PEEK insulation film flame retardancy testing verifies the intrinsic self-extinguishing behavior of polyetheretherketone films through standardized procedures such as UL94 vertical burning, limiting oxygen index (LOI), and glow-wire evaluation. This article details the experimental design, specimen preparation, test protocols, acceptance criteria, and typical results used to qualify PEEK films for high-temperature electrical insulation. Understanding these methods helps engineers select and certify flame-retardant insulation materials for demanding electronics, aerospace, and new-energy applications.
Technical Overview
PEEK (polyetheretherketone) is a semi-crystalline aromatic thermoplastic widely specified as a high-temperature electrical insulation material. Its rigid molecular backbone, built from phenylene rings connected by ether and ketone linkages, delivers outstanding thermal stability and intrinsic flame retardancy without any halogenated or phosphorus-based additive. When exposed to a flame, PEEK forms a protective carbonaceous char layer that insulates the underlying polymer, interrupts heat feedback, and suppresses further combustion.
The objective of this experiment is to characterize and verify the flame retardant performance of PEEK insulation films under controlled laboratory conditions. The test program evaluates ignition resistance, self-extinguishing behavior, dripping tendency, oxygen demand for sustained burning, and thermal decomposition characteristics, providing the data required for material qualification in safety-critical electrical and electronic systems.
- Glass transition temperature of approximately 143 °C and melting point of approximately 343 °C
- Limiting oxygen index (LOI) of approximately 35 percent, well above the 21 percent oxygen content of air
- Halogen-free formulation with inherently low smoke and toxic gas emission
- Stable electrical insulation properties retained throughout and after flame exposure
Testing Samples
Specimen Geometry and Thickness Series
For UL94 vertical burn testing, PEEK insulation film is cut into strips measuring 125 mm in length and 13 mm in width, with the machine direction and transverse direction identified separately. A thickness series of 25, 50, 75, 100, 125, and 250 micrometers is prepared to evaluate the relationship between gauge and burn classification. Ten specimens per thickness are supplied, organized as two condition sets of five specimens each in accordance with UL94 requirements. For films thinner than 0.4 mm, the upper 6 mm of each specimen is supported by a clamp adapter so that the specimen hangs vertically without excessive curling.
Specimen Conditioning
All specimens are conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity for a minimum of 48 hours prior to testing, consistent with ASTM D618 and IEC 60695-11-10 practice. An additional specimen set may be thermally aged at 70 °C for 168 hours to assess whether long-term heat exposure alters the burning behavior of the film.
Traceability and Dimensional Verification
Each specimen is assigned a unique identifier linking it to its film lot, thickness, and orientation. Thickness is measured at five positions using a calibrated micrometer, and specimens deviating more than 10 percent from nominal thickness are excluded. This traceability ensures that any afterflame anomaly observed during testing can be correlated with material or processing variables.
Testing Items
The flame retardancy experiment covers the following test items, each mapped to an internationally recognized standard:
- UL94 vertical burning classification (V-0, V-1, or V-2) per UL94 and IEC 60695-11-10
- Limiting oxygen index (LOI) per ISO 4589-2 and ASTM D2863
- Glow-wire flammability index (GWFI) per IEC 60695-2-12 and glow-wire ignition temperature (GWIT) per IEC 60695-2-13
- Thermal decomposition stability by thermogravimetric analysis (TGA) per ISO 11358
- Smoke production screening per ISO 5659-2, where required by end-application specifications
Testing Methods
UL94 Vertical Burn Test
Each conditioned specimen is suspended vertically with its lower edge 10 mm above the top of a burner tube. A nominal 20 mm blue flame is applied to the bottom edge of the specimen for 10 seconds, then withdrawn, and the afterflame time t1 is recorded. After the specimen stops burning, the flame is applied a second time for 10 seconds, and the afterflame time t2 and afterglow time t3 are recorded. A layer of surgical cotton is placed 300 mm below the specimen to detect flaming drips. To achieve the V-0 classification, the following criteria must all be satisfied:
- Individual afterflame time (t1 or t2) does not exceed 10 seconds for any specimen
- Total afterflame time for the five specimens of a condition set does not exceed 50 seconds
- No specimen exhibits afterflame or afterglow propagating up to the clamp
- No flaming particles or drips ignite the cotton indicator
- Any afterglow extinguishes within 30 seconds after removal of the second ignition source
In typical results for unfilled PEEK film, individual afterflame times remain below 5 seconds, cumulative burn times per five-specimen set stay under 25 seconds, and no flaming drips are observed, supporting V-0 classification across the tested thickness range.
Limiting Oxygen Index (LOI) Test
LOI testing is performed in a glass chimney with a controlled upward flow of an oxygen and nitrogen mixture. Thin PEEK film specimens are supported on a mandrel per ISO 4589-2 provisions for flexible materials. The specimen is ignited at its top edge, and the operator determines whether the flame propagates downward for at least 50 mm or burns for at least 180 seconds. The oxygen concentration is adjusted stepwise until the boundary condition is bracketed, and the LOI is calculated as the volume percentage of oxygen in the gas mixture at that boundary. PEEK insulation film typically records an LOI between 34 and 37 percent, confirming self-extinguishing behavior in normal atmospheric conditions without any added flame retardant.
