Thermal conductivity measurement of plastics is essential for evaluating heat transfer performance in electronic packaging, thermal interface materials, cable insulation, and thermally conductive polymer compounds. This article provides a comprehensive technical overview of the determination of plastic thermal conductivity, covering testing principles, applicable standards such as ISO 22007, ASTM C177, ASTM C518, and ASTM E1461, specimen requirements, and method selection guidance. Whether you are developing engineering polymers or outsourcing testing to accredited third-party laboratories, this guide will help you choose the right thermal characterization approach.
Technical Overview
Thermal conductivity, denoted as λ (lambda) and expressed in W/(m·K), quantifies a material's ability to conduct heat. For most unfilled and amorphous plastics, thermal conductivity falls within a narrow range of approximately 0.10 to 0.50 W/(m·K), which is roughly 1,000 times lower than that of metals. However, modern thermally conductive plastics—compounds filled with thermally conductive fillers such as alumina, boron nitride, graphite, or aluminum nitride—can achieve thermal conductivity values from 1 W/(m·K) up to 20 W/(m·K) or higher, making them viable replacements for metal heat spreaders and housings in weight-sensitive and electrically sensitive applications.
The determination of plastic thermal conductivity is a core requirement in material development, incoming material inspection, quality control, and product reliability assessment. Because plastics are low-conductivity, temperature-sensitive, and often anisotropic (especially fiber- or flake-filled compounds), selecting an appropriate measurement method and specimen geometry is critical to obtaining accurate and reproducible data.
Thermal conductivity measurement techniques are broadly divided into two categories: steady-state methods, in which a constant temperature gradient is established across the specimen, and transient methods, in which the temperature response to a controlled heat pulse or heating step is analyzed. Closely related properties frequently measured alongside thermal conductivity include thermal diffusivity α (mm²/s) and specific heat capacity cp (J/(g·K)), which are linked through the fundamental relationship λ = α · ρ · cp, where ρ is density.
Testing Samples
Flat Plaques and Sheet Specimens for Steady-State Methods
Guarded hot plate and heat flow meter methods require flat specimens with smooth, parallel faces, typically square plates of 100 × 100 mm to 300 × 300 mm with thicknesses ranging from a few millimeters up to 50 mm. Face parallelism and flatness tolerances are essential, because air gaps between the specimen and the instrument plates introduce significant contact resistance errors, particularly for low-conductivity materials.
Disc Specimens for Laser Flash Analysis
The laser flash method uses small disc-shaped specimens, commonly 10 mm to 12.7 mm in diameter and 1 mm to 3 mm thick. Because most plastics are semi-transparent to infrared radiation, specimens are usually coated with a thin layer of graphite or sputtered gold to ensure opaque, uniform energy absorption at the front face and reliable infrared detection at the rear face.
Paired Block Specimens for Transient Plane Source Testing
The transient plane source (hot disc) method places a double-spiral nickel sensor between two pieces of the same material, each with at least two flat, parallel faces. Typical specimen dimensions are 20 mm to 50 mm across with thicknesses of 5 mm to 20 mm. This geometry also enables characterization of anisotropic conductivity by orienting the sensor relative to injection-molding or extrusion directions.
Liquids, Pastes, Melts, and Compounds
Uncured encapsulants, thermal greases, polymer melts, and powder compounds can be measured with transient line-source (hot-wire or needle-probe) techniques, which immerse a thin heated probe directly into the sample. This approach is also suitable for quality-control screening of raw pellets and molding compounds when molded specimens are not yet available.
Testing Items
Typical measurement parameters reported for plastic thermal characterization include:
- Thermal conductivity λ, in W/(m·K), reported at one or more specified temperatures
- Thermal diffusivity α, in mm²/s
- Specific heat capacity cp, in J/(g·K), typically by differential scanning calorimetry (ISO 11357-4) or derived from transient measurements
- Density ρ at the test temperature, in g/cm³, required for calculating conductivity from diffusivity
- In-plane versus through-plane (directional) thermal conductivity for anisotropic composites
- Thermal conductivity versus temperature curves across service ranges such as -40 °C to 200 °C
- Thermal resistance R, in m²·K/W, for insulation and structural applications
Testing Methods
Guarded Hot Plate Method (ISO 8302, ASTM C177, GB/T 10294)
The guarded hot plate is the primary absolute steady-state method and serves as the reference technique for thermal conductivity in the approximate range of 0.02 to 2 W/(m·K). A heated main plate surrounded by a guard ring creates one-dimensional axial heat flow through the specimen stack toward cooled plates, and conductivity is calculated directly from the electrical power input, specimen geometry, and surface temperature difference using Fourier's law: λ = Q · d / (A · ΔT). It offers the highest accuracy, typically 1 to 3 percent, but requires long equilibration times and large, thick specimens.
Heat Flow Meter Method (ISO 8301, ASTM C518, GB/T 10295)
The heat flow meter is a faster steady-state comparative method in which calibrated heat flux transducers measure the heat flow through a specimen sandwiched between temperature-controlled plates. The apparatus must be calibrated using certified reference materials traceable to national metrology institutes. It is well suited for routine quality control of insulation boards, foams, and low-to-medium conductivity plastics, with typical measurement uncertainty of 2 to 5 percent.
