Accurate determination of the thermal conductivity of plastics is critical for material selection in electronics, electric vehicles, LED lighting, and thermal insulation applications. This article explains the internationally recognized test methods, including the guarded hot plate, heat flow meter, transient plane source, and laser flash techniques, together with the relevant ISO, ASTM, and GB/T standards. It also provides practical guidance on sample preparation, measurement uncertainty, and how to select the right method for each plastic material class.
Technical Overview
Thermal conductivity, denoted by the symbol λ (or k) and expressed in W/(m·K), is the fundamental material property that quantifies the ability of a substance to conduct heat. For plastics, this property spans an unusually wide range: unfilled polymers typically exhibit values between 0.10 and 0.50 W/(m·K) at room temperature, thermally conductive engineering compounds filled with aluminum nitride, boron nitride, alumina, or graphite can reach 1 to 20 W/(m·K) or higher, and polymer foams used for insulation can fall below 0.030 W/(m·K). Accurate measurement across this range requires matching the test method to the material, geometry, and conductivity level.
Several characteristics of plastics make thermal measurement particularly challenging. Their low absolute heat fluxes demand highly sensitive instrumentation, their thermal conductivity varies significantly with temperature, polymer chains and oriented fillers create strong anisotropy between through-plane and in-plane directions, and the soft viscoelastic surfaces introduce interfacial contact resistance at the sensor or plate interface. Moisture absorption in polyamides and other hygroscopic resins further complicates reproducibility.
The internationally recognized framework for accurate determination consists of the ISO 22007 series dedicated to plastics, the steady-state guarded hot plate and heat flow meter methods described in ISO 8302, ISO 8301, ASTM C177, and ASTM C518, the laser flash technique covered by ISO 22007-4 and ASTM E1461, and the corresponding GB/T 10294 and GB/T 10295 national standards. Accredited third-party laboratories apply these methods with traceable calibration to deliver defensible results for product development, certification, and quality control.
Testing Samples
Neat Resin Plates and Compression-Molded Sheets
Unfilled thermoplastics such as PE, PP, PS, PVC, PC, PMMA, and epoxy or silicone casting resins are usually supplied as flat plates with parallel faces. For steady-state plate methods, specimens of approximately 100 to 300 mm square and 5 to 50 mm thick are preferred, because the low heat flux of unfilled polymers requires a generous metering area. Surfaces must be machined or molded flat within tight parallelism tolerances to avoid air gaps that bias results upward or downward.
Mineral-Filled and Fiber-Reinforced Compounds
Thermally conductive grades of PA6, PA66, PPS, PBT, PET, and PC, filled with aluminum nitride, boron nitride, alumina, graphite, or glass fibers, are typically evaluated on injection-molded plaques or compression-molded slabs. Because filler orientation during molding creates pronounced anisotropy, specimens should be tested in both the through-plane and in-plane directions and the orientation clearly reported. Discs of 10 to 12.7 mm diameter and 1 to 3 mm thickness are standard for the laser flash method.
Films, Thin Sheets, and Coatings
Polymer films, adhesive layers, thermal interface pads, potting compounds, and conformal coatings down to sub-millimeter thickness are measured with transient methods. A stack of multiple film layers can be used to reach a measurable total thickness, provided interlayer air is excluded by applying modest uniform pressure or a thin layer of thermal contact medium.
Foams and Insulation Boards
PU, XPS, EPS, PE, and phenolic foams are tested as large-format boards, typically 200 to 300 mm square and 10 to 50 mm thick, using the guarded hot plate or heat flow meter method. Specimens must be conditioned to equilibrium, because cell gas composition and moisture strongly influence the apparent conductivity of closed-cell foams.
Testing Items
The core measurands for thermal characterization of plastics, and the quantities routinely reported by accredited third-party laboratories, are listed below.
