Determination of Flame Retardancy of Insulation Films for New Energy Vehicles

Flame retardancy determination of insulation films is a critical safety verification step for new energy vehicles, where polymer films provide dielectric protection for battery cells, busbars, and high-voltage components. This article explains the applicable standards, sample types, test items, and test methods—including UL 94 vertical burning, glow wire, and limiting oxygen index testing—commonly performed by accredited third-party laboratories. Understanding these procedures helps battery and vehicle manufacturers qualify materials, mitigate thermal runaway risk, and comply with EV safety regulations.

Technical Overview

New energy vehicles (NEVs), including battery electric vehicles (BEVs), plug-in hybrid electric vehicles (PHEVs), and fuel cell electric vehicles (FCEVs), operate on high-voltage platforms of 400 V to 800 V or higher. Within these systems, polymer insulation films act as dielectric barriers and protective layers between battery cells, on busbars and high-voltage harnesses, and inside electric drive motors and power electronics modules. Because these films are located in close proximity to energy-dense lithium-ion cells, their combustion behavior directly influences the propagation of thermal runaway events.

Flame retardancy determination evaluates whether an insulation film will self-extinguish after removal of an ignition source, whether it produces flaming drips capable of igniting adjacent materials, and how it behaves under prolonged thermal radiation. The results support compliance with battery safety regulations such as GB 38031 for electric vehicle traction batteries, in which manufacturers must demonstrate that fire and explosion risks are controlled and that occupants receive adequate warning before hazardous conditions develop. Component-level flame retardancy data for insulation films are therefore an essential part of the overall safety evidence chain for an NEV battery system.

Typical qualification targets for insulation films include a UL 94 V-0 or UL 94 VTM-0 classification, a limiting oxygen index (LOI) of 28% or above, and satisfactory glow wire performance. Testing is generally carried out by accredited third-party laboratories in accordance with internationally recognized standards published by UL, IEC, ISO, ASTM, EN, and GB.

Testing Samples

Battery Cell Insulation Films (PET Blue Film)

Polyethylene terephthalate (PET) films coated with UV-cured or thermally cured flame-retardant lacquers are widely used as cell-level insulation on prismatic and cylindrical lithium-ion cells. The blue or black coating layer, typically a few micrometers thick, must be evaluated together with the PET substrate because the flame retardancy of the composite structure, rather than the base film alone, determines the end-use performance. Specimens are prepared in the coated condition and, where relevant, after simulated processing such as cell wrapping, heat aging, and electrolyte exposure.

Polyimide (PI) Insulation Films

Polyimide films are used for motor slot insulation, flexible printed circuit substrates, barrière layers in power modules, and high-temperature adhesive tapes. PI films are inherently flame retardant, with limiting oxygen index values commonly in the range of 36% to 38%, but verification against UL 94 and IEC 60695 series methods is still required for qualification. Specimens are cut to the dimensions specified by the relevant test method and conditioned in a standard atmosphere before testing.

Mica-Based and Ceramic-Coated Films

Mica composite films and ceramic-coated PET or PI films serve as fire-protection barriers between cells and between the module and the pack enclosure. These materials are evaluated both for their ability to extinguish after ignition and for structural integrity after flame exposure, since their function is to delay heat transfer during a thermal runaway event.

Composite Laminates and Insulating Tapes

Multilayer laminates combining PET, polypropylene (PP), polyethylene naphthalate (PEN), and adhesive layers are used for busbar wrapping, cell-to-cell separation, and module-level encapsulation. Because adhesives can significantly alter burning behavior, specimens are tested in the final laminate construction, including any pressure-sensitive adhesive layers.

