PEEK film physical properties testing evaluates the mechanical, thermal, electrical, and dimensional performance of polyetheretherketone film using standardized experimental methods such as ASTM D882 tensile testing, DSC thermal analysis, and ASTM D149 dielectric breakdown measurement. This technical article explains the sample types, test items, procedures, typical results, and application relevance of PEEK film physical property experiments. Understanding these experiments helps engineers and quality professionals reliably qualify PEEK film for electronics, aerospace, energy, and medical applications.
Technical Overview
PEEK (polyetheretherketone) is a semi-crystalline aromatic thermoplastic in the polyaryletherketone (PAEK) family, and its film form is one of the most versatile high-performance polymer films available today. PEEK film combines a glass transition temperature of approximately 143 °C, a melting point near 343 °C, and continuous service capability up to 240 °C with high tensile strength, inherent flame retardancy, outstanding chemical resistance, and excellent dielectric behavior. Because final film properties depend strongly on processing history—especially orientation, quench rate, and annealing—systematic physical property experiments are essential for lot acceptance, quality control, and new grade development.
A complete experimental program for PEEK film typically covers dimensional characteristics, mechanical performance, thermal transitions and stability, electrical insulation behavior, surface properties, and optical quality. This article summarizes representative sample types, the standard test items and methods, typical experimental results, and the industrial applications that depend on them.
Testing Samples
Test specimens are cut from production rolls or laboratory cast sheets at representative locations (leading edge, middle, trailing edge) and are identified by grade, thickness, crystalline state, and thermal history. Thicknesses commonly range from 5 μm to 500 μm. Unless otherwise specified, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 40 hours in accordance with ASTM D618 or ISO 291 before testing.
Virgin Amorphous Cast PEEK Film
Amorphous film is produced by melt extrusion followed by rapid quenching below the glass transition temperature, which suppresses crystallization. It is transparent to translucent amber, exhibits low crystallinity (typically below 10 %), and shows very high elongation at break, often exceeding 150 %. It is selected for thermoforming, laminating, and applications where formability matters. When heated above Tg during thermal testing, amorphous film will cold-crystallize, an effect that must be considered when interpreting DSC and heat-shrinkage data.
Semi-Crystalline (Annealed) PEEK Film
Semi-crystalline film is achieved by annealing at approximately 200–300 °C or by controlled slow cooling, yielding crystallinity of roughly 20–35 %. It is tan to opaque, stiffer, and offers superior solvent resistance, hydrolytic stability, and long-term heat performance. This is the standard state for electrical insulation grades, and most published specification values refer to it.
Biaxially Oriented PEEK (BOPEEK) Film
Biaxially oriented film is stretched above Tg in the machine and transverse directions and then heat-set. Orientation raises tensile strength and modulus dramatically and controls thermal shrinkage. All physical tests on oriented film are reported separately for the machine direction (MD) and transverse direction (TD) because properties are anisotropic.
Filled and Modified PEEK Film
Modified grades include PTFE- or graphite-lubricated films for low friction, pigmented films for color coding, and carbon-loaded grades for electrical conductivity. These samples undergo the baseline physical property suite plus targeted tests such as coefficient of friction or surface resistivity.
Testing Items
The core physical property experiments performed on PEEK film include the following items:
- Thickness and thickness uniformity across the web and roll length
- Density and degree of crystallinity
- Tensile strength, tensile modulus (Young's modulus), and elongation at break
- Tear resistance (Elmendorf), puncture resistance, and MIT folding endurance
- Coefficient of friction and surface energy (wettability)
- Glass transition temperature, melting point, and heat of fusion by DSC
- Thermal degradation onset temperature and char yield by TGA
- Coefficient of linear thermal expansion (CTE) by TMA
- Heat shrinkage and dimensional stability at elevated temperature
- Dielectric strength, dielectric constant, dissipation factor, and volume/surface resistivity
- Water absorption and moisture-related dimensional effects
- Light transmittance and haze for transparent grades
- Flammability rating (UL 94) and limiting oxygen index (LOI)
Testing Methods
Specimen Conditioning and Preparation
All specimens are die-cut with sharp, burr-free edges because nicks trigger premature tear failures. Conditioning follows ASTM D618 Procedure A (23 °C / 50 % RH, 40 h) or ISO 291 (23/50). For anisotropic oriented films, matched MD and TD specimen sets are prepared. When moisture-sensitive comparisons are required, an additional dried sub-set (for example, 3 h at 150 °C in vacuum) may be included, with the conditioning state clearly reported.
