line Source Method (ASTM D5930, ASTM D5334)

A needle probe containing a heater wire and thermocouple is inserted into or placed on the specimen. The temperature response to constant heating is analyzed to yield thermal conductivity. The method is quick and convenient for melts, viscous polymers, elastomers, and powders, with an approximate range of 0.1 to 5 W/(m·K).

Method Selection Guidelines

• Insulation foams and low-conductivity plastics: guarded hot plate (ISO 8302, ASTM C177) or heat flow meter (ASTM C518, ASTM E1530)
• General engineering plastics at room temperature: guarded heat flow meter (ASTM E1530)
• Thermally conductive compounds above 2 W/(m·K): laser flash (ISO 22007-4, ASTM E1461) or transient plane source (ISO 22007-2)
• Rapid screening and multi-property measurement: transient plane source (ISO 22007-2)
• Polymer melts and curing systems: transient line source (ASTM D5930)
• Elevated or cryogenic temperature mapping: laser flash (ASTM E1461)

Applications

Thermal conductivity data for plastics support design, qualification, and regulatory compliance across many sectors:

• Electronics and semiconductor packaging: heat spreaders, LED substrates, thermal interface pads, connector housings, and 5G device components requiring thermally conductive engineering plastics
• Electric vehicle and battery systems: battery module enclosures, cooling components, cable insulation, and flame-retardant structural parts where heat dissipation and thermal runaway prevention are critical
• Building and construction: insulation boards, window profiles, and piping systems evaluated for energy efficiency and compliance with thermal performance regulations
• Home appliances and HVAC: housings, gaskets, and components where thermal insulation or controlled heat transfer affects safety and energy consumption
• Aerospace and automotive: lightweight composite structures, interior panels, and under-hood components subjected to wide temperature ranges
• Medical devices: instrument components and sterilizable parts requiring predictable thermal behavior
• Material research and development: formulation optimization of filled compounds, verification of simulation inputs for molding and thermal analysis software, and incoming material quality control

FAQ

What is the typical thermal conductivity of common plastics?

Most unfilled plastics fall between 0.10 and 0.50 W/(m·K) at room temperature. For example, PP and PE are approximately 0.22 to 0.45 W/(m·K), PS is near 0.12 W/(m·K), PC is near 0.20 W/(m·K), and PA6 is approximately 0.25 to 0.35 W/(m·K). Highly filled thermal management compounds can reach 5 to 20 W/(m·K).

How do I choose between steady-state and transient methods?

Steady-state methods such as ASTM E1530 provide well-established results at fixed temperatures and are ideal for insulation and mid-range conductivity materials. Transient methods such as ISO 22007-2 and ISO 22007-4 are faster, cover wider conductivity and temperature ranges, and deliver diffusivity and heat capacity data simultaneously. For regulatory comparisons, always confirm the standard specified by the customer or end-use specification.

What temperature range can be tested?

Heat flow meter instruments typically operate from -20 °C to 200 °C or higher, while laser flash systems can cover approximately -125 °C to 1100 °C for specialized applications. The applicable range depends on the instrument configuration and the thermal stability of the polymer.

How much sample is required?

Requirements vary by method: laser flash needs a small disc of about 12.7 mm diameter and 1 to 3 mm thickness; heat flow meter methods typically require 50 mm diameter discs or squares 2 to 25 mm thick; transient plane source testing requires two matching pieces at least 20 to 40 mm across. Duplicated or triplicated specimens are recommended to verify repeatability.

Why do results differ between laboratories or methods?

Differences can arise from specimen preparation, anisotropy of filled compounds, contact resistance, moisture conditioning, and the inherent uncertainty of each technique. Reporting the test standard, temperature, specimen orientation, and conditioning history is essential for meaningful comparison. Interlaboratory reproducibility of 5% to 10% is common for plastics.

Can finished products be tested instead of molded plaques?

Yes, provided a suitably flat and parallel test area can be obtained. Sections cut from finished parts can be evaluated, but geometry, thickness limitations, and material anisotropy introduced by processing must be considered. One-sided transient sensors are particularly useful for finished surfaces.

How long does thermal conductivity testing take?

Transient measurements such as hot disc or laser flash typically take from several minutes to under an hour per temperature point, including preparation. Steady-state measurements require thermal equilibrium at each condition and may take two to eight hours per point. Complete test reports from accredited third-party laboratories are generally issued within five to ten working days.