- Thermal conductivity λ, in W/(m·K), at one or more specified mean temperatures
- Thermal diffusivity α, in mm²/s, describing transient heat propagation
- Specific heat capacity cp, in J/(g·K), usually determined by differential scanning calorimetry
- Density ρ at the test temperature, required to link the three properties through the relation λ = α × ρ × cp
- Thermal resistance and thermal impedance of layered assemblies and interface materials
- In-plane versus through-plane conductivity for anisotropic filled compounds
- Temperature-dependent conductivity curves, for example from -40 °C to 200 °C
Optional supplementary testing includes thermal aging stability, conductivity after humidity conditioning, and comparative evaluation before and after thermal cycling, which are frequently requested for automotive and electronics qualification programs.
Testing Methods
Guarded Hot Plate Method (ISO 8302 / ASTM C177 / GB/T 10294)
The guarded hot plate is the primary absolute method for low-conductivity materials. A centrally metered hot plate, surrounded by a guard ring that eliminates lateral heat leakage, is sandwiched between two specimen plates and cold plates, establishing one-dimensional steady-state heat flow. Because the method requires no calibration against reference materials, it achieves uncertainties of approximately 1 to 2 percent and serves as the benchmark for insulating plastics and foams below roughly 2 W/(m·K). Its drawbacks are long stabilization times of several hours and the need for large, perfectly flat specimens.
Heat Flow Meter Method (ISO 8301 / ASTM C518 / GB/T 10295)
The heat flow meter places calibrated heat flux transducers between the specimen and the temperature-controlled plates. After calibration with certified reference materials traceable to national metrology institutes, it reproduces guarded hot plate results within about 2 to 5 percent while cutting test time substantially. It is the workhorse method for routine quality control of insulation boards, polymer foams, and low-conductivity plastics, and it readily generates λ-versus-temperature profiles.
Transient Plane Source Method (ISO 22007-2)
The transient plane source technique embeds a double-spiral nickel sensor, protected by a thin insulating film, between two specimens with flat faces. A constant electrical heating step is applied, and the sensor simultaneously records the temperature response, from which conductivity and diffusivity are extracted by fitting the theoretical model. The method covers an exceptional range of approximately 0.005 to 500 W/(m·K), needs only small specimens such as 20 to 50 mm squares, completes a measurement in seconds to minutes, and offers dedicated modules for measuring anisotropic materials in both principal directions. It is the preferred routine method for plastics under the ISO 22007 series.
Laser Flash Method (ISO 22007-4 / ASTM E1461)
In the laser flash method, a short energy pulse irradiates the front face of a small disc specimen while an infrared detector records the temperature rise on the rear face. The half-rise time yields thermal diffusivity, and thermal conductivity is then calculated from λ = α × ρ × cp using separately measured density and specific heat. The method requires only small specimens, is non-contact, is essentially free of contact resistance errors, and covers moderate to highly conductive filled plastics as well as elevated temperatures up to several hundred degrees Celsius. Its accuracy is governed by the uncertainty of the cp and density inputs, so simultaneous or in-house DSC measurement is strongly recommended.
Comparative and Supplementary Techniques
Steady-state comparative methods such as ASTM E1225 and the transient comparative method of ISO 22007-6 place the specimen in series with a reference material of known conductivity, offering a practical route for rod-shaped or irregular specimens. The modified transient plane source technique of ASTM D7984 extends transient testing to thin, flexible, and textile-like polymer constructions. These methods are valuable for screening, although their uncertainties are generally wider than those of the primary techniques.
Practical Measures for Accurate Results
Regardless of the method chosen, the following practices are decisive for accuracy and interlaboratory reproducibility.
- Condition specimens to standard atmosphere, typically 23 ± 2 °C and 50 ± 5 % relative humidity per ISO 291, and dry hygroscopic resins such as polyamide beforehand
- Machine or mold faces flat and parallel, and measure actual thickness at multiple points, since thickness error translates directly into conductivity error
- Apply thin, controlled layers of thermal contact media or defined clamping pressure to suppress interfacial resistance in plate and laser flash measurements
- Verify sensor and apparatus calibration with certified reference materials whose conductivity brackets the expected specimen value
- Report the mean test temperature, method and standard, specimen orientation, and number of replicates, and express results with a stated measurement uncertainty
- Characterize anisotropic filled compounds in both through-plane and in-plane directions rather than assuming isotropy
Applications
Accurate thermal conductivity data for plastics underpin design and compliance decisions across a broad range of industries.