Testing Items

Flame retardancy determination for NEV insulation films typically covers the following test items, selected according to the film function, thickness, and the OEM material specification:

• UL 94 vertical burning classification (V-0, V-1, V-2)
• UL 94 thin-material vertical classification (VTM-0, VTM-1, VTM-2) for flexible films that distort under a standard burner flame
• UL 94 500 W vertical burning classification (5VA, 5VB) for enclosure-grade and barrier materials
• UL 94 horizontal burning classification (HB, HB40, HB75)
• Glow wire flammability index (GWFI) per IEC 60695-2-12
• Glow wire ignition temperature (GWIT) per IEC 60695-2-13
• Needle flame test per IEC 60695-11-5
• Limiting oxygen index (LOI) per ISO 4589-2, GB/T 2406.2, and ASTM D2863
• Cone calorimeter characterization (time to ignition, peak heat release rate, total heat release, smoke production) per ISO 5660-1
• Horizontal burning of vehicle interior materials per GB 8410 and ISO 3795, where films are used in occupant-facing applications

Recorded parameters include individual and total afterflame time, afterglow time, ignition of the indicator cotton by flaming drips, burn rate over the gauge length, specimen distortion and shrinkage, and whether burning reaches the clamp or the 100 mm benchmark.

Testing Methods

Vertical Burning Test (UL 94 V-0/V-1/V-2, IEC 60695-11-10, GB/T 2408)

Bar specimens measuring 125 mm × 13 mm are cut at the minimum and maximum supplied thicknesses. After conditioning at 23 ± 2 °C and 50 ± 5% relative humidity for at least 48 hours, each specimen is clamped vertically and exposed twice to a 20 mm (50 W) methane burner flame for 10 seconds. For a V-0 classification, the afterflame time after each application must not exceed 10 seconds, the total afterflame time for all ten applications must not exceed 50 seconds, flaming particles must not ignite the dry surgical cotton placed below the specimen, and the afterflame plus afterglow must not exceed 30 seconds after the second application. GB/T 2408 defines equivalent vertical classifications aligned with the IEC 60695-11-10 method.

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

Flexible films below approximately 0.25 mm in thickness often twist, shrink, or wrap around the burner flame during standard vertical testing. For these materials, specimens are wrapped around a mandrel and rolled into a self-supporting cylindrical configuration before being tested vertically. Timing criteria for VTM-0, VTM-1, and VTM-2 mirror those of the corresponding V classifications, with additional attention to whether burning fragments or hot particles ignite the cotton indicator. This method is the primary route for qualifying thin PET cell-insulation films, where VTM-0 is the usual OEM requirement.

Horizontal Burning Test (UL 94 HB, GB/T 2408)

In the horizontal configuration, a specimen clamped horizontally is ignited at one end by the 20 mm burner flame. The material is classified HB if burning does not pass the 100 mm reference mark, or if the burn rate between the 25 mm and 100 mm marks does not exceed 40 mm/min for thicknesses of 3 mm to 13 mm, or 75 mm/min for thicknesses below 3 mm. GB/T 2408 also defines HB40 and HB75 classifications using the same burn-rate thresholds. Horizontal data are useful for ranking materials and for interior applications evaluated under GB 8410 and ISO 3795.

Glow Wire Tests (IEC 60695-2 Series)

The glow wire tests simulate thermal stress from overloaded or glowing components. In the glow wire flammability index (GWFI) test, a nickel-chromium glow wire heated to a specified temperature (550 °C to 960 °C) is pressed against the specimen for 30 seconds, and the highest temperature at which the specimen does not ignite or self-extinguishes within 30 seconds after removal is determined. The glow wire ignition temperature (GWIT) is the glow wire temperature 25 K above the highest temperature at which ignition of the specimen does not occur during the 30 second contact. Battery system specifications frequently require GWFI of 960 °C and GWIT of 775 °C or 850 °C for insulating parts located near live connections.

Needle Flame Test (IEC 60695-11-5)

The needle flame test applies a small 12 mm diffusion flame from a 0.5 mm orifice burner to the specimen for 10 to 30 seconds. It evaluates fire hazard from small ignition sources such as component failures. Acceptance is judged by flame-out time, whether the wrapping tissue below the specimen is ignited, and whether a pine board backing board is charred.

