The Limiting Oxygen Index (LOI) test is one of the most widely used methods for evaluating the flame-retardant performance of rubber foam materials such as EPDM, NBR/PVC, and chloroprene foams. Performed in accordance with ISO 4589-2, ASTM D2863, and GB/T 2406.2, the test quantifies the minimum oxygen concentration required to sustain combustion, providing essential data for product development, quality control, and regulatory compliance. This guide explains the testing principle, sample requirements, test procedures, result interpretation, and typical applications of LOI testing for rubber foam.
Technical Overview
The Limiting Oxygen Index (LOI), also known as the Oxygen Index (OI) or Critical Oxygen Index (COI), is defined as the minimum concentration of oxygen, expressed as a volume percentage, in a flowing mixture of oxygen and nitrogen that will just support sustained flaming combustion of a vertically mounted test specimen. Since normal atmospheric air contains approximately 20.95% oxygen, a rubber foam material with an LOI value above 21% will tend to self-extinguish when ignited in ambient air. The higher the LOI value, the better the flame-retardant performance of the material.
For rubber foam products used in building insulation, HVAC systems, transportation, and electrical enclosures, the LOI test serves as a fundamental screening tool for flammability. Because cellular rubber has a large surface area and low density, its burning behavior differs significantly from solid rubber, making dedicated oxygen index testing on the actual foam grade essential for accurate fire-risk assessment.
- Primary standards: ISO 4589-2, ASTM D2863, and GB/T 2406.2 (which is technically aligned with ISO 4589-2).
- Test atmosphere: a controlled upward flow of oxygen and nitrogen gas mixture at 23 ± 2 °C.
- Result expression: LOI = 100 × V(O₂) / [V(O₂) + V(N₂)], reported to the nearest 0.1%.
- Supplementary standards: ISO 4589-3 for LOI determination at elevated temperatures, applicable when service conditions involve heat exposure.
Testing Samples
EPDM Rubber Foam
Closed-cell EPDM (ethylene propylene diene monomer) foam is widely used for sealing strips, expansion joints, and outdoor thermal insulation. Untreated EPDM foam typically shows a moderate LOI, while flame-retardant grades loaded with inorganic fillers such as aluminum hydroxide or magnesium hydroxide can reach LOI values of 28% to 35%.
NBR and NBR/PVC Rubber Foam
Nitrile butadiene rubber (NBR) foam, often blended with PVC, is the dominant material for air-conditioning and refrigeration pipe insulation. Building regulations in many markets require this material to achieve an LOI of 32% or higher to qualify for low-flammability classifications, which is typically achieved through halogenated or halogen-free flame-retardant systems.
Chloroprene (Neoprene) Rubber Foam
Chloroprene rubber (CR) foam, used in wetsuits, gaskets, and vibration-damping pads, inherits a degree of inherent flame retardancy from its chlorine content, with LOI values commonly in the range of 26% to 32% depending on formulation and density.
Silicone Rubber Foam
Silicone foams used in rail transit, aerospace, and fire-stop sealing applications exhibit high thermal stability and can achieve LOI values above 30%, making them suitable for environments with strict smoke and toxicity requirements.
Sample Preparation and Conditioning
Typical specimen dimensions for rigid cellular materials follow ISO 4589-2 Type I geometry: 80 mm to 150 mm in length, 10 mm ± 0.5 mm in width, and 10 mm ± 0.5 mm in thickness; thinner Type III specimens (10 mm × 4 mm) may be used when product thickness is limited. ASTM D2863 specimens are commonly 70 mm to 150 mm long, 6.5 mm ± 0.5 mm wide, and 3 mm ± 0.5 mm thick. Specimens must be cut cleanly without crushing or tearing the cell structure, and conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 88 hours prior to testing in accordance with ISO 291 or ASTM D618.
Testing Items
An LOI test program for rubber foam generally covers the following items:
- Determination of the Limiting Oxygen Index value at room temperature (23 °C).
- Observation of ignition behavior, including ease of ignition and flame spread along the specimen surface.
- Recording of after-flame time and after-glow time following removal of the ignition source.
- Optional determination of the temperature index (LOI at elevated temperature) per ISO 4589-3.
- Assessment of whether melting, dripping, or char formation occurs during combustion.
Testing Methods
Test Apparatus
The LOI apparatus consists of a heat-resistant glass chimney (typically 450 mm to 500 mm high) of specified diameter, a specimen holder that clamps the specimen vertically in the center of the chimney, a precision gas mixing and flow control system with calibrated oxygen and nitrogen flowmeters, and a small hydrogen or propane flame igniter. An oxygen analyzer may be used to verify the actual oxygen concentration of the gas mixture delivered to the chimney.
