Nitrile Rubber (NBR) Combustion Performance Testing: Standards, Methods, and Technical Guide

Nitrile rubber (NBR) combustion performance testing evaluates the ignitability, flame spread, heat release, smoke production, and toxic gas emission characteristics of nitrile butadiene rubber materials used in automotive, rail, oil and gas, and electrical applications. This technical guide explains the standard test methods—including oxygen index (LOI), UL 94 vertical/horizontal burning, smoke density, and cone calorimetry—along with sample requirements, acceptance criteria, and regulatory frameworks that govern flammability compliance for NBR compounds and finished products.

Technical Overview

Nitrile rubber (NBR, acrylonitrile-butadiene rubber) is a synthetic elastomer produced by the copolymerization of acrylonitrile (ACN, typically 18–50%) and butadiene. It is widely valued for its excellent resistance to oils, fuels, and hydraulic fluids, which makes it the material of choice for seals, gaskets, hoses, and diaphragms across many industries. However, like most hydrocarbon-based elastomers, standard NBR is inherently flammable.

The limiting oxygen index (LOI) of unfilled or conventionally filled NBR typically falls in the range of 17–20%, which is below the ambient atmospheric oxygen concentration of approximately 21%. This means NBR will continue to burn in air after ignition, producing a self-sustaining, sooty flame accompanied by dense black smoke. During combustion, the acrylonitrile segments of the polymer chain can release hazardous gases including hydrogen cyanide (HCN), carbon monoxide (CO), and nitrogen oxides (NOx), making smoke toxicity evaluation a critical part of any comprehensive fire hazard assessment.

Combustion performance testing of NBR therefore serves several purposes: characterizing the baseline flammability of the compound, verifying the effectiveness of flame-retardant formulations (e.g., systems incorporating aluminum trihydrate ATH, magnesium hydroxide MDH, intumescent agents, or halogen-antimony synergists), and demonstrating compliance with the fire safety regulations of end-use industries such as automotive interior materials, rail transit vehicles, building products, and electrical equipment.

Testing Samples

Combustion tests require properly prepared and conditioned specimens. Because unvulcanized rubber cannot hold a stable geometry, virtually all flammability testing is performed on fully vulcanized (cured) NBR materials. Specimens are typically die-cut from molded plaques or taken directly from finished products, then conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for at least 40 hours (per ISO 291 or ASTM D618) prior to testing.

Vulcanized Standard Plaques and Sheets

Compression-molded or injection-molded NBR sheets, usually 1–13 mm thick depending on the test standard, are the most common sample form. These allow consistent comparison between formulations and are used for oxygen index, UL 94, and cone calorimeter testing. Test specimens are cut to the precise dimensions required by each method, with edges trimmed square and free of flash or cracks.

Finished Rubber Products

Seals, O-rings, gaskets, fuel filler hoses, diaphragms, protective sleeves, and cable sheathing can be tested in their end-use configuration or as cut sections. For automotive interior parts, FMVSS 302 / ISO 3795 specimens of 356 mm × 102 mm are cut from the actual component wherever possible, since surface texture, curvature, laminations, and attachments can influence burn behavior.

Cellular and Sponge NBR Materials

Expanded (foam) NBR used for thermal insulation and acoustic damping is tested as finished sheet or tube stock. Because closed-cell foam burns differently from dense rubber—often with faster flame spread and dripping—dedicated classifications under EN 13501-1 or EN 45545-2 may apply, and specimen orientation (horizontal versus vertical) must follow the product installation condition.

Testing Items

A complete combustion performance evaluation of NBR typically includes the following measurable parameters:

• Limiting oxygen index (LOI) – the minimum volumetric oxygen concentration required to support candle-like burning
• Horizontal burning rate (mm/min) – flame propagation speed along the specimen length
• Vertical burning classification – afterflame time, afterglow time, and ignition of cotton indicator by flaming drips
• UL 94 rating (HB, V-2, V-1, V-0, or 5VA/5VB)
• Ignition time and piloted ignition temperature under radiant heating
• Peak heat release rate (pkHRR) and total heat release (THR)
• Mass loss rate and residue (char) yield
• Specific optical smoke density (Ds, Ds max, VOF4)
• Smoke toxicity – concentrations of HCN, CO, CO₂, NOx, and hydrocarbons in the effluent
• Flame spread index and surface flammability characteristics
• Glow wire flammability index (GWFI) and glow wire ignition temperature (GWIT)
• Needle-flame test resistance for small electrical components

Testing Methods

Testing is conducted according to internationally recognized standards. The selection of methods depends on the regulatory framework of the target application.

Limiting Oxygen Index (ASTM D2863 / ISO 4589-2)

A bar specimen (typically 80–150 mm long × 10 mm wide × 4 mm thick) is mounted vertically in a glass chimney and ignited at its top edge with a small diffusion flame. The oxygen concentration in the oxygen–nitrogen mixture is adjusted stepwise until the critical concentration is found at which the specimen either burns for 3 minutes or consumes 50 mm of its length. Flame-retardant NBR compounds for rail applications are commonly required to reach an LOI of 28–32% or higher.

Horizontal and Vertical Burning (UL 94 / IEC 60695-11-10 and -11-20)

In the UL 94 vertical test, a 125 mm × 13 mm specimen clamped vertically is exposed twice to a 20 mm (50 W) premixed flame for 10 seconds each. Classification as V-0, V-1, or V-2 depends on afterflame time, total afterflame across both ignitions, afterglow, and whether flaming drips ignite surgical cotton placed below. The horizontal (HB) test records the linear burning rate. Because unfilled NBR generally does not self-extinguish, achieving V-0 requires substantial flame-retardant loading.

