Determination of the Oxygen Index of Butyl Rubber: Standards, Methods, and Applications

The oxygen index test is one of the most widely used methods for evaluating the flammability characteristics of polymeric materials, including butyl rubber. This article provides a comprehensive technical guide to butyl rubber oxygen index determination, covering the underlying principles, applicable international standards, sample preparation requirements, step-by-step test procedures, and practical applications in quality control and flame-retardant compound development.

Technical Overview

Butyl rubber (IIR) is a synthetic elastomer produced by copolymerizing isobutylene with a small fraction (typically 1.5–4.5%) of isoprene. It is valued for its exceptionally low gas permeability, excellent vibration damping, strong resistance to heat aging, ozone, and many chemicals, and good weathering stability. These properties make it indispensable in tire inner liners and tubes, pharmaceutical closures, sealing systems, and vibration isolation components.

Like most hydrocarbon-based elastomers, butyl rubber is combustible. The oxygen index, also known as the limiting oxygen index (LOI), is a standardized laboratory measure of a material's relative flammability. It 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 candle-like combustion of a vertically mounted sample after ignition from the top. Because ambient air contains approximately 20.9% oxygen, materials with an LOI below 21% will generally continue to burn in air, whereas materials with an LOI above approximately 25–27% are considered difficult to ignite or self-extinguishing under ambient conditions.

Unfilled butyl rubber gum typically exhibits an LOI in the range of approximately 17–19%, which classifies it as a combustible material. Halogenated derivatives such as bromobutyl rubber (BIIR) and chlorobutyl rubber (CIIR) show moderately higher values, generally around 20–25%, due to the flame-inhibiting effect of halogen atoms released during decomposition. Compounding with flame retardants, halogen sources, antimony trioxide synergists, intumescent systems, or high loadings of inorganic fillers can raise the LOI of butyl rubber compounds substantially, sometimes exceeding 30% in engineered formulations.

Testing Samples

Raw Butyl Rubber Gum

Unvulcanized butyl rubber polymer can be evaluated, although results on raw gum are mainly used for polymer comparison and research. Since raw gum lacks dimensional stability at elevated temperature, it is common to press it into plaques of defined thickness before cutting test specimens.

Vulcanized (Cured) Butyl Rubber Compounds

The most frequently tested form is the fully cured compound, which reflects the realistic flammability performance of the finished material. Specimens are cut from vulcanized sheets or slabs prepared under standardized cure conditions so that crosslink density and filler dispersion are representative of production material.

Halogenated Butyl Rubber (BIIR and CIIR)

Brominated and chlorinated butyl rubbers are tested to quantify the flame-retardant contribution of halogenation. Comparative LOI data between IIR and its halogenated grades help formulators select base polymers for flame-resistant applications.

Fabricated Products

Finished articles such as inner tubes, tire inner liners, pharmaceutical stoppers, tank linings, gaskets, and cable components may be sampled directly, provided the material can be cut into specimens meeting the dimensional requirements of the test standard.

Testing Items

A typical butyl rubber oxygen index determination report includes the following measured and derived parameters:

• Limiting oxygen index (LOI), reported as volume percent of oxygen in the O2/N2 mixture
• Ignition behavior, including ease of ignition and the ignition time applied by the burner
• Afterflame time and afterglow time following removal of the ignition source
• Whether the specimen burns beyond the reference mark (typically 50 mm) within the specified 3-minute observation period
• Observable burning characteristics such as melting, dripping, sooting, smoke density, and char formation
• Specimen dimensions, orientation, and conditioning history, which must accompany the result
• For filled or anisotropic compounds, any difference between top-surface and edge ignition results

Testing Methods

Applicable Standards

The oxygen index test is governed by several internationally recognized standards, the most important being ASTM D2863 (Standard Test Method for Measuring the Minimum Oxygen Concentration to Support Candle-Like Combustion of Plastics), ISO 4589-2 (Plastics – Determination of burning behaviour by oxygen index), and the Chinese national standard GB/T 2406.2, which is technically aligned with ISO 4589-2. These standards describe essentially the same apparatus and general procedure, differing mainly in certain dimensional tolerances and verification requirements. Laboratories should select the standard specified by the customer, the product specification, or the relevant regulatory framework.

Apparatus

The test is performed in an oxygen index chamber consisting of a heat-resistant glass chimney (typically 70–100 mm in diameter and 450–600 mm high), a specimen holder that supports the sample vertically at the base of the chimney, a mixing device that delivers a precisely controlled oxygen/nitrogen flow upward through the chimney at a defined total flow rate, calibrated oxygen and nitrogen flowmeters or mass flow controllers, an oxygen analyzer for verifying mixture concentration, and a small propane or butane burner with a defined flame for igniting the specimen tip. Scales, timers, and marked reference points on the specimen complete the setup.