Test Procedure
The conditioned specimen is clamped vertically in the holder with at least 50 mm of specimen exposed above the top of the chimney. A mixture of oxygen and nitrogen is passed through the chimney at a total flow rate of 40 mm/s ± 2 mm/s linear velocity for at least 30 seconds to purge the system. The top of the specimen is then ignited using the igniter, applying the flame for a maximum of 30 seconds while the gas flow continues. The tester observes whether the specimen burns for 180 seconds or more, or consumes a burning length of 50 mm, as specified in ISO 4589-2 for Type I to Type IV specimens.
The oxygen concentration is adjusted stepwise, typically in increments of 0.2% near the final value, until the critical concentration is bracketed between concentrations that just pass and just fail the burning criterion. The "pass" response is defined as burning that does not reach the specified time or length criteria. At least five valid specimens are tested at each bracketing concentration, and the LOI is calculated from the volumetric flow rates of the two gases.
Result Interpretation
The reported LOI value is used to rank the flammability of rubber foam materials as a general guide:
- LOI below 21%: readily flammable in normal air; generally unacceptable for building or transport applications.
- LOI of 21% to 24%: self-extinguishing tendency in air; suitable only for low-risk uses.
- LOI of 24% to 28%: moderately flame retardant; common for general industrial sealing products.
- LOI of 28% to 32%: flame retardant; typical requirement for rail vehicle and appliance components.
- LOI above 32%: highly flame retardant; commonly specified for building insulation foam and fire-stop materials.
It should be noted that LOI is a small-scale laboratory measurement. Regulatory approval of rubber foam products usually requires supplementary tests such as UL 94 horizontal and vertical burning, smoke density, and toxic gas emission testing, depending on the end-use specification.
Applications
LOI testing of rubber foam supports material qualification and quality control across many industries:
- Building and construction: fire-performance classification of insulation boards, sealing strips, and expansion joint fillers under national building codes.
- HVAC and refrigeration: verification of flame-retardant grades of NBR/PVC pipe insulation for chillers, ductwork, and chilled-water systems.
- Automotive and rail transit: evaluation of door and window seals, firewall insulation, and interior components against flame-safety requirements for rolling stock and vehicles.
- Marine and offshore: qualification of low-flame-spread insulation and gasketing materials for ships and platforms.
- Electrical and electronics: screening of foam gaskets and cable management components used near energized equipment.
- Research and development: comparison of flame-retardant formulations, verification of filler loading effects, and assessment of aging on combustion performance.
FAQ
What is a good LOI value for rubber foam?
For most regulated applications, an LOI of 28% or higher is considered flame retardant. Building insulation foam in many markets is specified at 32% or above, while general-purpose sealing foam may only require 24% to 27%. The exact acceptance limit depends on the applicable product standard and end-use regulation.
Which standards govern rubber foam LOI testing?
The three principal standards are ISO 4589-2 (plastics — determination of burning behaviour by oxygen index, part 2: ambient-temperature test), ASTM D2863, and GB/T 2406.2. These methods are technically equivalent for most rigid cellular materials, though specimen dimensions and pass/fail criteria differ slightly, so the correct standard must be agreed with the purchaser or regulator before testing.
How does foam density affect the LOI result?
Lower-density foam has a higher surface-to-mass ratio and a larger volume of trapped air, which generally lowers the LOI value compared with a denser compound of the same formulation. Cell structure, skin formation during molding, and specimen thickness all influence results, which is why specimens must represent the actual finished product.
How is LOI different from the UL 94 test?
LOI quantifies the minimum oxygen concentration needed to sustain burning and yields a numerical value suitable for ranking materials, whereas UL 94 evaluates burning behavior, after-flame time, and dripping in ambient air and assigns a classification such as HBF, V-0, or V-2. The two tests are complementary and are frequently both required in full material flammability assessments.
Can aged rubber foam be tested?
Yes. Heat aging, UV exposure, and humidity cycling can deplete or migrate flame-retardant additives in foam, so accelerated aging followed by LOI retesting is a common method for verifying long-term fire performance, particularly for products with design service lives of 10 to 25 years.
How should samples be submitted to an accredited third-party laboratory?
Submit flat, undeformed panels or finished parts large enough to yield at least 15 specimens of the required dimensions, representing current production formulation and curing conditions. Provide the material designation, nominal density, thickness, target standard, and any conditioning history so that the laboratory can prepare and condition specimens correctly.