Thermal conductivity is one of the most important thermal properties of plastics, directly determining their performance in thermal management, heat insulation, and electronic packaging applications. This article presents a professional analysis of thermal conductivity determination for plastic materials, covering measurement principles, standard test methods, sample requirements, testing parameters, and typical industrial applications. By understanding the available techniques and their applicable ranges, engineers and quality professionals can select the most suitable testing approach and accurately interpret the resulting data.
Technical Overview
Thermal conductivity, denoted by the symbol λ (or k) and expressed in W/(m·K), quantifies a material's ability to conduct heat. Plastics are intrinsically thermal insulators, with unfilled polymers typically exhibiting thermal conductivity values between 0.1 and 0.5 W/(m·K). In recent years, thermally conductive plastics filled with ceramic or carbon-based fillers, such as boron nitride, aluminum nitride, alumina, graphite, graphene, or metal powders, have been developed with conductivity values reaching 1 to 20 W/(m·K) or higher, making them viable alternatives to metals in many heat dissipation designs.
Thermal conductivity determination for plastics is closely related to two other thermal parameters: thermal diffusivity (α, in mm²/s) and specific heat capacity (Cp, in J/(g·K)). These quantities are linked by the fundamental relationship:
λ = α × ρ × Cp, where ρ is the density of the material.
Because polymer molecular chains and filler particles often orient during processing, plastic materials frequently display thermal anisotropy, meaning the in-plane (lateral) conductivity may differ significantly from the through-plane (thickness) direction. Thermal conductivity also varies with temperature, so test results must always be reported together with the measurement temperature. Reliable determination therefore requires appropriate specimen preparation, correct method selection, and strict adherence to recognized international and national standards such as ISO 22007, ASTM C177, ASTM C518, ASTM D5930, ASTM E1461, and the corresponding GB standards.
Testing Samples
Unfilled Thermoplastic Resins
Common engineering materials such as polyethylene (PE), polypropylene (PP), polystyrene (PS), polyamide (PA), polycarbonate (PC), polyvinyl chloride (PVC), ABS, and PTFE can be tested as injection-molded plaques, compression-molded sheets, or machined coupons. Specimens must have smooth, flat, parallel surfaces to ensure intimate contact with the measurement sensors or plates. Surface burrs, warpage, and curvature are the most common causes of measurement error.
Thermally Conductive Compounds and Filled Plastics
Filled compounds containing boron nitride, aluminum nitride, alumina, graphite, carbon fibers, or metallic fillers typically exhibit strong anisotropy. For these materials, specimens should be prepared under conditions representative of the actual molding process, because filler orientation during injection or compression molding substantially affects the measured conductivity. When possible, both through-plane and in-plane directions should be characterized.
Films and Thin Sheets
Thin polymer films used in electronics, flexible circuits, and thermal interface applications require special handling. Multiple film layers may be stacked to reach the minimum thickness required by certain methods, provided that entrapped air is eliminated. Layered specimens should be tested with methods suitable for thin samples, such as the laser flash method or specialized transient techniques.
Granules, Powders, and Pastes
Raw resin granules, powders, pastes, and gels can be measured using dedicated sample holders or cells that apply a controlled compaction pressure. Because packing density strongly influences the apparent thermal conductivity of loose materials, the bulk density of the specimen must be recorded and reported alongside the conductivity value.
Testing Items
A comprehensive thermal characterization program for plastic materials generally includes the following measurable parameters:
- Thermal conductivity (λ), expressed in W/(m·K), measured over a specified temperature range, typically from -20 °C to 300 °C depending on the method and instrument capability.
- Thermal diffusivity (α), expressed in mm²/s, obtained from transient methods such as laser flash or temperature wave analysis.
- Specific heat capacity (Cp), expressed in J/(g·K), determined by differential scanning calorimetry (DSC) in accordance with ISO 11357 or ASTM E1269, or measured simultaneously by certain transient methods.
- Density (ρ), expressed in g/cm³, measured per ISO 1183 or ASTM D792 as required for conductivity calculation.
- Anisotropic conductivity characterization, distinguishing through-plane and in-plane values for oriented or highly filled materials.
- Temperature-dependent conductivity curves showing how the material behaves across the intended service temperature range.
Testing Methods
Thermal conductivity determination methods fall into two categories: steady-state techniques, which measure heat flux after thermal equilibrium is established, and transient techniques, which analyze the temperature response to a short heating pulse. Each method has distinct advantages, applicable conductivity ranges, and specimen requirements.
Guarded Hot Plate Method (ISO 8302, ASTM C177, GB/T 10294)
The guarded hot plate method is an absolute steady-state technique considered the benchmark for thermal conductivity measurement. A heated main plate surrounded by a guard ring is sandwiched between two identical specimens, and the heat flow through the specimens is measured after equilibrium. The method offers excellent accuracy for insulating and low-conductivity materials but requires large, precisely machined specimens and long equilibrium times, often several hours per temperature point.
Heat Flow Meter Method (ISO 8301, ASTM C518, GB/T 10295)
The heat flow meter method is a comparative steady-state technique in which the specimen is placed between two plates at different temperatures and one or more calibrated heat flux transducers measure the heat flow through the sample. Calibration is performed using certified reference materials traceable to recognized metrology institutes. This method is faster than the guarded hot plate and well suited for routine quality control of insulation boards, foam sheets, and low-conductivity plastics.