- Electric vehicle battery modules, where flame-retardant and thermally conductive PA6, PPS, or PC housings and cooling components manage cell temperature gradients
- LED luminaires and power electronics, where filled plastic heat sinks, substrates, and enclosures replace metal to reduce weight and cost
- Thermal interface materials, gap pads, adhesives, encapsulants, and potting compounds for semiconductor packaging and power modules
- Wire and cable insulation, connectors, and breaker housings evaluated under electrical equipment safety requirements referenced by IEC and UL standards
- Building insulation boards of PU, XPS, and EPS, where declared λ values determine energy compliance calculations
- Household appliance housings, cookware handles, and consumer electronics enclosures requiring controlled heat dissipation
- Research and development of new filler systems, compound formulation, and injection molding process optimization
Typical thermal conductivity targets illustrate the practical significance of measurement accuracy: unfilled ABS around 0.17 W/(m·K), filled PA6 with 50 percent aluminum nitride around 3 to 5 W/(m·K), and graphite-loaded compounds exceeding 15 W/(m·K) in-plane. A relative error of just 10 percent can decide whether a design meets its thermal budget, which is why certified testing by accredited third-party laboratories is routinely specified in supply agreements and qualification protocols.
FAQ
What is the most suitable method for measuring the thermal conductivity of plastics?
For unfilled plastics, foams, and insulation boards up to roughly 2 W/(m·K), the guarded hot plate and heat flow meter methods deliver the best accuracy. For filled, reinforced, and moderately to highly conductive plastics, the transient plane source method under ISO 22007-2 and the laser flash method under ISO 22007-4 are the most efficient and widely accepted choices.
Why do thermal conductivity results sometimes differ between laboratories?
Differences usually arise from specimen anisotropy and orientation, uncorrected contact resistance, different mean test temperatures, assumptions about specific heat capacity in diffusivity-based calculations, and differing moisture conditioning. Specifying the exact standard, orientation, temperature, and conditioning state in the test plan minimizes discrepancies.
What sample size and thickness are required?
Laser flash testing uses small discs of typically 12.7 mm diameter and 1 to 3 mm thickness. Transient plane source testing requires two specimens with matching flat faces, commonly 20 to 50 mm across and a few millimeters thick. Steady-state plate methods need larger plates, typically 100 to 300 mm square, with thickness selected so that the specimen represents the bulk material.
What temperature range can be tested?
Most commercial instrumentation covers approximately -50 °C to 300 °C, which comfortably brackets the service range of nearly all thermoplastics. Laser flash systems extend to much higher temperatures, useful for thermosets and ceramic-filled compounds, while insulation testing is typically performed from -10 °C to 40 °C mean temperature in accordance with product standards.
How is thermal conductivity different from thermal diffusivity?
Thermal conductivity quantifies steady heat flow through a material, while thermal diffusivity describes how quickly temperature changes propagate during transient conditions. They are linked by the equation λ = α × ρ × cp, so diffusivity measurements require density and specific heat data to yield a conductivity value.
What accuracy can be expected from a professional measurement?
Guarded hot plate measurements on insulating plastics can reach uncertainties near 1 to 2 percent, calibrated heat flow meter and transient plane source measurements typically fall within 3 to 5 percent, and laser flash results carry roughly 5 percent uncertainty when high-quality cp and density inputs are available. Accredited third-party laboratories report these uncertainties alongside the measured values.
How many replicates are needed for a reliable result?
Good practice calls for at least three specimens per material and direction, plus repeat measurements on each specimen. For anisotropic compounds, separate specimen sets oriented in the through-plane and in-plane directions should be prepared, since processing-induced filler alignment can cause differences exceeding a factor of two.