Automotive Interior Flammability (FMVSS 302 / ISO 3795)

A 356 mm × 102 mm specimen is held horizontally and exposed to a Bunsen burner flame at one end for 15 seconds. The burn rate is measured between gauge marks 254 mm apart. For most vehicle categories the maximum permitted horizontal burn rate is 102 mm/min, or the material must self-extinguish before the first mark. NBR-based components such as fuel hose covers, grommets, and sound-damping layers must pass this requirement for vehicle homologation.

Smoke Density (ISO 5659-2 / ASTM E662)

A 75 mm × 75 mm specimen is exposed in a closed chamber to radiant heat (25 kW/m², with or without piloted flame). A vertical light beam measures transmittance, from which the specific optical density is calculated. Key outputs include Ds max within 10 minutes and the cumulative VOF4 value, both heavily weighted in railway standard EN 45545-2, where NBR is penalized by its high soot yield.

Cone Calorimetry (ISO 5660-1 / ASTM E1354)

A 100 mm × 100 mm specimen is irradiated at 25–50 kW/m² in an open combustion circuit. Oxygen consumption calorimetry yields the heat release rate curve, peak HRR, total heat released, effective heat of combustion, ignition time, smoke production rate (SPR), and CO/CO₂ yields. This method is central to performance-based fire engineering assessment of NBR products.

Smoke Toxicity and Gas Analysis

Effluent gases from furnace or smoke chamber tests (e.g., EN ISO 5659-2 with FTIR analysis, or the ISO 19700 tube furnace) are quantified for HCN, CO, NO₂, and HBr/HCl where halogenated flame retardants are present. The acrylonitrile content of NBR makes HCN evolution a distinguishing hazard relative to natural rubber or SBR, and the conventional toxic gas index CITG under EN 45545-2 must be verified for rail vehicle components.

Electrical and Component-Level Tests

NBR cable jackets and sealing elements in electrical equipment are evaluated by glow wire tests (IEC 60695-2-10/-11/-12/-13), the needle flame test (IEC 60695-11-5), and cable flame propagation tests (IEC 60332-1, -2, and -3 series). Aerospace interior NBR parts may additionally require FAR 25.853 vertical, 60-degree, and 12-second burn tests.

Applications

Combustion performance testing of NBR supports safety compliance and material qualification in the following sectors:

• Automotive – fuel and vapor hoses, filler neck seals, grommets, and interior acoustic components verified under FMVSS 302 / ISO 3795 and OEM specifications
• Rail transit – door and window sealing profiles, floor underlays, and HVAC components classified per EN 45545-2 (LOI, smoke density, and toxicity requirement sets R1–R28)
• Oil and gas – flame-retardant NBR seals, packer elements, and blowout preventer components for offshore installations subjected to ISO 13763 / NORSOK-related fire safety programs
• Mining – fire-resistant NBR/PVC blend conveyor belt cover compounds tested per EN 12882 and related flame propagation standards
• Electrical and electronics – cable sheathing, gaskets, and strain reliefs qualified through UL 94, glow wire, and IEC 60332 series testing
• Building and HVAC – NBR foam thermal insulation tubes and sheets assessed under EN 13501-1 (Euroclass) and, in some markets, ASTM E84 surface burning characteristics
• Aerospace – interior seals and cushioning elements meeting FAR 25.853 / ABD 0031 flammability, smoke density, and toxicity criteria

FAQ

What is the typical limiting oxygen index of nitrile rubber?

Standard NBR compounds generally exhibit an LOI of 17–20%, meaning they burn readily in normal air. Through flame-retardant modification, LOI values of 28–35% are achievable, which satisfies the requirements of most rail and industrial specifications.

Can NBR achieve a UL 94 V-0 rating without flame retardants?

No. Unmodified NBR continues to burn after removal of the ignition source and cannot meet the afterflame time and non-dripping criteria of V-0. V-0 performance requires formulated flame-retardant packages, typically ATH or magnesium hydroxide at high loadings, often combined with synergists.

Which hazardous gases are produced when NBR burns?

Combustion products include carbon monoxide, carbon dioxide, soot, unburned hydrocarbons, nitrogen oxides, and—specific to the acrylonitrile component—hydrogen cyanide. For this reason, smoke toxicity testing is strongly recommended whenever NBR is used in enclosed or occupied spaces.

Does acrylonitrile content influence combustion behavior?

Yes. Higher ACN content marginally increases LOI and char formation while reducing heat release, but it also increases the potential yield of HCN in the smoke. Formulators must balance oil resistance (higher ACN) against fire hazard profile (lower ACN or effective FR additives).

How should specimens be prepared and how many are required?

Specimens must be fully vulcanized, die-cut to standard dimensions (for example 125 mm × 13 mm for UL 94, 356 mm × 102 mm for FMVSS 302, 100 mm × 100 mm for cone calorimetry), and conditioned at 23 °C / 50% RH for a minimum of 40 hours. Most standards require 5–10 replicate specimens per condition, plus spares, since LOI and UL 94 testing involve iterative or double-ignition procedures.

What is the difference between horizontal and vertical burning tests?

Horizontal tests (HB, FMVSS 302) measure the linear burn rate of a specimen ignited at one end while lying flat; they represent milder exposure and are used for automotive interiors. Vertical tests (UL 94 V ratings) ignite the bottom of an upright specimen and assess afterflame time, afterglow, and flaming drips under more severe conditions, providing a more demanding classification for electrical and electronic applications.