Colored Insulation Film Flame Retardant Testing: Standards, Methods, and Compliance

Colored insulation film flame retardant testing evaluates the ignition resistance and flame propagation behavior of pigmented PET, PI, PVC, and polycarbonate films used in electronics, lithium-ion batteries, and electrical equipment. This technical guide explains the key test items, internationally recognized standards such as UL 94, IEC 60695, ISO 4589, and GB/T 2408, and the sample preparation, procedures, and acceptance criteria used by accredited third-party laboratories to verify flame retardant compliance.

Technical Overview

Colored insulation films are thin polymer materials, typically polyethylene terephthalate (PET), polyimide (PI), polyvinyl chloride (PVC), or polycarbonate (PC), that incorporate pigments or dyes for color coding, circuit identification, and aesthetic functions while providing reliable electrical insulation. They are widely used in lithium-ion cell wrapping, flexible printed circuits, transformer and motor insulation, cable bundling, and consumer electronics. Because these films often operate adjacent to live conductors, heat-generating components, and energy-dense battery cells, their resistance to ignition and flame propagation is a critical safety property.

Flame retardant testing subjects film specimens to controlled ignition sources under standardized laboratory conditions and classifies the observed burning behavior into ratings such as UL 94 V-0, V-1, V-2, HB, 5VA, and VTM classifications. Testing is generally performed by accredited third-party laboratories in accordance with standards published by UL, IEC, ISO, ASTM, EN, JIS, and GB (Chinese national standards). Test results support product certification, regulatory compliance, incoming material inspection, and supplier qualification for manufacturers across the electronics and electrical industries.

Testing Samples

Typical sample types submitted for flame retardant testing include the following categories:

Colored PET Insulation Films

Blue, green, red, or black PET films with thicknesses typically ranging from 20 micrometers to 350 micrometers, widely used as lithium-ion cell wrap and general-purpose dielectric insulation. Specimens for UL 94 vertical burning are cut to 125 mm by 13 mm at both the minimum and maximum production thickness.

Polyimide (PI) Films

High-temperature amber or pigmented PI films used in flexible printed circuits, motor slot liners, and busbar insulation, where flame resistance must be combined with long-term thermal endurance.

PVC and Polycarbonate Films

Colored PVC films for insulating tape and wire wrapping applications, and PC films used as dielectric barriers, display insulation layers, and structural insulating parts in power electronics.

Composite and Coated Films

Multilayer film structures, flame-retardant coated films, and adhesive-backed insulating tapes, where the substrate, coating, and adhesive jointly determine the overall burning behavior.

Specimen conditioning is an integral part of sample preparation. Most standards require conditioning at 23 ± 2 °C and 50 ± 5% relative humidity for at least 48 hours. UL 94 additionally requires a set of specimens oven-aged at 70 °C for 168 hours followed by cooling in a desiccator before testing, in order to evaluate thermal aging effects on flammability performance.

Testing Items

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

The core classification test for insulation films, performed per UL 94, IEC 60695-11-10, GB/T 5169.16, and GB/T 2408. A 20 mm high methane flame is applied twice for 10 seconds to the lower edge of a vertically mounted specimen, and afterflame time, afterglow duration, and flaming drips are recorded and compared against classification criteria.

Horizontal Burning Test (UL 94 HB)

A lower-severity classification per UL 94, IEC 60695-11-10, and ASTM D635, measuring the linear burning rate of a horizontally mounted specimen between the 25 mm and 100 mm gauge marks.

Thin-Film Vertical Test (UL 94 VTM)

UL 94 VTM-0, VTM-1, and VTM-2 classifications and ISO 9773 apply specifically to thin, flexible films that distort or shrink during standard vertical testing. Specimens are rolled into a cylindrical form around a mandrel before flame application.

500 W Flame Tests (UL 94 5VA / 5VB)

More severe burning tests per UL 94 and IEC 60695-11-20 (GB/T 5169.17), performed on bar and plaque specimens, and frequently required for enclosures and load-bearing insulating components.

Glow Wire Tests (GWFI / GWIT)

Glow wire flammability index and glow wire ignition temperature per IEC 60695-2-12 and IEC 60695-2-13, evaluating ignition behavior against a heated resistance element. These tests are commonly mandated for insulation materials in unattended household appliances.

Limiting Oxygen Index (LOI)

Per ISO 4589-2, ASTM D2863, and GB/T 2406.2, the LOI determines the minimum oxygen concentration in an oxygen and nitrogen mixture required to sustain candle-like burning, providing a useful quantitative index for material comparison and flame-retardant formulation development.

Smoke Density and Combustion Gas Analysis

Specific optical density of smoke per ISO 5659-2, ASTM E662, and GB/T 8323, combined with qualitative and quantitative analysis of toxic and corrosive gases such as hydrogen chloride, hydrogen bromide, hydrogen fluoride, hydrogen cyanide, and carbon monoxide released during combustion.

