PET High-Temperature Insulation Film Combustion Testing: Standards, Methods, and Flammability Rating

PET high-temperature insulation film combustion testing evaluates the flammability characteristics of polyethylene terephthalate films used in electrical and electronic insulation systems, ensuring compliance with internationally recognized safety standards such as UL 94, IEC 60695-11-10, and related test protocols. This guide covers sample preparation, key test items, burning test procedures, classification criteria, and typical applications of PET insulation films. Understanding these combustion test methods helps engineers and quality teams select certified materials for transformers, motors, batteries, and flexible printed circuits.

Technical Overview

PET (polyethylene terephthalate) high-temperature insulation film is a biaxially oriented polyester film, widely recognized under trade names such as Mylar, Melinex, and Hostaphan. It is one of the most widely used flexible insulation materials in the electrical and electronics industry, offering an excellent combination of high dielectric strength (approximately 150 kV/mm or higher for thin gauges), high tensile strength (typically 170–230 MPa), good chemical resistance, and dimensional stability. Standard PET films support continuous operating temperatures of approximately 105–120 °C, while thermally stabilized grades can be qualified for long-term service at 130–155 °C.

Like most thermoplastics, PET is inherently combustible. When exposed to an ignition source, it softens, melts at approximately 255–265 °C, and can produce flaming molten drips that may propagate fire to adjacent components. Because PET films are frequently installed in motors, transformers, battery packs, and printed circuit assemblies where electrical arcing or overheating can occur, combustion testing is a mandatory part of material qualification and end-product safety certification under UL, IEC, and GB frameworks.

Combustion testing of PET insulation film quantifies parameters such as afterflame time, afterglow time, flaming drip behavior, and burning rate, and assigns a flammability rating (for example V-0 or VTM-0) that system designers use to demonstrate compliance with fire hazard requirements. The results also guide the development of flame-retardant film formulations, including halogen-free phosphorus-based modified grades increasingly demanded by the lithium battery and consumer electronics sectors.

Testing Samples

Standard BOPET Insulation Film

Unmodified biaxially oriented PET film represents the baseline sample type. Common electrical grades range from 12 μm to 250 μm in thickness, supplied as transparent or hazy rolls. These samples establish the intrinsic flammability behavior of the material and typically form the reference point against which modified grades are compared.

Flame-Retardant Modified PET Film

Flame-retardant (FR) PET films incorporate additive or reactive flame retardants—most commonly halogen-free phosphorus compounds, phosphinate salts, or synergistic inorganic fillers—to suppress ignition, reduce afterflame time, and prevent flaming drips. These samples are tested to verify whether the modification achieves the target rating, such as VTM-0 or V-0, without unacceptable degradation of mechanical or electrical properties.

Specimen Preparation and Conditioning

Specimens are die-cut cleanly from representative film samples, free of creases, contamination, and edge defects. Standard specimen geometries include the following:

• UL 94 vertical (V series) and horizontal (HB) tests: 125 mm × 13 mm strips, cut in both machine and transverse directions where required.
• UL 94 thin-material vertical (VTM series) tests: 200 mm × 50 mm specimens rolled longitudinally around a 13 mm mandrel and secured to form a self-supporting cylinder.
• Limiting oxygen index tests: strip specimens per ASTM D2863 or ISO 4589-2, using rolled or supported configurations appropriate for thin films.

Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 24 h (commonly 48 h). Additional conditions may be specified by the client, such as testing in the as-received dry state or after thermal aging representative of the intended service life.

Testing Items

The combustion test program for PET high-temperature insulation film evaluates the following parameters, which together determine the assigned flammability classification:

• Afterflame time after the first flame application (t1).
• Afterflame time after the second flame application (t2).
• Afterglow time following flame extinction (glowing combustion without flame).
• Occurrence of flaming drips and ignition of the surgical cotton indicator placed below the specimen.
• Whether the flame front reaches the specimen clamp (vertical tests) or the 100 mm reference mark (horizontal test).
• Linear burning rate in millimeters per minute (horizontal test).
• Limiting oxygen index (LOI), expressed as the minimum oxygen volume concentration supporting combustion.
• Qualitative observations, including shrinkage, distortion, melting behavior, soot and smoke generation, and char formation.

Testing Methods

Equipment and Environmental Conditions

Testing is performed in a ventilated enclosure or fume hood with the draft closed during specimen exposure. The ignition source is a laboratory Tirrell-type burner with wing top, fueled by methane of at least 98 % purity or controlled natural gas, producing a calibrated 20 mm blue flame. Digital timers record afterflame and afterglow durations, and a layer of dry surgical cotton is positioned 300 mm below the specimen to capture dripping material. A minimum of five specimens per set is tested, with a second set evaluated where results are marginal.

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

The specimen is clamped vertically with its long axis aligned downward and its lower edge approximately 10 mm above the burner top. The 20 mm blue flame is applied to the bottom edge of the specimen for 10 seconds, then withdrawn while the afterflame time t1 is recorded. After the flame self-extinguishes, the burner flame is reapplied for a second 10-second interval, and the afterflame time t2 and afterglow time are recorded. Throughout both applications, the tester observes whether flaming drips ignite the cotton and whether the specimen burns up to the clamp.

