PEEK insulation film is a high-performance engineering thermoplastic widely used in electric vehicles, aerospace, and advanced electronics where reliable thermal management is critical. Accurate thermal conductivity testing of PEEK films ensures that materials meet stringent design requirements for heat dissipation and electrical insulation. This article provides a comprehensive guide to PEEK insulation film thermal conductivity testing, covering sample requirements, testing items, international standards, methods, and applications.
Technical Overview
PEEK (polyetheretherketone) is a semi-crystalline, high-performance thermoplastic known for its exceptional combination of thermal stability, mechanical strength, chemical resistance, and excellent electrical insulation properties. With a glass transition temperature of approximately 143°C, a melting point of 343°C, and a continuous service temperature of up to 250–260°C, PEEK films have become indispensable in demanding applications such as electric vehicle battery insulation, flexible printed circuits, aerospace wiring, and semiconductor manufacturing equipment.
The thermal conductivity of unfilled PEEK film typically ranges from 0.20 to 0.30 W/(m·K), while thermally enhanced grades containing boron nitride (BN), aluminum nitride (AlN), or alumina (Al₂O₃) fillers can reach values above 1–5 W/(m·K). Because even small variations in crystallinity, filler loading, orientation, and film thickness can significantly influence heat transfer behavior, precise and standardized thermal conductivity testing is essential for material qualification, quality control, and thermal simulation inputs.
Thermal conductivity (λ) is rarely measured directly for polymer films. Instead, it is derived from the fundamental relationship: λ = α × ρ × Cp, where α is thermal diffusivity, ρ is density, and Cp is specific heat capacity. Modern testing laboratories therefore combine diffusivity measurement techniques, such as the laser flash method, with density determination and calorimetric analysis to deliver accurate, traceable results.
Testing Samples
Unfilled (Neat) PEEK Films
Virgin PEEK insulation films, typically available in thicknesses from 6 μm to 500 μm, represent the baseline material category. Testing these films establishes reference thermal properties and enables evaluation of the effects of crystallinity and processing conditions. Thin films below 50 μm are usually tested as stacked multi-layer specimens to achieve the minimum thickness required by standard instrumentation.
Filled and Thermally Enhanced PEEK Films
These composite films incorporate thermally conductive ceramic or mineral fillers to improve heat dissipation while maintaining electrical insulation. Because platelet-shaped fillers such as hexagonal boron nitride tend to align during film extrusion or casting, these materials frequently exhibit strong anisotropy, meaning in-plane thermal conductivity may be several times higher than through-plane conductivity. Both directions should be characterized for design-critical applications.
Multilayer and Coated Composite Films
PEEK films laminated with polyimide, coated with adhesives, or combined with metal foils require careful specimen preparation to ensure the test captures the effective thermal behavior of the complete stack. Interfaces between layers can introduce additional thermal resistance that must be considered when interpreting results.
Sample Preparation Requirements
- Specimens must be flat, uniform in thickness (typically ±2% variation), and free of wrinkles, bubbles, or voids.
- Standard disc specimens of 10–12.7 mm diameter are used for laser flash testing; larger pieces (typically 20 mm × 20 mm or greater) are required for transient plane source methods.
- All specimens should be conditioned and dried (for example, 2–4 hours at 150°C or per ASTM D618) to eliminate absorbed moisture that could distort results.
- Film thickness is measured at multiple points using a precision micrometer or optical profiler, and the average value is used in calculations.
Testing Items
A complete thermal characterization program for PEEK insulation film typically includes the following measurable parameters:
Thermal conductivity (λ, W/(m·K)) — the primary reported parameter, determined in both through-plane and in-plane directions where anisotropy is expected.
Thermal diffusivity (α, mm²/s) — measured directly by transient or flash methods, reflecting how quickly heat propagates through the film.
Specific heat capacity (Cp, J/(g·K)) — determined by differential scanning calorimetry (DSC) according to ASTM E1269 or ISO 11357-4.
Density (ρ, g/cm³) — measured at the test temperature using Archimedes' principle (ASTM D792) or pycnometry, since polymer density varies with temperature.
Temperature-dependent thermal property curves — measurements at multiple set points (for example, 25°C, 100°C, 150°C, 200°C, and 250°C) to capture behavior across the full service range.
Thermal stability indicators — supporting data such as glass transition temperature, melting point, and crystallinity from DSC analysis per ASTM D3418.
