Nitrile Butadiene Rubber (NBR) is a synthetic elastomer widely used in components that must withstand contact with petroleum-based fluids. This material, often referred to simply as Nitrile or Buna-N, has become the industry standard for seals, gaskets, and hoses in automotive and aerospace fuel systems. The short answer to whether nitrile rubber is fuel-resistant is yes, it exhibits good to excellent resistance to hydrocarbon fuels. NBR’s formulation balances mechanical properties and chemical stability, making it a reliable choice for preventing leaks in systems carrying traditional oils and fuels.
Understanding Nitrile Rubber Composition
Nitrile Butadiene Rubber is a copolymer created through the polymerization of two monomers: butadiene and acrylonitrile (ACN). This composition determines the material’s resistance profile, which trades off fuel stability against physical flexibility. The butadiene component provides the rubber-like elasticity and flexibility, while the acrylonitrile content dictates the material’s ability to resist non-polar solvents like petroleum fuels.
Manufacturers grade NBR compounds based on their ACN content, which typically ranges from 18% to over 50%. A higher percentage of acrylonitrile introduces more polar nitrile groups into the polymer chain. These semi-polar groups resist the absorption of non-polar hydrocarbon molecules found in fuels, thereby limiting the material’s volume swell and degradation.
A high-nitrile compound, containing over 45% ACN, offers superior fuel resistance but becomes stiffer and loses flexibility at low temperatures. Conversely, a low-nitrile compound, with less than 30% ACN, retains better low-temperature flexibility but experiences greater swelling when exposed to fuel. Most commercial applications use a medium-nitrile grade, balancing good fuel resistance with adequate low-temperature performance and elasticity.
Compatibility with Different Fuel Types
Nitrile rubber demonstrates excellent compatibility with common petroleum-based fuels and fluids, which are non-polar or low-polarity solvents. This includes standard gasoline, diesel fuel, kerosene, and lubricating oils. In these environments, the semi-polar NBR matrix effectively repels the fluid, leading to minimal volume change and maintaining the structural integrity of seals and hoses.
The resistance extends to aliphatic hydrocarbons, which are the main components of mineral oils and many hydraulic fluids. This makes NBR a default choice for applications like O-rings in oil filtration systems and fuel injector seals where continuous immersion is expected. The material’s ability to resist significant swelling (often less than 5% volume change in pure diesel) allows it to maintain its sealing force and prevents failure.
However, NBR’s performance degrades significantly when exposed to newer, more aggressive fuel formulations, especially those containing polar additives. Highly oxygenated fuels, such as gasoline blended with high percentages of ethanol (like E85, which contains up to 85% ethanol), pose a substantial challenge. Ethanol is a polar solvent that aggressively attacks the semi-polar nitrile polymer chain, causing severe swelling, loss of hardness, and a rapid decline in mechanical strength.
Similarly, other polar or semi-polar chemicals compromise NBR’s structure, including biodiesel (which contains fatty acid methyl esters), ketones, esters, and aromatic or chlorinated hydrocarbons. These substances act as plasticizers, breaking down the polymer network, which can lead to a volume swell exceeding 50% in extreme cases. This severe degradation means NBR is not recommended for systems handling pure biofuels or fuels with a high aromatic content.
External Conditions Affecting Performance
The performance of nitrile rubber in a fuel environment is not solely determined by the fuel’s chemical makeup; external conditions also play a role. Temperature is a primary factor, as elevated heat significantly accelerates the rate of chemical attack and fluid absorption. While NBR can typically operate up to 120°C, exposure to fuel at the upper end of its temperature range will result in faster degradation and greater volume swell compared to room-temperature exposure.
The manufacturing process, specifically vulcanization or curing, is also a determinant of the material’s final resistance. Vulcanization introduces cross-links within the polymer structure, transforming the rubber from a plastic-like material into a highly elastic one. A properly cured NBR compound, especially one using a sulfur-based system, develops a tighter cross-linked network that resists solvent penetration much more effectively.
This cross-linking explains why a finished NBR component can exhibit a swelling rate of less than 10% in a hydrocarbon solvent, while the un-vulcanized base polymer might swell by over 50%. The duration of exposure is also a factor, as long-term immersion allows for greater fluid absorption and chemical degradation over time. Furthermore, the material’s initial hardness, measured by durometer, affects its final performance, as harder compounds generally resist swelling more effectively due to their higher initial density.