Flame Retardancy Performance Testing of Thermally Conductive Gels for Photovoltaic Modules

Flame retardancy performance testing of thermally conductive gels is a critical safety evaluation for photovoltaic modules, ensuring that thermal interface materials used in junction boxes and cell assemblies do not ignite or propagate fire under electrical fault conditions. This article provides a comprehensive technical guide to test samples, testing items, methods, and applicable standards such as UL 94, IEC 60695, and ISO 4589, helping manufacturers and quality engineers select validated materials for reliable PV module design.

Technical Overview

Thermally conductive gels are low-modulus, curable silicone or organic polymer materials filled with high-thermal-conductivity ceramic particles such as aluminum oxide, aluminum nitride, or boron nitride. In photovoltaic (PV) modules, these gels are widely used to fill gaps between bypass diodes, junction box internals, busbars, and enclosure walls, transferring waste heat away from heat-sensitive components while maintaining electrical insulation and mechanical stress relief.

Why Flame Retardancy Is Critical in PV Modules

PV modules operate for 25 years or more under high DC voltages, elevated temperatures, and cyclic thermal stress. Electrical faults such as hot spots, arc faults, diode failure, and insulation breakdown can generate localized ignition sources in direct contact with the thermal gel. If the gel sustains combustion or produces flaming drips, a localized fault can escalate into module-level fire, threatening roofs, buildings, and personnel. Flame retardancy testing therefore verifies that the material self-extinguishes quickly, does not ignite adjacent components, and minimizes heat and smoke release.

Regulatory and Standards Context

Flame retardancy evaluation of thermal gels is anchored in internationally recognized fire hazard testing standards, primarily the UL 94 and IEC 60695 series, supplemented by ISO 4589 for limiting oxygen index and IEC 61730 for overall PV module safety. Accredited third-party laboratories perform these tests under ISO/IEC 17025 quality systems, generating reports accepted by module makers, certifiers, and downstream system integrators worldwide.

Testing Samples

Proper specimen preparation directly determines the validity of flame retardancy results. Because gels are typically applied and cured in place, representative test plaques must be produced using the same formulation, mixing ratio, cure schedule, and thickness as the actual production process.

Cured Standard Plaques

Gel is cast into flat molds and cured into free-standing sheets, then cut into rectangular specimens. For UL 94 vertical and horizontal burning tests, specimens of 125 mm × 13 mm are prepared at the minimum and maximum thicknesses intended for use. For limiting oxygen index testing per ISO 4589-2, specimens of 70–150 mm × 10 mm × 4 mm (or the actual application thickness) are used.

Application-Configured Specimens

Glow wire and needle flame tests are frequently performed on the gel in its end-use configuration, for example cured onto a junction box base, terminal carrier, or metal substrate. This evaluates the interaction between the gel, the substrate, and nearby plastic components rather than the material in isolation.

Aged and Environmental-Conditioned Specimens

To simulate long-term service, specimens may be thermally aged (for example 168 h at elevated temperature), exposed to damp heat (85 °C / 85 % RH), or UV-condensation cycling before flammability testing. This reveals whether flame retardant additives migrate, deplete, or degrade over the module lifetime.

Specimen Conditioning

Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 48 hours in accordance with IEC 60695-11-10 and UL 94 requirements. Separate specimen sets may be oven-aged to evaluate retention of flame retardant performance.

Testing Items

Depending on the application, safety standard, and customer specification, the following flammability characteristics of thermally conductive gels are evaluated:

• Vertical burning classification (UL 94 V-0, V-1, V-2) per UL 94 and IEC 60695-11-10
• Horizontal burning classification (UL 94 HB) per UL 94 and IEC 60695-11-10
• Needle flame test per IEC 60695-11-5
• Glow wire flammability index (GWFI) per IEC 60695-2-12
• Glow wire ignition temperature (GWIT) per IEC 60695-2-13
• Glow wire test on end-use assemblies (GWT) per IEC 60695-2-11
• Limiting oxygen index (LOI) per ISO 4589-2, ASTM D2863, and GB/T 2406.2
• Horizontal and vertical burning classification per GB/T 2408
• Burning behavior of thin materials per IEC 60695-11-20 and IEC 60695-11-21
• Heat release rate and total heat release via cone calorimetry per ISO 5660-1
• Smoke production and smoke density per ISO 5659-2
• Flaming drip behavior and ignition of underlying cotton indicators
• Comparative tracking index for combined fire and electrical safety per IEC 60112

Testing Methods

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

A specimen is clamped vertically, and a 20 mm blue flame from a methane burner is applied to the lower edge for 10 seconds, removed, and then reapplied for a second 10-second interval after the specimen stops burning. Afterflame time, afterglow time, and flaming drips are recorded for a set of five specimens. Classification criteria include: for V-0, afterflame time no more than 10 seconds per application, total afterflame time no more than 50 seconds for all specimens, and no flaming drips that ignite cotton; V-1 permits longer burn times but still prohibits igniting drips; V-2 permits flaming drips that may ignite the cotton. Most PV junction box applications require the gel and adjacent plastics to achieve V-0 at the relevant thickness.

UL 94 Horizontal Burning Test (HB)

The specimen is mounted horizontally and ignited at one end. The burning rate is measured between reference marks, or the specimen must self-extinguish before reaching the final mark. HB requirements differ by thickness: for specimens 3.0 mm to 13 mm thick the burning rate must not exceed 40 mm/min across a 75 mm span, while thinner specimens must not exceed 75 mm/min. HB is generally the minimum baseline for less critical locations.

