Oxygen Index Testing of Thick-Type Fireproof Coatings: Standards, Methods, and Evaluation

Oxygen index testing, also known as the Limiting Oxygen Index (LOI) test, is a widely used laboratory method for evaluating the intrinsic flame-retardant performance of thick-type fireproof coating films. By determining the minimum oxygen concentration required to sustain candle-like combustion, this test provides manufacturers, specifiers, and accredited third-party laboratories with a reliable, quantitative index of the ignition resistance of coating materials used in passive fire protection. This article explains the principle, applicable standards, sample preparation, test procedure, result interpretation, and typical applications of oxygen index testing for thick-type fireproof coatings.

Technical Overview

Thick-type fireproof coatings, also referred to as non-intumescent or heavy-duty fire-resistive coatings, are cementitious or gypsum-based protective materials applied to structural steel and concrete at dry film thicknesses typically above 7 mm. These coatings protect substrates through low thermal conductivity, heat-absorbing decomposition, and insulating mineral aggregates such as expanded perlite and vermiculite, rather than through intumescent expansion. Their fire performance is primarily demonstrated through fire-resistance testing in accordance with standards such as GB 14907, EN 13381, and ASTM E119.

The oxygen index test complements fire-resistance testing by characterizing the ignitability and self-extinguishing behavior of the coating film itself. The Limiting Oxygen Index (LOI) is defined as the minimum concentration of oxygen, expressed as a volume percentage in an oxygen and nitrogen mixture, that will just support candle-like combustion of a vertically mounted specimen. Because ambient air contains approximately 20.9 percent oxygen, materials with an LOI significantly above 21 percent will not continue burning in ordinary air once the ignition source is removed.

The oxygen index method for thick-type fireproof coating films is generally performed in accordance with the following equivalent or harmonized standards:

• GB/T 2406.2-2009 — Plastics — Determination of burning behaviour by oxygen index — Part 2: Ambient-temperature test
• 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 (Oxygen Index)
• ISO 4589-3 — Ambient to elevated temperature determination, applicable when high-temperature ignition behavior is of interest

For predominantly inorganic thick-type coatings, the LOI value is usually very high because mineral binders and aggregates do not sustain combustion. A measured LOI result therefore serves as an effective quality-control indicator for verifying that organic binder modifications, additives, or production batch variations have not degraded the flame-retardant character of the coating system.

Testing Samples

Free Film Specimens

Because the coating must be evaluated as a standalone material, a free film is prepared by casting the liquid or paste coating into a mold, leveling it to a uniform thickness, and allowing it to cure fully under the conditions specified by the manufacturer. The cured sheet is then cut into rectangular bar specimens. The preferred specimen geometry under ISO 4589-2 and GB/T 2406.2 is:

• Length: 80 mm to 150 mm
• Width: 10 mm, with a width of 20 mm permitted for flexible or film materials that tend to distort
• Thickness: 4 mm is preferred; other thicknesses may be tested and must be reported

Specimen edges should be smooth and free of cracks, bubbles, and unbonded aggregate particles, since surface defects can alter ignition behavior and reduce repeatability.

Specimen Conditioning

Specimens are conditioned in a controlled atmosphere of 23 ± 2 °C and 50 ± 5 percent relative humidity, in accordance with ISO 291 or the conditioning clause of the governing test standard, for a minimum of 88 hours prior to testing. This step stabilizes the moisture content of the hygroscopic mineral constituents and ensures reproducible results.

Specimen Quantity

A complete LOI determination normally consumes 10 to 15 specimens. The initial estimation of the oxygen concentration requires several specimens, and the final bracketing sequence requires additional bars tested at closely spaced oxygen increments. Laboratories should prepare spare specimens in case of retests.

Testing Items

The core testing item for this program is the determination of the Limiting Oxygen Index of the cured thick-type fireproof coating film at ambient temperature. Supporting observations and optional items include:

• Limiting Oxygen Index (LOI) expressed as the volume percentage of oxygen in the O2/N2 mixture, reported to one decimal place
• Afterflame time and afterglow behavior following removal of the ignition source
• Observation of flaming drips or flaming particles, where applicable
• Comparison of LOI values before and after conditioning treatments such as water immersion, humidity cycling, or artificial aging, when durability assessment is required
• Optional elevated-temperature oxygen index testing in accordance with ISO 4589-3

Testing Methods

Apparatus

The oxygen index apparatus consists of a heat-resistant glass chimney of specified dimensions, a specimen holder that clamps the lower portion of the bar and supports it vertically on the chimney axis, a precision gas mixing and metering system for oxygen and nitrogen, a small propane or hydrogen flame igniter, and an oxygen analyzer for verifying the delivered mixture concentration. The linear gas flow velocity through the chimney is maintained at 40 mm/s, and the gas mixture temperature is held at 23 ± 2 °C.