Limiting Oxygen Index (ISO 4589-2, GB/T 2406.2, ASTM D2863)

The limiting oxygen index is the minimum concentration of oxygen in an oxygen/nitrogen mixture that will just support candle-like combustion of a vertically mounted specimen for 3 minutes or over a 50 mm burn length. The test is performed across a series of gas concentrations, and the LOI is determined by the bracketing method. Insulation films for NEV applications are typically required to achieve LOI values of at least 28%, with high-performance PI and mica-based films exceeding 35%, indicating that the material will not continue burning in normal air (approximately 21% oxygen).

Cone Calorimeter Testing (ISO 5660-1)

Cone calorimetry provides quantitative fire performance data under radiant heating representative of a developing fire or a neighboring cell venting hot gases. Film specimens of 100 mm × 100 mm are mounted horizontally and exposed to irradiance levels of 25, 35, or 50 kW/m². Measured outputs include time to ignition, peak heat release rate, total heat release, fire growth rate index, and smoke production rate. These parameters are increasingly used in pack-level thermal propagation simulations and in comparative screening of flame-retardant coating formulations.

Applications

Flame retardancy determination data for insulation films are applied throughout the NEV development and production cycle:

• Material qualification and supplier approval for cell insulation coatings, PI films, mica barriers, and insulating tapes against OEM and industry material specifications
• Compliance evidence for GB 38031 battery safety requirements and equivalent international electric vehicle safety regulations addressing thermal propagation and fire risk
• Design verification for cell-to-cell barrier films, busbar wrap films, module encapsulation layers, and enclosure-related insulating sheets used to delay or prevent thermal runaway propagation
• Motor and power electronics insulation validation, including slot liners, phase insulation, and flexible circuit covers operating near high-temperature components
• Incoming inspection and periodic production surveillance testing to confirm that lot-to-lot flame retardancy remains consistent with the qualified baseline
• Failure analysis support when field or production anomalies involve charring, tracking, or suspected combustion of insulating materials
• Research and development of new flame-retardant coating chemistries, halogen-free formulations, and ceramic-filled films, using LOI and cone calorimeter data to compare performance

FAQ

What flame retardancy rating is typically required for battery cell insulation film?

Most OEM specifications require a UL 94 VTM-0 classification for thin flexible films below approximately 0.25 mm, and V-0 for thicker sheets, together with a limiting oxygen index of at least 28% to 32%. Glow wire requirements such as GWFI 960 °C may be added for parts near electrical connections.

Why is the VTM method used instead of the standard V-0 method for thin films?

Very thin films tend to shrink, curl, or wrap around the burner flame during standard vertical burning, making the afterflame measurement unreliable. The VTM method rolls the film into a self-supporting cylindrical specimen so that the classification criteria can be applied reproducibly. VTM-0 corresponds to V-0 severity.

How many specimens are needed for a UL 94 test program?

A vertical burning test generally requires two sets of five bar specimens per thickness, plus additional specimens for the cotton-ignition observation. For VTM testing, rolled specimens are prepared in a similar quantity. Laboratories typically request surplus material to allow retesting in case of borderline results.

Does the coating side orientation matter when testing coated insulation films?

Yes. When flame-retardant coating is applied to one side, specimens should be tested with the flame applied to the coated surface as well as in the reverse orientation, because the end-use installation may expose either side to an ignition source.

What is the difference between GWIT and GWFI?

GWIT (glow wire ignition temperature) is the highest glow wire temperature at which the specimen does not ignite during contact, increased by 25 K, and reflects resistance to ignition. GWFI (glow wire flammability index) is the highest temperature at which the specimen either does not ignite or self-extinguishes within 30 seconds after the glow wire is removed, reflecting the ability to stop burning once ignited.

How do film test results support battery pack safety compliance?

Component-level flame retardancy data demonstrate that insulation films will not act as fire spread paths during a cell failure. Combined with cell, module, and pack level thermal propagation testing, these data form part of the technical documentation required to show that thermal event risks are managed in accordance with applicable EV battery safety regulations.