Nitrile butadiene rubber (NBR) combustion performance testing evaluates the flammability, flame propagation, smoke generation, and heat release characteristics of nitrile rubber materials used in seals, hoses, cables, and industrial components. This technical article outlines the key testing standards, sample requirements, test methods, and application scenarios to help manufacturers select the appropriate flame retardancy evaluation approach for NBR products.
Technical Overview
Nitrile butadiene rubber (NBR), a copolymer of acrylonitrile and butadiene, is widely valued for its excellent oil resistance, fuel resistance, and mechanical durability. However, like most hydrocarbon-based elastomers, NBR is inherently flammable, with a typical limiting oxygen index (LOI) of approximately 17–19%, which is well below the 21% oxygen concentration of ambient air. The high butadiene content in the polymer backbone provides abundant combustible hydrocarbon segments, while the acrylonitrile content influences both thermal decomposition behavior and smoke emission characteristics. As a result, unmodified NBR ignites readily, burns rapidly, and drips molten burning particles, which limits its use in applications with fire safety requirements.
Combustion performance testing of NBR is therefore essential for material qualification, regulatory compliance, and product development. Through systematic evaluation using methods such as the limiting oxygen index test, horizontal and vertical burning tests, cone calorimetry, and smoke density measurement, manufacturers can characterize the fire hazard of NBR compounds, verify the effectiveness of flame-retardant formulations, and demonstrate conformity with industry standards in automotive, electrical, aerospace, and industrial sectors.
Testing Samples
Vulcanized NBR Test Plates
Standard compression-molded or injection-molded vulcanized sheets are the most common sample form. Plates are typically prepared at thicknesses of 1.0 mm, 2.0 mm, or 3.0 mm, depending on the selected test method. Proper vulcanization conditions must be documented, as under-cured or over-cured samples exhibit inconsistent burning behavior.
Flame-Retardant Modified NBR Compounds
Formulations containing halogenated flame retardants, phosphorus-based additives, inorganic fillers such as aluminum hydroxide or magnesium hydroxide, intumescent systems, or nano-fillers are tested to quantify improvements in LOI, ignition time, and heat release rate compared with the baseline compound.
NBR Foam and Sponge Materials
Cellular NBR products used for thermal insulation, such as pipe insulation tubes and sheets, require dedicated combustion evaluation due to their high surface area and low density, which significantly increase flammability relative to solid elastomers.
Finished NBR Products
Commercial items including O-rings, gaskets, fuel hoses, diaphragms, conveyor belts, and cable sheathing can be tested either by cutting specimens from the product or by testing representative plaque material, subject to the requirements of the applicable specification.
Testing Items
Combustion performance evaluation of NBR typically covers the following parameters:
- Limiting oxygen index (LOI) — the minimum oxygen concentration required to sustain candle-like burning
- Horizontal burning characteristics — burning rate, flame spread distance, and damaged length
- Vertical burning characteristics — after-flame time, after-glow time, ignition of cotton indicator, and dripping behavior
- Ignition time and critical heat flux for ignition
- Heat release rate (HRR), peak heat release rate (PHRR), and total heat release (THR)
- Smoke production rate and total smoke production
- Smoke density and light transmittance through smoke
- Mass loss rate during combustion
- Toxic and corrosive gas emissions, including HCN, CO, NOx, and HCl from certain formulations
- Effective heat of combustion and char residue formation
- Glow-wire ignitability and needle-flame behavior for electrical applications
Testing Methods
Limiting Oxygen Index (LOI) Testing
LOI testing is performed in accordance with ASTM D2863 or ISO 4589-2. A bar-shaped specimen is clamped vertically in a glass chimney and ignited at its top with a small flame in a controlled mixture of oxygen and nitrogen. The oxygen concentration is adjusted stepwise to determine the minimum level that supports burning over a specified distance or time. LOI provides a quantitative, reproducible measure of intrinsic flammability and is widely used for ranking NBR compounds. Materials with LOI above approximately 22% are generally considered somewhat flame-resistant, while flame-retardant NBR formulations can achieve LOI values of 27–35%.
Horizontal and Vertical Burning Tests
Horizontal burning tests per ASTM D635 or ISO 3795 (used for automotive interior materials) measure the burning rate of specimens oriented horizontally, reporting either the flame spread speed or extinguishment before a marked gauge length. Vertical burning tests per UL 94, ASTM D3801, or IEC 60695-11-10 classify materials into ratings such as V-0, V-1, V-2, or HB based on after-flame time, after-glow duration, and whether flaming drips ignite cotton placed beneath the specimen. Thin NBR sheets intended for electrical insulation are commonly evaluated under these procedures.
