Determination of the Oxygen Index of Foamed Rubber: Principles, Standards, Procedures and Applicatio

The oxygen index (Limiting Oxygen Index, LOI) test is one of the most widely used methods for evaluating the flammability of foamed rubber materials, quantifying the minimum oxygen concentration required to sustain candle-like combustion. This article provides a comprehensive technical overview of foamed rubber oxygen index determination, covering applicable standards such as ISO 4589-2, ASTM D2863 and GB/T 10707, sample requirements, test procedures, result interpretation and typical industrial applications. Whether you are a materials engineer, quality control specialist or product developer, this guide will help you understand how LOI testing supports flame-retardant design and regulatory compliance for cellular rubber products.

Technical Overview

Foamed rubber (also referred to as cellular rubber or sponge rubber) is a lightweight elastomeric material characterized by a gas-filled cellular structure. Its large surface area, low density and high air content make it considerably more combustible than dense solid rubber, which is why flammability assessment is a critical step in material qualification for many industries. The oxygen index test measures the minimum volumetric concentration of oxygen, in a flowing mixture of oxygen and nitrogen at 23 °C, that will just support combustion of a vertically mounted specimen after ignition with a small pilot flame.

The result is expressed as the Limiting Oxygen Index (LOI) in percent. Because normal ambient air contains approximately 20.9% oxygen, materials with an LOI below 21% will generally continue to burn in air once ignited, while materials with an LOI above 27% are commonly regarded as self-extinguishing or flame retardant. Foamed rubber products intended for construction sealing, transportation, electrical enclosures and thermal insulation frequently must demonstrate LOI values of 28% or higher, often in combination with other fire performance tests, to satisfy customer specifications or regulatory requirements. Oxygen index determination for foamed rubber is typically performed by accredited third-party laboratories or in-house fire testing facilities using internationally recognized methods.

Testing Samples

Specimen selection and preparation have a decisive influence on the reproducibility of oxygen index results, particularly for cellular materials. Test pieces shall be representative of the finished product, free of skin damage, collapsed cells, contamination and processing defects.

Closed-Cell Foamed Rubber

Closed-cell materials such as EPDM sponge, neoprene (CR) foam and foamed nitrile rubber are typically supplied as sheets, tubes or profiles. Specimens are cut with sharp dies or blades to avoid compressing or sealing the cells, and cut surfaces must be smooth and perpendicular to the faces.

Open-Cell Sponge Rubber

Open-cell materials, such as flexible polyurethane-ether elastomer blends and natural rubber sponge, may require greater care during cutting because of their soft, resilient structure. If the material cannot support its own weight in the vertical chimney, a support may be used in accordance with the applicable standard.

Flame-Retardant Modified Compounds

Foamed rubber formulations containing halogenated or halogen-free flame retardants, intumescent packages or mineral fillers are tested in their final cured and post-cured state, since retardant efficiency depends on the complete compound and its cellular morphology.

Specimen Dimensions and Conditioning

• For cellular materials tested to ISO 4589-2, the preferred specimen type is generally 80 mm to 150 mm in length with a 10 mm × 10 mm cross-section.
• ASTM D2863 specifies comparable vertical bar specimens, with dimensions selected according to the material type and the reporting requirements.
• GB/T 10707, the Chinese national method for rubber flammability by oxygen index, defines corresponding rectangular bar specimens for vulcanized and thermoplastic rubber.
• A minimum of 15 usable specimens is normally required to complete the concentration series.
• Specimens shall be conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for the period defined by the standard before testing.

Testing Items

The core deliverable of the test is the Limiting Oxygen Index of the foamed rubber, reported as a volume percentage to one decimal place. Supplementary information recorded during testing includes:

• Burning behavior observations, such as flaming, glowing, dripping, sooting and afterflame characteristics.
• Whether combustion reached the 50 mm measuring mark or exceeded the 180-second burning criterion.
• Specimen type, dimensions, orientation and surface condition (skinned or cut face exposed to ignition).
• The test standard applied (ISO 4589-2, ASTM D2863, GB/T 10707) and the ignition procedure used.
• Optional elevated-temperature oxygen index determination in accordance with ISO 4589-3 where service conditions demand it.

Testing Methods

Principle of the Oxygen Index Test

A rectangular specimen is clamped vertically in a heat-resistant glass chimney. A controlled mixture of oxygen and nitrogen flows upward through the chimney at a specified linear velocity. A small pilot flame is applied to the top of the specimen. If the specimen burns beyond the defined criterion, the oxygen concentration is reduced; if it extinguishes prematurely, the concentration is increased. The oxygen concentration is progressively adjusted until the critical concentration that just sustains combustion is established.