Glow-Wire Testing
Glow-wire testing simulates thermal stress from overheated or glowing electrical components. A glow wire controlled by a thermocouple is heated to a specified temperature between 550 °C and 960 °C and pressed against the specimen with a force of 1 N for 30 seconds. For GWFI, the specimen must not ignite, or any flame must self-extinguish within 30 seconds without igniting the tissue paper placed beneath. For GWIT, the temperature is raised in increments to determine the lowest value at which ignition occurs. PEEK films commonly achieve a GWFI of 960 °C and a GWIT of 750 °C or higher, satisfying the requirements of unattended household appliance standards.
Thermogravimetric Analysis (TGA)
TGA characterizes the thermal decomposition profile that underpins the flame retardant mechanism. A specimen of approximately 8 mg is heated at 10 °C/min to 800 °C under nitrogen, followed by a switch to air to evaluate char oxidation. PEEK films typically show a temperature of 5 percent mass loss above 500 °C, an onset of rapid decomposition near 575 °C, and a char residue of approximately 50 percent at 800 °C under nitrogen. The high char yield directly correlates with the elevated LOI, because the insulating char layer restricts volatile fuel transport to the flame front.
Smoke Density Assessment
Where end-use specifications demand it, smoke generation is measured in a single-chamber apparatus per ISO 5659-2 at irradiance levels of 25 and 50 kW/m² in both flaming and non-flaming modes. PEEK ranks among the lowest smoke emitting engineering polymers, with specific optical density values far below those of halogenated insulation materials, and it releases no hydrogen halides because the formulation is intrinsically halogen-free.
Summary of Typical Experimental Results
- UL94 rating of V-0 achieved at tested thicknesses of 50 micrometers and above, with no flaming drips in any trial
- LOI of 34 to 37 percent, with a mean value near 35 percent
- GWFI of 960 °C and GWIT of 750 °C or higher for standard film grades
- TGA 5 percent mass-loss temperature above 500 °C and char residue near 50 percent under nitrogen
- Low specific optical density in smoke chamber testing compared with conventional flame-retarded films
Results can vary slightly with film crystallinity, orientation, thickness, pigment loading, and surface contamination, so certification testing should always use specimens representative of the production material.
Applications
The verified flame retardancy of PEEK insulation film enables its use in applications where electrical insulation, high temperature endurance, and fire safety must coexist. Its self-extinguishing nature, absence of halogen additives, and low smoke output make it especially valuable in enclosed systems and passenger environments.
- Flexible printed circuit substrates and coverlay films for high-density, high-temperature electronics
- Aerospace wire wrap and cable insulation, including installations subject to flammability and smoke requirements
- Electric vehicle battery modules, busbar wraps, and thermal runaway barrier layers
- Motor and generator slot insulation, phase insulation, and magnet wire wrap in traction equipment
- Insulation for high-temperature sensors, semiconductor handling carriers, and downhole electronics
In each of these applications, the experimental data from UL94, LOI, and glow-wire testing provides documented evidence that the insulation layer will not propagate flame following an electrical fault or external ignition event.
FAQ
Does PEEK insulation film require added flame retardants to pass UL94 V-0?
No. PEEK achieves its V-0 classification through its aromatic polymer structure alone. No halogenated, phosphorus, or inorganic flame retardant additives are required, which eliminates halogen acid gas release during combustion and simplifies regulatory compliance.
What flame retardancy rating does PEEK film typically achieve?
Standard unfilled PEEK films achieve UL94 V-0 at commonly tested gauges, an LOI around 35 percent, a GWFI of 960 °C, and a GWIT of approximately 750 °C. Exact values should always be confirmed by testing the specific film grade and thickness used in the design.
How does PEEK film compare with polyimide film in flame retardancy?
Both materials are intrinsically self-extinguishing with comparable LOI values in the mid-thirties. PEEK additionally offers robust melt integrity and very low smoke emission, while polyimide chars without melting. Selection between the two usually depends on combined requirements for dielectric performance, mechanical toughness, and processing method.
Can very thin PEEK films below 50 micrometers be tested under UL94?
Yes. Thin and flexible specimens are tested with appropriate clamping support so they hang vertically, and the minimum classified thickness must be stated on the material certification. Designers should verify the rating at the exact thickness they intend to use, since burn behavior can change at very low gauges.
Why does PEEK burn so little even without additives?
Upon heating, the aromatic backbone undergoes crosslinking and charring rather than depolymerizing into flammable volatiles. The resulting char layer shields the polymer from heat and oxygen, which raises the oxygen concentration needed to sustain burning and produces the short afterflame times observed in testing.