Cone Calorimetry
Cone calorimetry according to ISO 5660-1 or ASTM E1354 exposes a 100 mm × 100 mm specimen to a controlled radiant heat flux, typically 25–75 kW/m², and measures heat release through oxygen consumption calorimetry. Key outputs include time to ignition, peak heat release rate, total heat released, mass loss rate, and smoke production rate. This method provides the most comprehensive quantitative fire hazard data for NBR and is invaluable for research and development of flame-retardant formulations.
Smoke Density Testing
Smoke generation is assessed using the NBS smoke chamber method (ASTM E662 or ISO 5659-2). A specimen is exposed to a radiant heat source in a closed chamber, either with or without an ignition pilot flame, and specific optical density is calculated from the attenuation of a light beam. Because nitrile rubber produces considerable smoke containing hydrogen cyanide and carbon monoxide from acrylonitrile decomposition, smoke density testing is critical for enclosed-space applications.
Glow-Wire and Needle-Flame Tests
For NBR components used in electrical and electronic equipment, glow-wire tests per IEC 60695-2-10 through 60695-2-13 evaluate ignitability (GWIT) and flammability (GWFI) at elevated temperatures up to 960°C. The needle-flame test per IEC 60695-2-2 simulates small fault-source flames that may occur in energized equipment.
Thermogravimetric Analysis (TGA)
Although not a direct combustion test, TGA under nitrogen or air characterizes the thermal decomposition profile of NBR, including onset degradation temperature, maximum mass loss rate, and char yield. These data complement flammability tests by explaining the thermal stability mechanisms of the compound.
Applications
Combustion performance testing of NBR supports fire safety compliance and material selection across multiple industries:
- Automotive — fuel hoses, fuel system seals, engine bay gaskets, and vibration-damping components requiring conformity with FMVSS 302 or ISO 3795
- Wire and cable — NBR-based or NBR/PVC blend sheathing and insulation requiring UL 94 V-class ratings and glow-wire compliance
- Thermal insulation — NBR foam pipe and sheet insulation requiring fire classification for building services installations
- Oil and gas — downhole sealing elements, blowout preventer components, and refinery gaskets exposed to ignition sources
- Aerospace and defense — fuel tank sealing compounds and elastomeric components requiring low flammability and low smoke toxicity
- Mining — flame-resistant conveyor belts and hydraulic seals in underground environments governed by strict fire regulations
- Consumer and industrial goods — appliance gaskets, gloves, and mats where flame retardancy is a market or regulatory requirement
FAQ
Why is NBR considered a flammable rubber?
NBR contains a high proportion of butadiene units with unsaturated carbon–carbon double bonds and hydrocarbon segments that decompose into combustible volatile products at elevated temperatures. Its natural LOI of roughly 17–19% is below the atmospheric oxygen level, meaning it will continue burning in air after ignition without flame-retardant modification.
How can the flame retardancy of NBR be improved?
Common strategies include incorporating halogen-antimony synergistic systems, phosphorus-based or nitrogen-phosphorus intumescent flame retardants, inorganic hydroxide fillers such as aluminum trihydrate and magnesium hydroxide, and blending with PVC, which also enhances ozone resistance. Loading levels, dispersion quality, and curing system compatibility must be optimized to balance flame retardancy with mechanical properties.
What specimen dimensions are required for LOI testing of NBR?
For rigid or self-supporting specimens, ASTM D2863 and ISO 4589-2 typically specify bars of 80–150 mm in length, 10 mm in width, and 4 mm in thickness. Flexible NBR may require special support frames to keep the specimen vertical during testing.
Which test should be selected first when evaluating NBR combustion performance?
LOI testing is generally the recommended starting point because it is fast, reproducible, and requires only a small sample quantity. The results guide subsequent testing, such as UL 94 vertical burning for classification or cone calorimetry for detailed fire hazard assessment.
Do NBR combustion tests address smoke toxicity?
Standard flammability tests such as LOI and UL 94 do not directly measure toxicity. For applications in enclosed or life-safety environments, additional gas analysis during combustion or smoke toxicity testing should be specified, particularly because acrylonitrile decomposition can generate hydrogen cyanide.
How does acrylonitrile content affect the burning behavior of NBR?
Higher acrylonitrile content generally increases char formation tendencies and oil resistance but also increases nitrogen-containing gaseous decomposition products, which can raise smoke toxicity. Lower acrylonitrile grades contain more butadiene and tend to burn more readily with higher heat release.