Transient Plane Source / Hot Disc Method (ISO 22007-2)
The transient plane source method records the temperature rise of a spiral sensor that simultaneously acts as heater and resistance thermometer while embedded between two specimen halves. By analyzing the time-dependent temperature increase over a defined time window, both thermal conductivity and diffusivity are obtained in a single measurement. The technique covers a wide conductivity range (approximately 0.01 to 500 W/(m·K)), requires small specimens, works from sub-ambient temperatures up to roughly 500 °C, and permits testing of anisotropic materials, making it highly popular for thermally conductive plastic compounds.
Transient Line Source Method (ASTM D5930, ASTM D7896)
The transient line source method, described for plastics in ASTM D5930, inserts a needle probe containing a heater wire and thermocouple into the sample or into a cavity molded into the specimen. Analysis of the linear portion of the temperature rise versus logarithmic heating time yields thermal conductivity. It is rapid, tolerant of imperfect specimen geometry, and ideal for melts, pastes, gels, powders, and in-process quality control.
Laser Flash Method (ISO 22007-4, ASTM E1461, GB/T 22588)
The laser flash method directly measures thermal diffusivity: a short laser pulse energizes the front face of a coated disc specimen, and an infrared detector records the rear-face temperature rise. Thermal conductivity is then calculated as λ = α · ρ · cp using independently measured density and specific heat capacity. Benefits include very small specimen volume, rapid testing, excellent high-temperature capability, and typical diffusivity uncertainty of 3 to 5 percent. It is the preferred technique for thin specimens, high-conductivity filled plastics, and temperature-dependent characterization, provided density and heat capacity data are available.
Method Selection Considerations
- Use guarded hot plate or heat flow meter methods for thick insulation-grade specimens and when the lowest uncertainty is required
- Use transient plane source methods for filled conductive plastics, anisotropic parts, and limited sample quantities
- Use laser flash analysis for thin discs, elevated-temperature studies, and diffusivity-focused evaluations
- Use transient line source methods for liquids, uncured compounds, and rapid screening
- Always report specimen orientation relative to processing direction, because injection-molded or extruded parts may show through-plane conductivity several times lower than in-plane conductivity
Applications
Thermal conductivity data for plastics directly support engineering decisions across many industries:
- Thermal management of power electronics, LED modules, and automotive control units, where conductive plastic housings and heat spreaders must dissipate heat while providing electrical insulation
- Electric vehicle battery systems, including cell holders, busbar insulation, potting compounds, and thermal interface materials
- Wire and cable insulation and jacketing, for which steady-state conductivity data feed ampacity and thermal-aging calculations
- Appliance and building product components, including foam insulation panels and energy-efficiency certifications
- Polymer compound development, comparing filler types, loadings, and processing conditions such as screw speed and molding parameters
- Reliability engineering and simulation, supplying accurate material properties for finite-element thermal models
FAQ
What is the typical thermal conductivity of common plastics?
Unfilled commodity and engineering plastics generally measure between 0.10 and 0.50 W/(m·K): polypropylene and polystyrene are near 0.12 to 0.17 W/(m·K), while materials such as PPS or PEEK are somewhat higher. Highly filled thermally conductive plastics reach 1 to 20 W/(m·K) depending on filler chemistry, loading, and orientation.
Which method should be chosen for thin films and small parts?
The laser flash method is preferred for specimens 1 to 3 mm thick, while the transient plane source method handles small blocks from roughly 20 mm across. For steady-state plate methods, thin specimens amplify the relative effect of contact resistance and are generally less suitable.
What temperature range can be tested?
Most commercial instruments cover approximately -30 °C to 300 °C, and laser flash systems can extend well beyond 500 °C. Practical limits for plastics are set by glass transition, melting, softening, and oxidation, so test temperatures should remain safely within the material's stable region.
Why do different methods sometimes give different results for the same plastic?
Divergence arises from specimen orientation in anisotropic materials, contact resistance, semi-transparency in radiation-based detection, differing sensitivity to in-plane versus through-plane heat flow, and whether conductivity is measured directly or derived from diffusivity. Harmonizing specimen preparation, orientation, and temperature is essential for cross-laboratory comparison.
What accuracy can be expected?
Guarded hot plate measurements achieve approximately 1 to 3 percent uncertainty, heat flow meter and transient plane source methods typically 2 to 5 percent, and laser-flash-derived conductivity 5 to 8 percent when uncertainties in density and specific heat are included. Reporting full measurement conditions and uncertainty budgets is standard practice for accredited third-party laboratories.
How should specimens be prepared before testing?
Specimens should be conditioned to a stable moisture and thermal state in accordance with the material specification, have flat and parallel faces within the tolerance stated by the applicable standard, be free of voids and sink marks at the measurement location, and be documented with molding direction, thickness, and any coatings applied for opacity.