Halogen Content and Restricted Substances

Fluorine, chlorine, bromine, and iodine content per EN 14582 combustion ion chromatography, together with RoHS restricted substance screening per IEC 62321, to verify halogen-free claims for pigmented insulation films.

Testing Methods

UL 94 Vertical Burning Procedure

Specimens are clamped vertically with the long axis aligned upward. A calibrated methane burner producing a 20 mm blue flame is applied to the lower edge for 10 seconds, removed, and re-applied for a second 10-second interval after burning ceases. A cotton indicator is placed below the specimen to capture flaming drips. For a V-0 rating, individual afterflame time must not exceed 10 seconds, the total afterflame time for all specimens must not exceed 50 seconds, and no flaming drips may ignite the cotton. V-1 permits afterflame times up to 30 seconds with a 250-second total, while V-2 shares the timing limits of V-1 but tolerates flaming drips.

Horizontal Burning Procedure

The specimen is mounted horizontally and a 25 mm flame is applied to the free end at a defined angle for up to 30 seconds or until the flame reaches the 25 mm mark. The time for the flame front to travel from the 25 mm to the 100 mm mark is recorded. The HB classification requires a burning rate not exceeding 40 mm/min for thicknesses between 3 mm and 13 mm, or 75 mm/min for thinner specimens, or self-extinguishment before the 100 mm mark.

Glow Wire Procedure

A nickel-chromium heating element maintained at temperatures between 550 °C and 960 °C is pressed against the specimen surface with defined force for 30 seconds. The GWFI is the highest temperature at which the specimen either does not ignite or self-extinguishes within 30 seconds without igniting the underlying tissue paper. The GWIT is reported as 25 K above the highest temperature at which no ignition of the specimen occurs during the specified contact period.

Limiting Oxygen Index Procedure

The specimen is mounted vertically in a borosilicate glass chimney and ignited at its top edge. The oxygen concentration in the flowing gas mixture is progressively adjusted to determine the minimum concentration at which the specimen sustains burning for 3 minutes or over a 50 mm length. Higher LOI values indicate greater intrinsic flame resistance.

Evaluation and Reporting

Test reports issued by accredited third-party laboratories document the specimen description, thickness, color, conditioning history, standards applied, observed afterflame and afterglow times, dripping behavior, and the final classification rating. Because pigments, thickness, and coating formulations all influence burning behavior, results are valid only for the specific color and thickness tested, and separate specimens must be evaluated for each production variant.

Applications

Flame retardant test reports for colored insulation films support quality assurance and compliance across many industries:

• Lithium-ion battery and pack manufacturing, where colored PET wrap films must satisfy vertical burning or VTM requirements for cell-level insulation safety.
• Consumer electronics and household appliances, where insulation materials must meet fire hazard requirements referenced in IEC 62368-1 and IEC 60335 series standards.
• Wire harness and cable wrapping in automotive electrical systems, with flammability performance referenced to FMVSS 302, ISO 3795, and GB 8410.
• Transformer, motor, and generator manufacturing for slot liners, phase insulation, and bar wrapping films.
• Power tools, chargers, LED drivers, and power supply modules requiring certified dielectric film barriers.
• Supplier qualification, incoming material inspection, and export compliance documentation for film manufacturers and converters.

FAQ

What is the difference between UL 94 V-0, V-1, and V-2?

V-0 is the most demanding rating, requiring individual afterflame times of 10 seconds or less and a combined afterflame time of 50 seconds or less with no flaming drips. V-1 allows afterflame times up to 30 seconds and a 250-second total, while V-2 uses the same timing limits as V-1 but permits flaming drips that may ignite the cotton indicator.

Does the color of the insulation film affect flame retardant test results?

Yes. Pigments and dyes can alter thermal decomposition pathways, char formation, and drip behavior. Each production color and thickness should therefore be tested independently, and a rating obtained for one color cannot be automatically extended to another.

Which standard applies to very thin insulation films?

Thin, flexible films that distort or shrink during standard vertical testing are evaluated using the UL 94 VTM method or ISO 9773, in which specimens are rolled around a mandrel to form a cylindrical geometry before flame application.

How many specimens are required for UL 94 vertical burning?

Typically five specimens per conditioning set are tested, with two sets required by UL 94, one conditioned at standard laboratory atmosphere and one oven-aged at 70 °C for 168 hours. Additional replacement specimens may be needed if borderline results occur.

How long does flame retardant testing take?

Standard flammability classification testing is usually completed within five to ten working days after sample receipt, depending on the number of test items, colors, and thicknesses submitted. Combined programs including smoke density, gas analysis, or halogen content require additional time.

Do halogen-free films still require flame retardant testing?

Yes. Halogen-free describes chemical composition, not fire performance. Independent of halogen content verification, films must still undergo burning tests to obtain the flammability classifications required by end-product safety standards.