Oxygen Index Testing of Rubber Compounds: Evaluating How Formulation Variables Influence Flame Retar

The limiting oxygen index (LOI) test is one of the most widely used small-scale methods for quantifying the flammability of rubber compounds and for measuring how individual formulation changes affect flame retardant performance. This technical article explains the test principle behind oxygen index testing in accordance with ISO 4589-2, ASTM D2863 and related standards, describes specimen requirements, test procedures and reporting, and reviews the key formulation variables that raise or lower the oxygen index of rubber materials.

Technical Overview

The limiting oxygen index (LOI) is defined as the minimum concentration of oxygen, expressed as a volume percent in a flowing mixture of oxygen and nitrogen, that will just support candle-like downward combustion of a vertically mounted specimen. Under ISO 4589-2 and ASTM D2863, a specimen is ignited at its top edge in a glass chimney; the oxygen concentration is progressively adjusted until the critical concentration is found at which the specimen either burns for three minutes or consumes a 50 mm length of material.

Because ambient air contains approximately 21 percent oxygen, a rubber compound with an LOI below 21 will generally continue to burn in air once ignited, while compounds with higher LOI values tend to self-extinguish. In practical material specifications, compounds with an LOI above 26 are commonly described as flame retardant, and values above 32 indicate strongly self-extinguishing behavior.

Oxygen index testing is particularly valuable for rubber development because the measured value is highly formulation dependent. Small changes in polymer grade, filler loading, plasticizer content, flame retardant package or cure system can shift the LOI by several percentage points, making the method an efficient screening tool for comparing compound variants before committing to larger-scale fire tests performed by accredited third-party laboratories.

• Governing standards include ISO 4589-2, ASTM D2863, GB/T 2406.2 and JIS K 7201-2.
• The test is performed under controlled laboratory conditions, typically at 23 °C, with a constant gas flow velocity of approximately 40 mm/s.
• LOI results are reproducible, quantitative and well suited to statistical comparison of formulation variants.
• The method applies to solid rubbers, thermoplastic elastomers, rubber-plastic blends and many flexible cellular materials.

Testing Samples

Molded Sheets and Cut Strip Specimens

The most common specimen form is a strip cut from a compression-molded or injection-molded plaque. ISO 4589-2 specifies strips of 80 to 150 mm in length, 10 mm in width and 4 mm in thickness, while ASTM D2863 typically uses strips of 70 to 150 mm in length, 6.5 mm in width and 3 mm in thickness. All faces must be clean, flat and free of flash, blisters or surface contamination, since surface defects strongly influence ignition behavior.

Extruded Rods and Tubes

Cylindrical rods and tubing sections, typically in the range of 3 to 12 mm in outside diameter, can be tested directly, which makes the method convenient for wire insulation and hose samples. The ends of tube specimens should be closed or mounted so that the internal bore does not act as a chimney during testing.

Finished and Semi-Finished Parts

Sections cut from production articles such as cable sheaths, conveyor belt covers, gaskets and molded seals may be tested provided a specimen of the required geometry can be obtained without delamination or heating that alters the compound. This is useful for verifying that factory processing has not degraded the flame retardant performance established during formulation development.

Formulation Comparison Coupons

For formulation studies, all candidate compounds should be molded under identical conditions of temperature, pressure and cure time into coupons of identical geometry, including identical thickness. Because LOI is thickness dependent, comparing specimens of different thickness will produce misleading conclusions about formulation effects.

Testing Items

A typical oxygen index test program for rubber formulation evaluation includes the following items.

• Limiting oxygen index value in percent oxygen, reported for each compound variant.
• Ignition behavior, including ease of ignition and whether the specimen burns with a steady flame, flashes or extinguishes.
• Afterflame and afterglow time after removal of the igniter flame.
• Observation of flaming drips or falling particles, which is critical for compounds containing significant plasticizer content.
• Mode of failure, distinguishing clean candle-like burning from catastrophic collapse, cracking or bubbling.
• Comparative LOI matrix across formulation variables such as flame retardant loading, filler type, plasticizer level and cure system.
• Optional verification after thermal aging or post-cure to confirm that flame retardant performance is retained in service.

Testing Methods

Apparatus and Test Conditions

The test employs a vertical glass chimney of approximately 70 to 100 mm diameter, a specimen holder that clamps the specimen at its lower portion, a regulated supply of oxygen and nitrogen with calibrated flow meters or mass flow controllers, and a small propane or butane flame igniter. Gas mixing and total flow are controlled so that the linear velocity through the chimney remains close to 40 mm/s throughout the test.

Specimen Preparation and Conditioning

Specimens are marked with reference lines 50 mm below the top edge to define the burning criterion. Immediately before testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5 percent relative humidity in accordance with ISO 291 or ASTM D618 for a minimum of 40 hours. For formulation studies, all variants should be conditioned identically, and at least five specimens per compound, and often ten or more during the oxygen concentration bracketing sequence, should be prepared.

Test Procedure

The specimen is clamped vertically in the chimney, the gas mixture is set to an estimated starting oxygen concentration, and the chimney is purged for at least 30 seconds to stabilize the atmosphere. The igniter flame is applied to the top of the specimen for up to five seconds, repeated if necessary, until ignition occurs. The observer records whether the specimen burns downward past the 50 mm mark or continues burning for three minutes. The oxygen concentration is then raised or lowered in progressively smaller steps, typically converging to 0.2 percent increments, until the critical concentration bracketing ignition and extinction is established. The LOI is calculated from the final series of oxygen concentrations using the bracketing procedure defined in ISO 4589-2 or ASTM D2863.