UL 94 Thin-Material Vertical Test (VTM-0 / VTM-1 / VTM-2)

Because thin films shrink, curl, or collapse when clamped as flat strips, the standard vertical configuration is unsuitable for most PET insulation films below approximately 0.25 mm. In the VTM procedure, the rolled cylindrical specimen is mounted vertically and the 20 mm blue flame is applied briefly to its lower edge in two consecutive applications using a shorter exposure than the standard V test, since thin films ignite readily. The same response parameters—afterflame time, afterglow, dripping, and cotton ignition—are recorded. VTM-0 corresponds to the most stringent classification, with individual afterflame times on the order of 5 seconds or less and no flaming drips; VTM-1 permits longer afterflame times without flaming drips; and VTM-2 additionally permits flaming drips that ignite the cotton indicator.

Horizontal Burning Test (HB)

The specimen is clamped horizontally with reference marks at 25 mm and 100 mm from the exposed end. The flame is applied at a 45° angle to the free end for 30 seconds or until the flame front reaches the 25 mm mark, after which the burning rate is measured. An HB classification is assigned when specimens self-extinguish before the 100 mm mark or burn at a rate not exceeding 40 mm/min for thicknesses of 3–13 mm and not exceeding 75 mm/min for thinner materials, and when no specimen burns beyond the 100 mm reference mark.

Limiting Oxygen Index Test (ASTM D2863 / ISO 4589-2)

The LOI test quantifies the minimum oxygen concentration, in a flowing oxygen–nitrogen mixture, required to support candle-like downward burning of a vertically mounted specimen ignited from the top. The oxygen concentration is adjusted stepwise until the limiting condition is determined. Standard BOPET film typically exhibits an LOI of approximately 20–22 %, indicating it burns in normal air, whereas effective flame-retardant grades achieve values of 28 % or higher. LOI data are particularly useful for comparing formulations and for quality control between production lots.

Classification Criteria Summary

For the standard vertical test, the principal classification requirements are as follows:

• V-0: individual afterflame time not exceeding 10 seconds; total afterflame time for all specimens not exceeding 50 seconds; no flaming drips igniting cotton; no sustained afterflame or afterglow longer than 30 seconds after the second application.
• V-1: individual afterflame time not exceeding 30 seconds; total afterflame time not exceeding 250 seconds; no flaming drips igniting cotton.
• V-2: identical timing requirements to V-1, but flaming drips that ignite the cotton indicator are permitted.
• All classes: the specimen must not burn up to the clamp, and no specimen may exhibit flaming combustion persisting beyond the specified limits after the second flame application.

It should be noted that these are small-scale laboratory tests. A UL 94 rating does not by itself predict the behavior of a material in a full-scale fire, and end-product assessments may additionally require glow-wire testing (IEC 60695-2 series), needle-flame testing (IEC 60695-11-5), or comparative tracking index evaluation depending on the application standard.

Applications

Combustion-tested PET high-temperature insulation film is specified wherever flexible electrical insulation must satisfy verified flammability requirements. Certified flame-retardant PET film is now a default material choice in several rapidly growing sectors, driven by insulation coordination and battery safety regulations.

• Slot liner, phase insulation, and interlayer insulation in electric motors, generators, and transformers.
• Insulation barriers, cell-to-cell wraps, and cover materials in lithium-ion battery modules and packs.
• Base films for flexible printed circuits, membrane switches, and pressure-sensitive adhesive tapes.
• Wrapping and binding tapes for wire and cable harnesses in automotive and appliance applications.
• Dielectric layers in film capacitors and surface protection films for power electronics.
• Component insulation and barrier films in power supplies, chargers, and photovoltaic modules.

FAQ

What flammability rating does standard PET insulation film achieve?

Unmodified BOPET film is combustible and typically achieves only an HB or VTM-2 classification, with flaming drips often observed. Flame-retardant modified PET films are formulated specifically to reach VTM-0 or V-0, which is the level generally required for battery insulation and for most certified electronic assemblies.

Why is the VTM procedure used instead of the standard UL 94 vertical test for thin films?

Thin PET films shrink away from the flame, curl, and lose their geometry when clamped as flat vertical strips, making the standard V test unrepeatable. The VTM method rolls the film into a self-supporting cylinder, preserving the specimen shape and producing a reproducible, comparable measure of flammability for thin-gauge materials.

Which standards govern the combustion testing of PET insulation film?

The principal standards are UL 94 and IEC 60695-11-10 for vertical and horizontal burning, ASTM D2863 and ISO 4589-2 for limiting oxygen index, and GB/T 2408 and GB/T 2406.2 as the corresponding Chinese national standards. End-product requirements may add glow-wire tests under the IEC 60695-2 series or GB/T 5169 series.

Does the flammability rating depend on film thickness?

Yes. A UL 94 or VTM rating applies only to the thickness actually tested, and flame performance generally improves with increasing thickness. Materials should therefore be tested at the minimum thickness intended for use, and certified ratings should never be extended to thinner gauges without supporting test data.

Is high-temperature performance the same as flame retardance?

No. Thermal endurance—such as a long-term temperature index of 120 °C or 155 °C—describes how the film ages and retains dielectric performance during prolonged heat exposure. Flammability ratings describe behavior under direct flame ignition. A thermally stable PET film can still burn, so both properties must be evaluated independently and both must meet the requirements of the target application standard.

Do flame-retardant additives affect the electrical properties of PET film?

Flame-retardant loading can modestly reduce dielectric strength or increase dissipation factor, and may influence tracking resistance. A qualified test program therefore pairs combustion testing with dielectric strength, insulation resistance, and electrical verification so that fire safety improvements do not compromise the insulation function of the film.