Testing Methods
Laser Flash Method (LFA)
The laser flash method, performed per ASTM E1461 or ISO 22007-4, is the industry-preferred technique for PEEK films. A short laser pulse heats the front surface of a disc specimen, and an infrared detector records the temperature response on the rear surface. From this curve, thermal diffusivity is calculated, and thermal conductivity is derived using measured density and specific heat. The method offers rapid measurement (typically under one second of data acquisition per shot), excellent accuracy (±3–5%), and a wide temperature range from room temperature to 300°C or higher. Thin PEEK films are tested as pressed or bonded stacks, or using specialized thin-film holders, to meet the recommended thickness-to-diameter ratio.
Transient Plane Source (Hot Disk / TPS)
ISO 22007-2 describes the transient plane source technique, in which a spiral nickel sensor sandwiched between two film specimens acts as both heat source and temperature monitor. This method directly yields thermal conductivity and diffusivity, works well for flexible films, and can measure in-plane, through-plane, and isotropic properties using different sensor geometries. It is particularly useful for anisotropic filled PEEK films.
Guarded Heat Flow Meter
ASTM E1530 covers the guarded heat flow meter method, in which a specimen is placed between temperature-controlled plates and steady-state heat flux is measured. This steady-state approach provides reliable results for thicker films (typically ≥0.5 mm) and slabs, and it serves as a valuable cross-check for transient techniques.
Measurement Workflow and Quality Assurance
- Instrument calibration is verified using certified reference materials such as NIST-traceable Pyroceram 9606 or graphite standards.
- Each specimen is measured a minimum of three times, and average values with standard deviations are reported.
- Semi-transparent films are coated with a thin graphite or gold layer to ensure uniform energy absorption during laser flash testing.
- Final reports include test standards, specimen geometry, test temperatures, individual and mean thermal conductivity values, and estimated measurement uncertainty.
Applications
Thermal conductivity test data for PEEK insulation film directly supports engineering decisions across several high-value industries:
Electric vehicle batteries — PEEK films serve as cell-to-cell insulation, thermal barriers, and component carrier films where validated thermal properties are critical for pack safety modeling.
Flexible printed circuits and high-temperature electronics — thermally enhanced PEEK films dissipate heat from power devices while providing reliable dielectric insulation.
Aerospace and defense wiring — lightweight PEEK insulation must maintain predictable thermal behavior across extreme environmental conditions.
Electric motor and generator insulation — slot liners and phase insulation materials require verified through-plane conductivity for winding thermal management.
Semiconductor manufacturing — PEEK films used in wafer handling, carrier tapes, and process equipment benefit from documented thermal data for process modeling.
5G and high-frequency communication modules — thin PEEK dielectric layers with tailored thermal conductivity help manage heat in densely packaged antenna and RF assemblies.
FAQ
What is the typical thermal conductivity of PEEK insulation film?
Unfilled PEEK film exhibits a thermal conductivity of approximately 0.20–0.30 W/(m·K) at room temperature. Filled grades containing boron nitride or other ceramic fillers can achieve through-plane values of 0.5–2 W/(m·K) and in-plane values exceeding 3–5 W/(m·K), depending on filler type, loading, and orientation.
Which testing method is most suitable for thin PEEK films?
For films thinner than 100 μm, the laser flash method applied to stacked specimens or the transient plane source method with a thin-film sensor is generally recommended. The choice depends on the required accuracy, anisotropy information, and available specimen area. Laboratories often combine both methods for cross-validation.
What sample size and thickness are required for testing?
Laser flash testing typically requires disc specimens of 10–12.7 mm diameter with a total thickness of 0.5–3 mm, achieved through stacking for thin films. Transient plane source testing usually requires two matching pieces of at least 20 mm × 20 mm. Exact requirements should be confirmed with the testing laboratory before submission.
Over what temperature range can PEEK film thermal conductivity be measured?
PEEK insulation film can typically be characterized from -50°C to 300°C, comfortably covering its continuous service temperature of 250–260°C. Multi-temperature profiles are recommended because polymer thermal conductivity varies measurably with temperature, particularly near the glass transition.
Why do in-plane and through-plane results differ for filled PEEK films?
Platelet and flake-shaped thermally conductive fillers align parallel to the film surface during extrusion or casting. This alignment creates efficient heat conduction pathways in the plane of the film while leaving the through-plane direction dominated by polymer-filler interfaces, producing significant anisotropy that must be reflected in thermal models.
How accurate are thermal conductivity measurements for PEEK films?
With proper calibration, specimen preparation, and conditioning, laser flash measurements typically achieve reproducibility within ±3–5%, while transient plane source measurements generally fall within ±5–10%. Reports from accredited laboratories should always include measurement uncertainty and the reference standards used.