Determination of Flame Retardancy Performance of Polyimide (PI) Films

Polyimide (PI) films are high-performance engineering materials widely used in flexible printed circuits, aerospace wiring, and electrical insulation, where flame retardancy is a critical safety requirement. Determining the flame retardant performance of PI films relies on standardized test methods such as UL 94 vertical burning, limiting oxygen index (LOI), and glow wire testing. This technical article provides a comprehensive guide to PI film flame retardancy measurement, covering sample types, testing items, test methods, evaluation standards, and typical industrial applications.

Technical Overview

Polyimide (PI) film is an aromatic, high-temperature engineering polymer produced by polycondensation of a dianhydride (typically PMDA) with a diamine (typically ODA), followed by imidization. It combines exceptional thermal stability, dielectric strength, chemical resistance, and mechanical toughness over a wide temperature range from approximately -269°C to +400°C. Because PI film is used in safety-critical electrical and electronic assemblies, its flame retardancy must be quantified and verified against internationally recognized flammability standards.

The flame retardancy of PI film is intrinsic rather than additive. The rigid aromatic imide backbone promotes rapid carbonization and char formation during pyrolysis, which shields the underlying polymer from heat and oxygen, suppresses flaming combustion, and prevents the release of flaming droplets. Typical indicators of the intrinsic flame resistance of PI film include a limiting oxygen index of approximately 36% to 40%, a UL 94 rating of V-0 or VTM-0 at commercial gauges, decomposition onset above 500°C in nitrogen, and char yields exceeding 50%. Flame retardancy determination therefore focuses on confirming these properties through controlled laboratory burning tests under repeatable, standardized conditions.

Testing Samples

General-Purpose PI Film (Standard Grade)

Unfilled, amber-colored polyimide film produced by standard casting and imidization, typically supplied in thicknesses from 7.5 μm to 125 μm. This is the baseline material for flame retardancy characterization and corresponds to widely used commercial grades such as Kapton HN-type film.

Corona-Resistant PI Film

Film modified with inorganic fillers such as alumina or silica to resist partial-discharge degradation in rotating machines. The filler content may influence ignition behavior and char morphology, so flame retardancy is verified separately for these grades.

Black and Pigmented PI Film

Carbon black or pigment-filled film used in optical applications, flexible displays, and light-shielding layers. Pigment loading and dispersion are recorded because they can affect afterflame time and drip behavior.

FPC-Grade PI Film

Film intended as base substrate or coverlay for flexible printed circuits, often in composite form with adhesives. Specimens are tested both as neat film and as laminated constructions, since adhesive layers and copper cladding alter the burning characteristics of the finished circuit material.

Flame-Retardant Modified PI Film

Grades formulated with phosphorus-containing, silicon-containing, or nanocomposite modifiers to further enhance char formation or smoke suppression. These are evaluated to demonstrate compliance with stringent halogen-free flame retardancy requirements in consumer electronics and rail or air transport applications.

Testing Items

The flame retardancy determination of PI film typically includes the following measurable parameters:

• UL 94 vertical burning classification (V-0, V-1, V-2) or thin-film classification (VTM-0, VTM-1, VTM-2)
• UL 94 horizontal burning classification (HB) and linear burning rate (mm/min)
• Limiting oxygen index (LOI, % oxygen concentration)
• Afterflame time (t1 and t2) and total afterflame time per specimen set
• Afterglow time and cotton ignition by flaming drips
• Glow wire flammability index (GWFI) and glow wire ignition temperature (GWIT)
• Time to ignition (TTI), peak heat release rate (pHRR), and total heat release (THR) by cone calorimetry
• Smoke production rate and specific optical density by smoke chamber testing
• Mass loss rate, char yield, and decomposition temperatures by thermogravimetric analysis as supporting data

Testing Methods

UL 94 Vertical Burning Test (V-0 / V-1 / V-2)

Conducted in accordance with UL 94 and IEC 60695-11-10. Specimens measuring 125 mm × 13 mm are cut at machine and transverse directions, conditioned at 23°C and 50% relative humidity for at least 48 hours, and additionally oven-aged at 70°C for 168 hours for the aged condition. A 20 mm (50 W) methane flame is applied twice to the lower edge of the vertically clamped specimen for 10 seconds each. Classification is based on afterflame time, sum of afterflame times across five specimens, afterglow, and whether flaming particles ignite surgical cotton placed below the specimen. A V-0 rating requires individual afterflame times of 10 seconds or less and a total not exceeding 50 seconds for ten specimens, with no flaming drips.

UL 94 VTM Thin-Material Vertical Test

Because PI films are frequently thinner than 0.25 mm, the standard V test is complemented by the VTM procedure for thin films, in which specimens are wrapped around a mandrel to provide mechanical rigidity. Unoriented film rolls are exposed to the 20 mm flame under criteria analogous to V-0, V-1, and V-2, yielding the VTM-0, VTM-1, or VTM-2 classification commonly cited for commercial PI film.

