Sealant Oxygen Index Combustion Testing: Limiting Oxygen Index (LOI) Methods, Standards, and Applica

The sealant oxygen index combustion test measures the limiting oxygen index (LOI) of a sealant, providing a quantitative indicator of flammability and flame-retardant performance in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2. This article explains the testing principle, sample preparation, test procedure, acceptance criteria, and typical applications for silicone, polyurethane, polysulfide, and hybrid sealants used in construction, transportation, and electronics.

Technical Overview

The oxygen index combustion test, more precisely known as the limiting oxygen index (LOI) test, is a small-scale laboratory method that determines the minimum concentration of oxygen, expressed as a volume percentage in an oxygen–nitrogen mixture, that will just support candle-like flaming combustion of a vertically mounted specimen under controlled conditions. For sealants—elastomeric jointing materials based on silicone, polyurethane, polysulfide, silyl-modified hybrid polymers, acrylic, or butyl chemistry—the LOI value is one of the most widely used indicators of intrinsic flammability and flame-retardant performance.

Because ambient air contains approximately 20.9% oxygen by volume, the LOI result provides an intuitive benchmark: a sealant with an LOI below 21% will continue to burn in normal air once ignited, whereas a sealant with an LOI above 21% tends to self-extinguish. In practical engineering specifications, LOI thresholds of 26–30% or higher are frequently required for flame-retardant and fire-resistant sealant products.

The test is governed by an internationally harmonized family of standards, principally ISO 4589-2 (Plastics—Determination of burning behaviour by oxygen index—Part 2: Ambient-temperature test), ASTM D2863 (Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics), and GB/T 2406.2, which is technically aligned with ISO 4589-2. A related method, ISO 4589-3, extends the measurement to elevated temperatures.

• Measured quantity: minimum oxygen volume fraction that just sustains flaming combustion
• Primary standards: ISO 4589-2, ASTM D2863, GB/T 2406.2, JIS K 7201-2
• Specimen orientation: vertical, with top-face ignition
• Reported result: an index expressed as percent oxygen by volume (for example, LOI = 31)
• Common acceptance thresholds: 26% or above for flame-retardant grades, 30% or above for highly flame-retardant grades in many sealant specifications

Testing Samples

Any sealant that can be cured into a coherent strip or sheet and cut into test specimens is suitable for oxygen index testing. The principal material families submitted to accredited third-party laboratories include the following.

Silicone Sealants

One-part room-temperature-vulcanizing (RTV-1), two-part (RTV-2), and structural glazing silicones typically achieve relatively high LOI values, commonly in the range of 25–35% depending on filler loading and flame-retardant additives, owing to the inorganic siloxane backbone and the formation of a silica-like char. Structural silicones used in curtain-wall construction and fire-rated silicone systems are among the sealant categories most frequently tested.

Polyurethane Sealants

Moisture-curing polyurethane sealants in standard, non-flame-retardant grades generally show LOI values of approximately 17–24%. Phosphorus-, nitrogen-, or halogen-containing additives and mineral fillers are used to raise the index for demanding applications such as insulated glazing, automotive assembly, and trafficable flooring joints.

Polysulfide Sealants

Two-part polysulfide sealants, widely used in insulating glass manufacture and fuel-tank sealing, contain sulfur in the polymer backbone and typically reach moderate LOI values, which can be further improved with aluminum hydroxide or other flame-retardant fillers.

Silyl-Modified Hybrid (MS) Sealants

Silane-modified polyether and hybrid sealants, valued for their solvent-free and isocyanate-free chemistry, are tested to document flame-retardant performance for construction and industrial joints. Formulations are frequently engineered to exceed an LOI of 27% through the use of hydrated mineral fillers and char-promoting additives.

Acrylic, Butyl, and Specialty Sealants

Waterborne acrylic sealants, butyl and hot-melt sealants, epoxy-based compounds, and intumescent or fire-stop sealants are also evaluated, particularly during product development and pre-compliance screening prior to large-scale fire-resistance testing of penetration sealing systems.

Specimen Preparation and Conditioning

Sealants are viscoelastic and cannot be tested as supplied; they must first be fully cured. Beads are extruded into polytetrafluoroethylene-lined molds or onto release paper to form sheets, cured under standard laboratory conditions of 23 ± 2 °C and 50 ± 5% relative humidity for the manufacturer's stated full-cure period—commonly 7 to 28 days for moisture-curing systems—and then cut to the standard specimen geometry. Typical dimensions are 80–150 mm long, 10 mm wide, and 4 mm thick per ISO 4589-2, or 70–150 mm long, 6.5 mm wide, and 3 mm thick per ASTM D2863, although other cross-sections may be agreed between the interested parties. Flexible specimens that cannot support themselves may be retained with fine wire support frames as permitted by the standards, and the use of any support must be clearly reported. All specimens are conditioned to equilibrium before testing.

Testing Items

• Determination of the limiting oxygen index (LOI) at ambient temperature
• Verification of compliance with specification limits, such as LOI of 26% or 30% and above
• Comparative evaluation of flame-retardant formulations, filler types, and filler loadings
• Quality-control verification of production batches and lot acceptance testing
• Supplier qualification and incoming material inspection programs
• Technical datasheet documentation and pre-compliance screening before fire classification tests
• Optional determination of LOI at elevated temperature in accordance with ISO 4589-3

Testing Methods

Apparatus

The test employs a vertical heat-resistant glass chimney, a specimen holder that clamps the specimen at its lower end, a precision oxygen–nitrogen gas mixing and metering system using calibrated rotameters or mass-flow controllers, often with an oxygen analyzer for verification, and a small propane or butane pilot-flame igniter. The gas mixture flows upward through the chimney at a constant total flow rate stipulated by the standard, creating a stable and quiescent controlled atmosphere around the specimen.