Transient Plane Source Method (ISO 22007-2, GB/T 32064)
The transient plane source method, commonly known as the hot disc technique, uses a spiral nickel sensor that acts as both heat source and temperature recorder. The sensor is placed between two specimen halves, a constant electrical power is applied, and the temperature rise is recorded over time. The method covers a wide conductivity range from approximately 0.01 to 500 W/(m·K), requires relatively small specimens, completes a measurement in seconds to minutes, and can measure bulk, in-plane, and through-plane conductivity with different sensor configurations. It is currently the most widely used technique for plastics, thermally conductive compounds, and rubbers.
Transient Line Source Method (ASTM D5930, ASTM D5334)
The transient line source method, also called the hot wire or modified transient line source method, employs a needle probe containing a heating wire and a thermocouple. The probe is inserted into the specimen or pressed between pieces of the material, and the temperature response to a controlled heating current is used to calculate conductivity. This method is convenient for elastomers, melts, viscous pastes, powders, and compressible materials, with a typical measurement range up to about 10 to 20 W/(m·K).
Laser Flash Method (ISO 22007-4, ASTM E1461, GB/T 22588)
The laser flash method measures thermal diffusivity rather than conductivity directly. A short laser pulse heats the front face of a small disc-shaped specimen, and an infrared detector records the temperature rise on the rear face. Thermal conductivity is then calculated using independently measured density and specific heat capacity. The method requires only small specimens, covers an exceptionally wide diffusivity range, is highly accurate, and is particularly suitable for thin samples, films, and highly conductive ceramic-filled plastics. It can also be operated at elevated temperatures up to 1000 °C or beyond with appropriate furnaces.
Method Selection Guidelines
- For unfilled plastics and insulating materials with λ below 2 W/(m·K), the heat flow meter and transient plane source methods provide fast and reliable results.
- For thermally conductive compounds above 2 W/(m·K), the transient plane source method with appropriate sensors or the laser flash method is recommended.
- For thin films and small specimens, the laser flash method is generally preferred.
- For melts, pastes, gels, and powders, the transient line source method is the most practical choice.
- For reference-level accuracy and research applications, the guarded hot plate method remains the authoritative technique.
Applications
Thermal conductivity data for plastics directly supports product design, material selection, and quality control across numerous industries:
- Electronic thermal management: heat dissipation housings, LED lamp components, heat sinks, radiator parts, and thermal interface pads for consumer electronics and power modules.
- New energy vehicles: battery module housings, cell holder frames, charging connectors, busbar insulation parts, and power electronics encapsulation requiring verified heat dissipation performance.
- 5G and telecommunications: high-frequency device enclosures and antenna components that combine thermal management with dimensional stability.
- Wire and cable: insulation and sheathing compounds evaluated for heat buildup and ampacity calculations.
- Building and construction: insulation boards, foam panels, piping, and window profiles where certified thermal performance is required by energy-efficiency regulations.
- Household appliances: heater housings, motor components, cookware handles, and internal parts exposed to sustained heat.
- Industrial equipment: pump and valve liners, heat exchanger components, and chemically resistant parts operating in thermal environments.
- Material research and development: verification of filler dispersion efficiency, comparison of formulation variants, and validation of simulation inputs for thermal analysis software.
FAQ
What are the typical thermal conductivity values of common plastics?
Unfilled plastics generally fall between 0.1 and 0.5 W/(m·K). Representative values include PP at approximately 0.10 to 0.22 W/(m·K), PS around 0.10 to 0.13 W/(m·K), PE at roughly 0.33 to 0.50 W/(m·K), PVC around 0.12 to 0.25 W/(m·K), PC and PA near 0.20 to 0.35 W/(m·K), and PTFE at approximately 0.25 W/(m·K). Highly filled thermally conductive compounds can reach 1 to 20 W/(m·K) or more.
Why do different test methods sometimes give different results for the same plastic?
Differences arise from several sources: steady-state methods measure over long times and large temperature gradients, while transient methods capture short-time responses; specimen thickness, contact resistance, filler orientation, and anisotropy also influence results. For meaningful comparison, always confirm that specimens, direction of measurement, and test temperatures are equivalent.
What sample size and thickness are required for testing?
Requirements depend on the method. The transient plane source method typically requires two pieces of at least 20 mm × 20 mm with thickness determined by sensor size, commonly 5 mm or more. The laser flash method uses small discs, frequently 12.7 mm in diameter and 1 to 3 mm thick. Steady-state methods require larger flat plaques, often 100 mm × 100 mm or greater. Accredited third-party laboratories can advise on exact dimensions for each standard.
Over what temperature range can plastics be tested?
Most routine measurements are performed at room temperature, approximately 23 °C to 25 °C. With suitable instruments, testing can extend from about -20 °C to 300 °C for the transient plane source method, and to several hundred degrees Celsius with the laser flash method, provided the material does not degrade, soften, or melt during the test.
How should anisotropic materials be tested and reported?
For injection-molded or highly filled plastics, specimens should be prepared and tested in both the through-plane and in-plane directions. Test reports should clearly state the measurement direction, molding conditions, and specimen orientation to ensure the data correctly represent the intended application.
What information should a complete test report include?
A professional report should specify the test standard and method, specimen description and preparation, dimensions and density, conditioning, test temperature, measured thermal conductivity with uncertainty, calculated diffusivity and specific heat where applicable, instrument calibration references, and the test date.