Apparatus

The test rig comprises a vertical glass chimney, a specimen holder, gas supply and metering systems for oxygen and nitrogen with calibrated flow controllers, a pilot flame igniter, and timing and marking devices. Periodic verification using reference materials of known oxygen index is required to confirm apparatus accuracy.

Test Procedure

• Mount the conditioned specimen vertically in the holder and position it centrally in the chimney.
• Establish the specified total gas flow through the chimney and allow the system to purge and stabilize.
• Apply the pilot flame to the top of the specimen following the ignition procedure of the selected standard, typically in short defined applications repeated if ignition does not occur.
• Record whether the specimen burns for the criterion time or over the criterion length after removal of the igniter.
• Adjust the oxygen concentration using an up-and-down (bracketing) approach until the transition between burning and non-burning responses is bracketed within the required step interval.
• Verify the result with repeated specimens at the critical concentration as specified by the method.

Calculation and Expression of Results

The Limiting Oxygen Index is calculated from the volume flow rates of oxygen and nitrogen at the critical concentration, expressed as the percentage of oxygen in the gas mixture. The report shall include the LOI value, the standard used, specimen type and dimensions, orientation, conditioning history and all relevant observations. Results for different specimen thicknesses, orientations or ignition procedures shall not be directly compared, as these parameters significantly affect cellular material behavior.

Influencing Factors and Quality Control

• Foam density and cell structure strongly affect LOI; lower-density foams generally exhibit lower oxygen indices than the same compound in solid form.
• Specimen thickness, skin versus cut surface exposure, and cutting quality influence burning behavior.
• Flame retardant loading, dispersion and post-curing state of the rubber compound determine the intrinsic flammability performance.
• Conditioning temperature and humidity affect moisture content, which can marginally alter results for hygroscopic compounds.
• Operator technique in flame application and criterion judgment is controlled through standardized procedures and laboratory proficiency checks.

Applications

Oxygen index data for foamed rubber are used across many sectors to qualify materials, compare flame-retardant formulations and demonstrate compliance with specifications:

• Construction sealing strips, expansion joint fillers and door and window gaskets requiring enhanced fire safety performance.
• HVAC and refrigeration insulation made from foamed elastomeric sheets and tubes.
• Automotive sealing systems, body sealing profiles, NVH components and battery pack cushioning foams.
• Railway and marine interior components where flame retardancy is a core safety requirement.
• Electrical and electronic enclosures, cable management cushions and equipment gaskets.
• Footwear midsoles and padding materials where ignition resistance is evaluated.
• Research and development programs screening halogen-free flame retardant packages for next-generation cellular rubber compounds.

FAQ

What oxygen index value is considered flame retardant for foamed rubber?

As a general industry guideline, an LOI of 21% or below means the material can burn in normal air, values between 21% and 27% indicate slow burning, and values above 27% are commonly classified as self-extinguishing or flame retardant. Many specifications for foamed rubber sealing and insulation products require LOI values of 28% or higher.

Which standards are used for foamed rubber oxygen index testing?

The most common methods are ISO 4589-2 (ambient-temperature oxygen index), ASTM D2863 (minimum oxygen concentration to support candle-like combustion) and GB/T 10707 (oxygen index method for rubber). These methods are technically equivalent in principle, but specimen details and procedural requirements differ, so the specified standard must always be stated with the result.

Why do foam thickness and orientation affect the oxygen index result?

Cellular materials burn differently depending on the exposed surface and cross-section. A cut open-cell face ignites more readily than an intact skin, and thicker specimens retain more heat, supporting sustained combustion. Consequently, only results obtained with identical specimen geometry and orientation are directly comparable.

How many specimens are needed for one oxygen index determination?

A complete determination typically requires at least 15 usable specimens, because the critical oxygen concentration is established through a series of burns at incrementally adjusted concentrations followed by confirmation runs at the bracketed concentration.

Can an LOI result be converted into a UL 94 rating?

No. Oxygen index and UL 94 vertical or horizontal burning tests measure different combustion responses under different conditions. LOI is a quantitative measure of oxygen concentration limits, whereas UL 94 evaluates burning, afterflame and dripping behavior in air. Both tests are often performed together to fully characterize foamed rubber fire performance.

How should foamed rubber specimens be prepared to obtain reliable results?

Specimens should be die-cut or precisely sliced with sharp tools to avoid cell compression and heat damage, taken from representative production material after full curing, and conditioned under the standard atmosphere before testing. Engaging an accredited third-party laboratory or a qualified in-house fire test facility with calibrated oxygen index apparatus ensures traceable and reproducible results.