Test Procedure

• Mount the conditioned specimen vertically in the holder with its upper end positioned within the chimney and above the gas outlet.
• Select an initial oxygen concentration based on prior knowledge of the material class.
• Apply the ignition flame to the top of the specimen in short cycles, typically 5 seconds on and 5 seconds off, until the specimen ignites, within the maximum application time allowed by the standard.
• Observe whether combustion meets the failure criterion: a burning time of 180 seconds per ISO 4589-2 or 3 minutes per ASTM D2863, or the destruction of a 50 mm length of specimen measured from the ignition end.
• If the criterion is met, reduce the oxygen concentration; if the flame self-extinguishes before the criterion is reached, increase the oxygen concentration, using progressively smaller steps.
• Bracket the transition between burning and non-burning behavior in fine increments, commonly 0.2% oxygen, until adjacent pass and fail concentrations are identified, then determine the LOI in accordance with the calculation rule of the applicable standard.

Multiple specimens, typically fifteen or more, are consumed as the oxygen concentration is successively adjusted, since elastomeric sealants often show some scatter in burning behavior. The test report states the LOI value, specimen geometry, cure and conditioning history, use of supports, ignition behavior, molten dripping or flaming drips, and any deviations from the standard.

Calculation and Expression of Results

The LOI is derived from the final bracketing oxygen concentrations and is reported as a volume percentage of oxygen, normally to one decimal place, for example LOI = 29.5%. Reports may also state the oxygen concentration interval within which the transition between sustained burning and self-extinguishing occurred, together with the number of specimens tested.

Factors Influencing Results

• Incomplete cure of moisture-reactive sealants, which alters pyrolysis and char formation behavior
• Specimen thickness, since thinner specimens generally ignite more readily and can yield lower LOI values
• Filler type and content, including aluminum hydroxide, magnesium hydroxide, and platinum-catalyzed char systems in silicones
• Melt flow and flaming drips, which can affect the burning criterion for thermoplastic-type sealants
• Gas calibration accuracy, flow stability, and chimney cleanliness
• Laboratory ambient conditions and the specimen conditioning history

Applications

Construction and Curtain Wall Engineering

Flame-retardant structural and weather-sealing silicones and hybrid sealants used in high-rise curtain walls, interior joint sealing, and fire-stop systems are qualified against LOI limits specified in project specifications and product standards. A verified oxygen index supports both design approval and site acceptance of the sealing materials.

Transportation

Sealants and adhesives used in rail vehicles, ships, buses, and aircraft cabins are screened with oxygen index testing alongside complementary flammability methods as part of comprehensive materials fire-safety qualification programs. The LOI provides a reproducible comparative metric when selecting materials for enclosed passenger environments.

Electrical and Electronics

Potting compounds, conformal coating sealants, gasketing materials, and battery-module sealing adhesives are evaluated for oxygen index as an indicator of self-extinguishing behavior near potential ignition sources such as terminals, connectors, and power electronics.

Product Development and Quality Control

Formulators use LOI data to optimize flame-retardant additive packages, benchmark competitive materials, verify batch-to-batch consistency, and substantiate datasheet claims. Independent verification by accredited third-party laboratories strengthens the credibility of reported flame-retardant performance for customers and regulatory reviewers.

FAQ

What does an LOI value actually tell me about a sealant?

The LOI tells you the minimum oxygen enrichment required for the sealant to sustain candle-like burning after ignition. Since normal air contains about 20.9% oxygen, an LOI above this level means the material will not easily continue burning in ambient air, while higher values indicate progressively greater flame resistance and self-extinguishing tendency.

Is there a universal pass/fail limit for the oxygen index test?

No single limit applies to all products. Acceptance thresholds are set by the purchaser, the product specification, or the relevant technical standard. Common industry practice treats LOI values of 26% and above as flame retardant and 30% and above as highly flame retardant, but the applicable specification always governs.

Which standard should I specify: ISO 4589-2, ASTM D2863, or GB/T 2406.2?

These standards are technically harmonized and produce comparable results. ISO 4589-2 and GB/T 2406.2 are typically specified in international projects and the Chinese market respectively, while ASTM D2863 prevails in North American specifications. Select the standard cited by the governing project or product specification.

Can fully flexible sealants be tested?

Yes. Flexible or elastomeric specimens that cannot remain upright on their own may be supported by fine wire frames as permitted by the standards, or thin cured films may be cast and tested by agreement. The use of any support must be documented in the test report because it can influence the result.

Does a high LOI mean the sealant is fireproof?

No. The oxygen index is a comparative, small-scale flammability screening test conducted on individual specimens under specific laboratory conditions. It does not by itself demonstrate fire resistance of a joint or penetration assembly, which must be verified through full-scale fire-resistance tests under the relevant fire-test standards.

How many specimens and how much material are required?

A typical determination consumes at least fifteen specimens, which for sealants requires a cured sheet of roughly 150 × 100 mm or larger depending on the specified cross-section. Because moisture-curing sealants must reach full cure before cutting, the overall laboratory timeline is usually governed by the cure period rather than by the test itself.