Reporting and Data Interpretation

The test report should state the standard used, specimen dimensions and thickness, conditioning history, ignition procedure, the calculated LOI value, and any observations such as dripping, afterglow or specimen shrinkage. Within a single laboratory, repeatability is typically on the order of plus or minus 0.5 to 1.0 percent oxygen, so formulation effects smaller than approximately one percentage point should be verified with additional specimens before being treated as significant.

Key Formulation Factors Influencing Oxygen Index

Base Polymer Selection

The polymer backbone establishes the starting point for flame retardancy. Natural rubber, SBR, BR and butyl rubbers typically exhibit LOI values of approximately 17 to 21, while nitrile rubber falls near 20 to 22. Polychloroprene, which contains chlorine atoms in the polymer chain, naturally reaches values of roughly 26 to 32, and fluorocarbon and platinum-cured silicone compounds can exceed 35 to 40. Selecting a more inherently flame resistant base polymer is often more efficient than overloading a hydrocarbon rubber with flame retardants.

Metal Hydroxide Fillers

Aluminum trihydrate (ATH) and magnesium hydroxide (MDH) improve the LOI through endothermic decomposition and the release of water vapor, simultaneously diluting combustible gases. ATH decomposes near 180 to 240 °C and requires high loadings, commonly above 100 phr in cable compounds, while MDH decomposes near 330 °C and suits higher processing temperatures. Both fillers also reduce smoke density, although high loadings reduce elongation and flexibility.

Halogen-Antimony Synergism

Halogenated flame retardants, including chlorinated paraffins and brominated additives, act in the gas phase by scavenging radical species, and their effect is strongly enhanced by antimony trioxide, which forms volatile antimony halides. This synergy can raise the LOI substantially at relatively low total loading, though regulatory programs such as RoHS and REACH increasingly drive compounders toward halogen-free alternatives.

Phosphorus and Intumescent Systems

Red phosphorus and ammonium polyphosphate promote char formation in the condensed phase and are effective in a number of rubber matrices, particularly in combination with metal hydroxides and nitrogen-based synergists that create an intumescent protective layer.

Plasticizers and Process Oils

Hydrocarbon process oils, ester plasticizers and aromatic oils are combustible fuel sources that reliably lower the LOI. Even moderate increases in oil loading can offset the benefit of a flame retardant package, so flame retardant compound design should minimize plasticizer content or select higher flash point, less volatile plasticizers.

Reinforcing Fillers and Cure System

Carbon black and silica have a modest positive influence on LOI by promoting char and reducing the combustible fraction, while finer particle fillers can improve char coherence. Peroxide cure systems and higher crosslink density can slightly improve LOI relative to conventional sulfur systems by delaying depolymerization and melt flow, and zinc oxide contributes to halogen-based flame retardant chemistry.

Applications

Oxygen index testing of rubber formulations supports a wide range of industrial applications where ignition resistance must be engineered into the compound.

• Wire and cable insulation and sheathing compounds, where LOI screening supports qualification under flame tests such as those in the IEC 60332 series.
• Mining and bulk handling conveyor belts, where flame resistant covers are required for underground safety compliance.
• Rail, marine and aerospace sealing systems, where compound variants are ranked by LOI during qualification programs.
• Automotive components such as hose covers, seals and underhood parts that require ignition resistance alongside fluid resistance.
• Building products, foam sealing tapes and expansion joint compounds where flame spread performance is specified.
• Quality control in compounding plants, where periodic LOI verification confirms batch-to-batch consistency of flame retardant additions.

FAQ

What LOI value is considered flame retardant for rubber?

Although classification limits vary by specification, compounds with an LOI of 26 or above are generally regarded as flame retardant, and values above 32 are considered strongly self-extinguishing. Many procurement specifications set minimum LOI targets between 28 and 35 depending on the application.

How many specimens are required for one LOI determination?

A minimum of five specimens is required for the final bracketing series, but a full determination from an unknown starting concentration often consumes ten to fifteen specimens. For formulation comparison, at least two full determinations per compound are recommended.

Do ISO 4589-2 and ASTM D2863 give the same results?

The two methods are technically similar, but differences in specimen dimensions, chimney design and procedural details can produce slightly different values. Comparisons between compounds should always be based on a single standard and identical specimen geometry.

Does specimen thickness affect the oxygen index?

Yes. LOI is geometry dependent, and most rubbers show a measurable thickness effect. Only results obtained on specimens of identical dimensions and conditioning history should be compared when evaluating formulation changes.

Can LOI results replace large-scale fire tests?

No. LOI is a small-scale screening and quality control tool that measures ease of ignition in a controlled atmosphere. It does not capture flame spread, heat release or smoke production, and it must be complemented by application-specific fire performance tests for final compliance.

Why does adding process oil reduce the LOI of a flame retardant compound?

Process oils and plasticizers are additional fuel that vaporizes and burns at relatively low temperatures, counteracting the gas-phase and condensed-phase action of flame retardants. In flame retardant design, plasticizer levels are minimized or replaced with less combustible alternatives.

Can halogen-free rubber reach high oxygen index values?

Yes. With high loadings of ATH or MDH, often combined with phosphorus or nitrogen synergists, halogen-free EPDM, EVM and silicone compounds can achieve LOI values well above 35, though the trade-off is higher hardness and lower elongation, which must be balanced in the overall formulation.