Specimen Preparation

Specimens are cut to the dimensions prescribed by the standard, commonly 80–150 mm in length, 10 mm in width, and 4–10 mm in thickness for rigid and semi-rigid materials. For butyl rubber, thickness at the upper end of the range is generally preferred because thin, flexible elastomer specimens may distort or curl during burning. Edges must be smooth and free of cracks. Prior to testing, specimens are conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum period (typically 40–96 hours depending on the standard) to ensure reproducible moisture content.

Test Procedure

• Mount the conditioned specimen vertically in the holder so that the top of the specimen is at least 100 mm below the top of the chimney.
• Purge the chimney with the oxygen/nitrogen mixture at the standard total flow rate for at least 30 seconds to establish a stable atmosphere.
• Apply the ignition flame to the top of the specimen for the prescribed time (typically up to 30 seconds, with brief removal intervals) until the specimen is ignited and burns with a sustained flame.
• Observe the burning behavior. The trial is recorded as "burning" if the flame front consumes 50 mm of the specimen or persists for 3 minutes; otherwise it is recorded as "not burning."
• Adjust the oxygen concentration upward after a "not burning" result and downward after a "burning" result, typically in increments of 0.5–1.0%, and repeat trials on fresh specimens.
• Apply the bracketing or incremental procedure defined in the standard to converge on the critical oxygen concentration, using at least three specimens at the final concentration to confirm reproducibility.
• Calculate the LOI as 100 × (O2 flow rate) / (O2 flow rate + N2 flow rate), expressed to one decimal place, and report the result together with the standard used, specimen dimensions, and conditioning details.

Factors Affecting Accuracy

Several variables influence the reliability of butyl rubber LOI results. Elastomers exhibit time-dependent flow and melting during the test, so specimen thickness and holder design significantly affect whether flaming drips carry combustion downward. Gas calibration accuracy, chimney flow uniformity, ambient drafts, and burner technique are common sources of variability between laboratories. Because butyl rubber burns with considerable smoke and soot, chimney cleanliness and consistent operator ignition practice are essential. Interlaboratory reproducibility is generally on the order of ±1–1.5 percentage points of oxygen, so small differences between formulations should be interpreted with statistical caution.

Applications

Oxygen index data for butyl rubber support a wide range of engineering, quality control, and regulatory activities:

• Flame-retardant compound development: screening halogen/antimony synergist systems, intumescent packages, metal hydroxide fillers, and char-forming additives to raise the LOI of butyl formulations.
• Tire and tube industry qualification: evaluating inner liners, inner tubes, and curing bladders, particularly for applications with flammability requirements.
• Pharmaceutical and food-contact elastomers: supporting material selection and change control for closures and gaskets where combustion behavior is documented.
• Sealing and construction applications: verifying the flammability classification of gaskets, membranes, pipe seals, and roof lining materials.
• Wire and cable insulation: assessing butyl-based or blended insulating and sheathing compounds against flame performance requirements.
• Comparative polymer studies: benchmarking IIR against BIIR, CIIR, EPDM, NR, and SBR to guide base-polymer selection.
• Compliance documentation: providing LOI values required by specifications, certification bodies, safety data sheets, and transportation regulations.

FAQ

What is a typical oxygen index value for butyl rubber? Unfilled butyl rubber gum generally has an LOI of approximately 17–19%. Halobutyl rubbers usually fall between 20% and 25%, while flame-retardant compounded butyl rubber can achieve values above 30% depending on the additive system.

Why is butyl rubber considered flammable despite its chemical resistance? Butyl rubber is a hydrocarbon polymer composed primarily of carbon and hydrogen. Upon pyrolysis it releases combustible volatile hydrocarbons that sustain flaming combustion in air (20.9% oxygen), so its intrinsic LOI is below the ambient oxygen level.

What is the pass criterion for the LOI test? The LOI test itself does not define pass or fail; it produces a numerical index. Pass/fail thresholds are set by product specifications or regulations. As a general engineering guideline, an LOI above 27% indicates a material that will not easily sustain combustion in normal air.

How can the LOI of butyl rubber be improved? Effective strategies include incorporating halogenated flame retardants with antimony trioxide synergists, aluminum hydroxide or magnesium hydroxide fillers at high loading, intumescent flame-retardant systems, phosphorus-based additives, and char-promoting reinforcing fillers, or selecting bromobutyl rubber as the base polymer.

Which standard should be used for testing? ASTM D2863, ISO 4589-2, and GB/T 2406.2 are the most widely applied standards and are technically similar. The choice should follow the requirement of the customer specification, end-use regulation, or contractual agreement.

Does sample thickness affect the LOI result? Yes. Thicker specimens tend to show slightly different, often higher, LOI values because of heat sink effects and reduced relative influence of drips and distortion. The standard and the reported specimen dimensions must always accompany the LOI value for valid comparison.