Test Procedure

The test is conducted as follows:

• Mount the conditioned specimen vertically in the holder and install it in the chimney so that the top of the specimen is at the defined position below the chimney outlet.
• Purge the chimney with the oxygen and nitrogen mixture at the specified flow rate and allow the system to stabilize.
• Apply the ignition flame to the top of the specimen for the prescribed contact time, then withdraw the igniter.
• Observe whether the specimen burns for the defined period, typically 180 seconds, or whether the flame front travels beyond the 50 mm measuring mark.
• Adjust the oxygen concentration upward or downward in defined steps and repeat the test on fresh specimens until the critical concentration region is bracketed.
• Complete the final determination using fine increments, typically 0.2 percent oxygen, and calculate the LOI value according to the statistical method specified in the standard.

Calculation and Expression of Results

The Limiting Oxygen Index is calculated as the volume fraction of oxygen in the gas mixture and is expressed as:

LOI = 100 × V(O2) / [V(O2) + V(N2)]

The result is reported to one decimal place, together with the specimen dimensions, orientation, conditioning history, test standard applied, and any deviations such as distortion, shrinkage, or intumescence observed during burning.

Interpretation of Results

Although the standards do not define pass or fail thresholds, the following interpretation is widely applied in industry:

• LOI below 21 percent: the material burns readily in air and is considered flammable
• LOI between 21 and 27 percent: the material burns slowly in air and is moderately flame retardant
• LOI of 27 percent and above: the material is generally self-extinguishing in air and is regarded as flame retardant
• LOI approaching or exceeding the practical measurement range: characteristic of highly inorganic, non-combustible coating films typical of well-formulated thick-type fireproof coatings

Applications

Oxygen index testing of thick-type fireproof coatings is applied across several engineering and commercial contexts:

• Formulation research and development, where LOI provides rapid feedback on the effectiveness of inorganic binders, mineral fillers, and flame-retardant additives
• Factory quality control, enabling batch-to-batch verification that coating films retain their non-combustible character
• Type testing and product certification support, where accredited third-party laboratories perform LOI determinations as part of comprehensive characterization of fire-resistive coating systems
• Steel structure fire protection in high-rise buildings, industrial plants, petrochemical facilities, and power stations, where specifiers request LOI data alongside fire-resistance ratings
• Tunnel and infrastructure coatings, where coatings may contain organic modification components whose combustion behavior must be monitored
• Comparison of coating systems before and after environmental aging, verifying that weathering, humidity, and freeze-thaw exposure do not degrade flame-retardant performance

FAQ

What is the oxygen index and why is it important for thick-type fireproof coatings?

The oxygen index, or LOI, is the minimum oxygen concentration in a flowing oxygen and nitrogen mixture that will just sustain combustion of a vertically held specimen. For thick-type fireproof coatings, which are expected to be non-combustible or self-extinguishing, the LOI confirms that the coating film itself does not contribute fuel to a fire and validates the quality of the inorganic formulation.

Which standards govern the oxygen index test?

The most commonly applied standards are GB/T 2406.2-2009, ISO 4589-2, and ASTM D2863, all of which define the ambient-temperature candle-like combustion method. ISO 4589-3 covers determinations at elevated temperatures. These standards are technically equivalent in principle, differing mainly in procedural details and the statistical treatment of the bracketing data.

Can the coating be tested directly on the steel substrate?

No. The oxygen index method requires free-standing bar specimens so that the measured combustion behavior reflects the coating material alone. Testing a coating attached to a metallic substrate would introduce heat-sink effects from the steel and produce invalid results, so a cured free film is always prepared and cut into specimens.

What LOI value is considered acceptable for a fireproof coating?

While the test standards do not set acceptance criteria, industry practice regards an LOI of 27 percent or higher as indicating a self-extinguishing, flame-retardant material. Properly formulated thick-type coatings, which are dominated by cement, gypsum, perlite, and vermiculite, typically yield very high LOI values because their inorganic constituents do not support sustained combustion.

Is the oxygen index test a substitute for a fire-resistance rating test?

No. The oxygen index characterizes the ignitability of a small specimen of the coating film, whereas fire-resistance testing evaluates the ability of the full coating system on a loaded steel or concrete member to delay temperature rise for ratings such as 1.0, 1.5, 2.0, 2.5, or 3.0 hours. The two tests answer different questions and are used together: LOI for material screening and quality control, fire-resistance testing for structural performance.

What factors can influence the test result?

Specimen thickness and density, completeness of curing, moisture content after conditioning, uniformity of fiber and aggregate dispersion, gas flow stability, and operator ignition technique all influence the measured value. Strict adherence to specimen preparation and conditioning requirements is essential for reproducible results, which is why testing by accredited third-party laboratories is recommended for certification purposes.

How long does an oxygen index test take?

A single determination typically requires 10 to 15 specimens and can be completed within a few hours once specimens have been conditioned. When free films must be cast and cured from liquid coating samples, the total timeline depends on the curing schedule of the product, and laboratories usually quote several days to two weeks including preparation and reporting.