Thickness and Density Measurement
Thickness is measured with a dead-weight dial micrometer or a non-contact optical/laser gauge per ASTM D6988 or ISO 4593, using at least ten positions across the web; mean, minimum, and maximum values are reported. Density is determined by hydrostatic weighing (ASTM D792, Method B) or density-gradient column (ISO 1183-2). Because PEEK density rises from about 1.26 g/cm³ for amorphous material to about 1.32 g/cm³ for highly crystalline material, density serves as a quick indirect crystallinity indicator.
Tensile Testing
Tensile properties are measured on a universal testing machine per ASTM D882 (thin plastic sheeting), ISO 527-3, or GB/T 1040.3. Rectangular strips 10–25 mm wide are tested at gauge lengths of 100 mm (or 50 mm for low-elongation material) with crosshead speeds of 25–500 mm/min selected according to expected elongation. The experiment yields tensile strength, strength at break, Young's modulus, and elongation at break; oriented films are tested in both MD and TD. Elevated-temperature tensile runs (for example, at 150 °C or 200 °C) quantify strength retention under service conditions.
Thermal Analysis (DSC, TGA, TMA)
Differential scanning calorimetry is performed per ASTM D3418 or ISO 11357 using 5–10 mg specimens under nitrogen at 10 °C/min in a heat–cool–heat cycle. The first heating reveals as-produced transitions (Tg near 143 °C, cold crystallization of amorphous film, and a melting endotherm near 343 °C), while the second heating reflects inherent behavior. Degree of crystallinity is calculated as Xc = (ΔHm − ΔHc) / ΔH°m × 100 %, using a 100 % crystalline PEEK melting enthalpy of 130 J/g. Thermogravimetric analysis (ASTM E1131 / ISO 11358) at 10 °C/min to 800 °C under nitrogen, followed by an air switch, establishes the 5 % mass-loss temperature (typically above 500 °C) and char yield. Thermomechanical analysis per ASTM E831 or ISO 11359-2 measures the CTE, typically 40–60 ppm/K below Tg, in both MD and TD.
Heat Shrinkage and Dimensional Stability
Free shrinkage is measured per ASTM D1204 by exposing marked 100 mm × 100 mm specimens to elevated temperature, for example 150 °C or 200 °C for 30 minutes, then calculating the percentage dimensional change in MD and TD after cooling. Heat-set oriented films are additionally tested near their heat-setting temperature to verify that residual shrinkage remains within specification, commonly below 2 %. Long-term aging studies (for example, 1000 h at 200 °C) track shrinkage, tensile retention, and discoloration.
Electrical Property Testing
Dielectric strength is determined per ASTM D149 or IEC 60243-1 using the short-time method with 25 mm cylindrical electrodes immersed in insulating oil. Because thin films are prone to pinhole artifacts, tests are performed either on single layers with careful electrode alignment or on defined multilayer stacks, reporting the breakdown field in kV/mm. Dielectric constant and dissipation factor are measured per ASTM D150 or IEC 60250 across frequencies from 60 Hz to 1 MHz. Volume and surface resistivity are measured per ASTM D257 or IEC 60093 using a guarded-electrode arrangement; PEEK film typically shows volume resistivity above 10^16 Ω·cm.