Glow Wire Tests

A nickel-chromium glow wire is electrically heated to a specified temperature (typically 550 °C to 960 °C) and pressed against the specimen for 30 seconds. In the flammability assessment, the specimen must not ignite, or any flame must self-extinguish within 30 seconds after the glow wire is removed, without igniting the tissue paper placed beneath. The glow wire ignition temperature (GWIT) is determined as the temperature 25 K above the highest temperature at which ignition does not occur during three consecutive tests, while the glow wire flammability index (GWFI) identifies the highest temperature at which the material meets the self-extinguishing criteria. PV junction box environments commonly demand GWIT of 750 °C or higher depending on the safety standard invoked.

Needle Flame Test

A small 12 mm high flame from a gas burner is applied to the specimen surface for times of 5, 10, 20, 30, 60, or 120 seconds. The specimen is evaluated for sustained flaming, whether flame or glowing particles ignite a tissue paper below, and whether burning drops occur. This test simulates small fault-level ignition sources such as overloaded connections inside a junction box.

Limiting Oxygen Index (LOI)

The specimen is mounted vertically in a glass chimney and ignited at its top in a controlled flow of oxygen and nitrogen. The minimum oxygen concentration, in percent by volume, that just supports candle-like burning for 3 minutes or over a 50 mm downward burn length is determined. Silicone-based thermal gels typically achieve LOI values above 25 %, and values above 28 % are commonly specified for demanding PV applications.

Cone Calorimetry and Smoke Measurement

In the cone calorimeter per ISO 5660-1, a 100 mm × 100 mm specimen is exposed to a radiant flux of 25–50 kW/m², and time to ignition, peak heat release rate, and total heat released are quantified. Smoke density per ISO 5659-2 measures specific optical density in a closed chamber. These quantitative fire hazard metrics support advanced material selection, fire modeling, and compliance with building-integrated PV requirements.

Test Sequence and Data Reporting

Testing is normally sequenced from screening (LOI, HB) to classification (UL 94 V-series) to end-product simulation (glow wire, needle flame). A complete report includes material identification, batch and cure data, specimen dimensions and thicknesses, conditioning history, test standards and severities, individual and averaged afterflame times, observations of drips and cotton ignition, and the resulting classification. Reports from accredited third-party laboratories provide the documentation needed for IEC 61730-based module safety certification and customer audits.

Applications

Flame retardancy testing of thermally conductive gels supports safety and reliability across the photovoltaic value chain and adjacent power electronics markets:

• PV junction boxes: potting and gap filling around bypass diodes and terminals where arc and hot-spot faults are credible ignition sources
• Module-level power electronics: thermal management of microinverters, power optimizers, and shutdown devices integrated into modules
• Building-integrated photovoltaics (BIPV): materials installed in roofs and facades where building codes impose stringent fire performance evidence
• High-power modules and tracking systems: increased current ratings demand verified flame retardant thermal interfaces near higher-energy fault scenarios
• Energy storage and battery systems: fire-safety-qualified thermal gels transferable between PV and stationary or mobile storage platforms
• Material development: benchmarking flame retardant formulations, filler loadings, and halogen-free additive systems for next-generation gels

FAQ

What flame retardancy rating should a thermal gel have for PV junction box use?

Most module makers and junction box specifications require at least UL 94 V-0 at the applied thickness, often combined with a glow wire ignition temperature of 750 °C or higher. The exact requirement depends on the applicable product safety standard and the fault analysis for the specific design, so the design specification should always be confirmed before testing.

Why must the gel be tested at the actual application thickness?

Flame propagation behavior is strongly thickness-dependent. Thin gel layers ignite faster and burn differently from thick sections, so standards require evaluation at both the minimum and maximum thickness used in production. Testing only a standard thick plaque can overstate performance of a thin in-field application layer.

How is a gel, which is soft and may flow, tested by UL 94 methods?

Fully cured gel plaques with sufficient self-supporting rigidity are cut into specimens and clamped so the upper portion is held while the free lower edge contacts the burner flame. For very soft or highly elastomeric gels, testing the material in the end-use configuration using glow wire and needle flame methods provides complementary, application-relevant evidence.

What is the difference between GWFI and GWIT?

GWFI (glow wire flammability index) is the highest glow wire temperature at which the material either does not ignite or self-extinguishes within 30 seconds without igniting tissue paper beneath. GWIT (glow wire ignition temperature) is the temperature 25 K above the highest temperature at which no ignition occurs during the test. GWFI addresses burning behavior after exposure, while GWIT addresses whether ignition happens at all.

How long does flame retardancy testing take?

Typical turnaround for a standard UL 94 vertical burning assessment including specimen preparation and conditioning is one to two weeks. Multi-item programs combining UL 94, glow wire, needle flame, and LOI, or those including thermal or damp-heat aging preconditioning, generally require two to four weeks depending on laboratory scheduling and the number of thicknesses evaluated.

Can test results from one gel formulation be extended to other thicknesses or colors?

No. Flammability classifications are valid only for the specific formulation, color, and thickness range tested. Changes in filler loading, flame retardant package, pigment, or application thickness require retesting to maintain a defensible safety file for the PV module.

Do flame retardancy tests replace full module safety certification?

No. Gel-level fire tests provide material-level evidence that feeds into overall module safety evaluation under standards such as IEC 61730. Full module certification additionally covers electrical, mechanical, thermal, and environmental qualifications performed on the assembled product.