UL 94 Horizontal Burning Test (HB)

Performed per IEC 60695-11-10 for comparative purposes or when only an HB classification is required. A horizontally mounted specimen is ignited at one end, and the burning rate over a 75 mm gauge length is measured. PI film typically extinguishes before the first reference mark, exceeding HB requirements by a wide margin.

Limiting Oxygen Index (LOI)

Determined according to ASTM D2863 or ISO 4589-2. Specimens of defined size (for example, 70 mm to 150 mm long and 10 mm wide, depending on film thickness and support method) are vertically clamped and ignited at the top in a glass chimney through which a precisely controlled nitrogen-oxygen mixture flows. The oxygen concentration at which candle-like burning is just sustained for 3 minutes or over 50 mm is determined by the bracketing method. PI film typically reports LOI values between 36% and 40%, well above the 21% oxygen content of air and the 28% threshold generally accepted as self-extinguishing.

Glow Wire Testing (GWFI / GWIT)

Conducted per IEC 60695-2-12 and IEC 60695-2-13 using test temperatures of 550°C, 650°C, 750°C, 850°C, or 960°C. A heated glow wire is applied to the film specimen or to a finished part for 30 seconds. The glow wire flammability index records the highest temperature at which the specimen does not ignite or self-extinguishes within 30 seconds after removal without igniting underlying tissue paper. The glow wire ignition temperature identifies the temperature 30 K above which the material does not ignite during application. PI film and PI-based laminates commonly satisfy 960°C GWFI requirements.

Cone Calorimetry

Performed according to ISO 5660-1 or ASTM E1354 at irradiance levels of 25 to 50 kW/m². Film specimens are mounted in a frame with a retainer grid to prevent distortion. The method quantifies time to ignition, peak and total heat release, effective heat of combustion, smoke production, and mass loss rate, providing fire engineering data for hazard assessment of PI film in assembled systems. PI film characteristically shows delayed ignition, low peak heat release rate, and high residual char mass.

Smoke Density and Supplementary Characterization

Specific optical density of smoke is measured in the NBS smoke chamber per ASTM E662 or ISO 5659-2 where low-smoke certification is required, for example in rail and aircraft interiors. Thermogravimetric analysis under nitrogen and air, together with microscale combustion calorimetry (ASTM D7309), may be reported as supporting evidence of thermal stability and reduced flammability potential.

Applications

Verified flame retardancy is a prerequisite for deploying PI film in the following sectors:

• Flexible printed circuits: base films, coverlays, and adhesive films requiring UL 94 VTM-0 or V-0 recognition for consumer electronics and 5G modules
• Aerospace and avionics: wire and cable wrap insulation, flexible heaters, and thermal control blankets requiring low smoke and self-extinguishing behavior
• Electric vehicle and lithium-ion batteries: cell insulation barriers, tab insulation tape, and busbar wrapping where arc and thermal runaway resistance are essential
• Motors, generators, and transformers: slot liner films, magnet wire wrap, and layer insulation verified by glow wire and LOI testing
• Displays and semiconductors: high-temperature carrier films and black PI for OLED and COF packaging
• Rail and mass transit interiors: halogen-free, low-smoke PI-based laminates and labels complying with fire safety codes

FAQ

What flame rating does PI film normally achieve? Standard unfilled PI film typically achieves UL 94 V-0 at thicker gauges and VTM-0 in the thin-film vertical test, with no flaming drips, because of its aromatic imide structure and high char yield.

Why is PI film inherently flame retardant without halogenated additives? The aromatic heterocyclic imide ring decomposes at high temperature into a thermally stable, carbonaceous char layer that insulates the polymer from heat and oxygen and suppresses the release of combustible volatiles, so no external flame retardant package is required.

What LOI value indicates good flame resistance for PI film? An LOI above 28% is generally considered self-extinguishing; PI film typically measures between 36% and 40%, indicating excellent intrinsic flame resistance.

Which standards govern PI film flammability testing? The principal standards are UL 94, IEC 60695-11-10 and IEC 60695-11-20 for flame tests, ASTM D2863 and ISO 4589-2 for oxygen index, IEC 60695-2 series for glow wire, and ISO 5660-1 or ASTM E1354 for cone calorimetry.

Does film thickness affect the test result? Yes. Very thin films can be consumed quickly and may distort during burning, which is why the UL 94 VTM procedure with mandrel support is used for films thinner than 0.25 mm, and why the tested thickness must always be reported with the classification.

Can PI film be certified as halogen-free flame retardant? Yes. PI achieves its flame retardancy from molecular structure alone, satisfying halogen-free requirements while simultaneously meeting demanding electrical and thermal specifications.