Surface, Optical, and Durability Testing
Coefficient of friction is measured per ASTM D1894 (film-to-steel and film-to-film). Elmendorf tear resistance follows ASTM D1922, MIT folding endurance follows ASTM D2176, and puncture resistance is measured with a probe fixture. Water absorption follows ASTM D570 (24 h immersion at 23 °C, plus saturation where required). Light transmittance and haze of transparent amorphous film are measured with an integrating-sphere hazemeter per ASTM D1003. Flammability is classified per UL 94 (V-0 achievable at thin gauges) and LOI per ASTM D2863, typically near 35 %.
Typical Experimental Results
- Density: 1.26 g/cm³ (amorphous) to 1.32 g/cm³ (highly crystalline)
- Tensile strength: 90–110 MPa for cast semi-crystalline film; 200 MPa or higher for oriented film
- Tensile modulus: 2.5–4.0 GPa for cast film; up to about 5 GPa for oriented film
- Elongation at break: 30–50 % for semi-crystalline film; over 150 % for amorphous film
- Tg of 143 °C; Tm of 340–345 °C; TGA 5 % mass-loss temperature above 500 °C in nitrogen
- Dielectric constant of 3.2–3.3 and dissipation factor of 0.001–0.005 (1 kHz to 1 MHz)
- Dielectric strength of approximately 150–250 kV/mm for 25 μm film
- Water absorption after 24 h: 0.1–0.5 %
- Heat shrinkage below 2 % after 150 °C for 30 min on heat-set film
- UL 94 V-0 rating; limiting oxygen index of approximately 35 %
Applications
The experimental data generated by these physical property tests directly support qualification of PEEK film in demanding end uses:
- Flexible and rigid-flex printed circuit substrates, adhesiveless coverlays, and high-temperature insulation layers
- Slot and phase insulation for electric motors, transformers, and e-mobility traction systems
- Lithium-ion battery cell and module insulation, barrier layers, and dielectric components
- Aerospace thermal and acoustic insulation, composite prepregs, and release films for composite curing
- Medical device components, sterilizable packaging, and biocompatible membrane supports
- Acoustic diaphragms for micro-speakers and precision sensing elements
- Semiconductor carrier tapes, high-temperature masking, and cleanroom process films
FAQ
What is the difference between testing amorphous and semi-crystalline PEEK film?
Amorphous film is ductile and transparent and will cold-crystallize when heated above Tg, so its DSC trace shows a crystallization exotherm and heat exposure changes its properties; semi-crystalline film is stiffer, opaque, and dimensionally stable, and its data reflect a fixed crystallinity. Test reports must always state the crystalline state and any annealing history.
Which standards govern tensile testing of PEEK film?
Thin PEEK film (below about 0.25 mm) is tested per ASTM D882 or ISO 527-3; thicker sheet is tested per ASTM D638 or ISO 527-1/-2. Elongation results depend strongly on gauge length and crosshead speed, so these parameters must accompany the reported values.
How is the crystallinity of PEEK film determined?
Crystallinity is most commonly calculated from DSC melting enthalpy: Xc = (ΔHm − ΔHc) / 130 J/g × 100 %, where ΔHc is the cold-crystallization enthalpy observed on first heating. Density measurement per ASTM D792 and wide-angle X-ray scattering provide independent cross-checks.
Why must specimens be conditioned before testing?
PEEK absorbs a small but measurable amount of moisture that plasticizes the amorphous phase and can shift Tg, modulus, and dielectric values. Standard conditioning at 23 °C and 50 % RH for 40 h ensures comparable, reproducible data between laboratories.
Can physical property experiments be performed at elevated temperature?
Yes. Tensile testing with an environmental chamber, DMA, and TMA allow properties to be mapped from −40 °C to beyond 250 °C, which is critical for aerospace and automotive applications. PEEK film typically retains a large fraction of its room-temperature strength up to its glass transition temperature.
What sample thickness is needed for dielectric strength testing?
Specimens of 25 μm to 100 μm are typical. Very thin films (below about 15 μm) are often tested as multilayer stacks to avoid artifacts from pinholes and handling damage, and the per-layer breakdown field is calculated from the total measured thickness.