The nitrile rubber (NBR) oil resistance swelling test is a critical quality control and material qualification procedure that measures how acrylonitrile-butadiene rubber responds to prolonged contact with oils, fuels, and hydraulic fluids. This article provides a comprehensive technical guide covering test standards such as ASTM D471, ISO 1817, and GB/T 1690, specimen preparation, test liquids, step-by-step methodology, and how to interpret volume swell, mass change, and mechanical property retention results.
Technical Overview
Nitrile rubber, commonly abbreviated as NBR or Buna-N, is a synthetic elastomer produced by the copolymerization of acrylonitrile (ACN) and butadiene. Its outstanding resistance to petroleum-based oils, fuels, and hydraulic fluids makes it one of the most widely used elastomers in sealing and fluid-handling applications. However, no rubber is completely immune to fluid contact, and the oil resistance swelling test is the standard method for quantifying how much fluid an NBR compound absorbs and how its properties change as a result.
Swelling occurs when fluid molecules penetrate the elastomer network and force the polymer chains apart. The degree of swelling is governed by the solubility parameter match between the rubber and the fluid, the acrylonitrile content of the polymer (typically ranging from 18% to 50%), the crosslink density of the cured compound, and the presence of extractable plasticizers or processing oils. A higher ACN content generally reduces swelling in non-polar petroleum fluids, which is why high-nitrile grades are specified for the most demanding fuel and oil exposure conditions.
The test is essential for material selection, compound development, batch release, and failure analysis. Excessive swelling causes seals to soften, lose dimensional stability, extrude under pressure, or lose mechanical strength, while excessive shrinkage (negative swell) indicates the extraction of plasticizers, which leads to hardening and cracking in service.
Testing Samples
Standard Laboratory Coupons
The preferred specimen is a flat rectangular coupon cut from a vulcanized sheet, typically 25 mm × 25 mm with a preferred thickness of 2 mm ± 0.2 mm. Straight-sided strips of approximately 25 mm in length cut from narrow slab stock are also acceptable. Specimens must be free of surface defects, voids, blisters, and released agents, as these flaws distort mass and volume measurements.
Finished Product Specimens
When testing actual components, whole parts such as O-rings, gaskets, diaphragms, or small valve seats may be immersed directly. For larger components, representative sections may be cut, provided that cut edges are smooth and the specimen mass is sufficient for accurate weighing. O-rings are typically tested intact, with the immersion fluid volume kept at at least 15 times the specimen volume and the fluid refreshed according to the governing standard.
Specimen Conditioning
Prior to testing, all specimens must be conditioned in a controlled atmosphere, normally 23 °C ± 2 °C and 50% ± 5% relative humidity, for a minimum of 16 hours in accordance with ISO 23529 or ASTM D618. Only specimens from the same compound and cure should be compared in a single test series.
Testing Items
A complete oil resistance evaluation typically includes the following measured parameters, each determined before and after fluid immersion:
- Volume change (percentage swell or shrinkage)
- Mass change (percentage mass gain or loss)
- Hardness change (Shore A, IRHD, or micro-hardness)
- Tensile strength change (retained percentage)
- Elongation at break change (retained percentage)
- One hundred percent modulus change where applicable
- Dimensional change (thickness, diameter, or linear swell)
- Visual assessment of surface condition, tackiness, cracking, or blistering
Volume change is the primary acceptance criterion in most specifications, while changes in hardness and tensile properties reveal whether the fluid has chemically degraded or merely physically plasticized the elastomer.
Testing Methods
Governing Standards
The three most widely applied standards for NBR oil immersion testing are ASTM D471 (Standard Test Method for Rubber Property: Effect of Liquids), ISO 1817 (Rubber, vulcanized or thermoplastic: Determination of effect of liquids), and GB/T 1690, the Chinese national standard for the determination of resistance of vulcanized rubber to liquids. These standards share the same fundamental approach and specify specimen geometry, immersion conditions, measurement procedures, and calculation methods.
Test Liquids
Standardized reference fluids are used so that results are reproducible between laboratories. The most common immersion media include:
- ASTM No. 1 oil (IRM 901): a low-swelling, high-aniline-point oil simulating light mineral oils
- ASTM No. 2 oil (IRM 902): a medium-aniline-point oil simulating general industrial lubricants
- ASTM No. 3 oil (IRM 903): a high-swelling, low-aniline-point oil that provides the most severe petroleum oil challenge
- ASTM Reference Fuels A, B, C, and D: defined isooctane, toluene, and diisobutylene blends simulating gasoline and oxygenated fuel exposure
- Actual service fluids: engine oils, hydraulic fluids, gear oils, or diesel fuel when simulating real operating conditions
ASTM No. 3 oil is the most frequently specified medium for NBR qualification because its strong swelling action clearly differentiates compound performance.
Test Conditions
Immersion tests are conducted at elevated temperatures to accelerate fluid uptake and replicate service conditions. Common test temperatures include 23 °C, 70 °C, 100 °C, 125 °C, and 150 °C, selected according to the intended application and the thermal limits of the fluid. Standard immersion durations are 22 hours, 70 hours, 168 hours (one week), and, for extended-life qualification, 1000 hours or longer. The fluid temperature must be maintained within ±2 °C of the specified set point throughout the exposure period.
Test Procedure
The general test sequence is as follows:
- Condition and mark each specimen, then record its initial hardness, dimensions, and tensile properties where required
- Weigh each specimen in air to obtain the initial mass m1, with a precision of 1 mg
- Determine the initial volume V1 by the hydrostatic (Archimedes) method, weighing the specimen in air and submerged in water, or by direct displacement measurement
- Immerse the specimens fully in the test fluid, ensuring at least 15 mL of fluid per gram of rubber and keeping specimens separated and away from the container walls
- Maintain the bath at the specified temperature for the specified duration, refreshing the fluid for long-duration tests as required by the standard
- Remove the specimens, quickly blot off surface fluid with filter paper without squeezing the rubber, and reweigh within 30 seconds for the volume determination and within 60 seconds for the mass measurement
- Re-measure hardness and tensile properties after a defined recovery period, typically 30 minutes to 60 minutes for hardness and after full conditioning for tensile testing, depending on the standard
Volume change is calculated as ΔV = (V2 − V1) / V1 × 100, and mass change is calculated as Δm = (m2 − m1) / m1 × 100, where subscript 1 denotes the initial value and subscript 2 the value after immersion. Positive values indicate swelling; negative values indicate extraction or shrinkage.
Interpretation of Results
For a well-formulated medium-ACN NBR compound, a typical volume change after 70 hours in ASTM No. 3 oil at 100 °C falls in the range of approximately +5% to +20%, while high-ACN compounds can achieve results below +10%. Many automotive and industrial specifications require volume swell within a defined band, commonly −5% to +25% or tighter, together with a minimum tensile strength retention of typically 60% to 80% of the original value. Excessive positive swell signals softening and possible seal extrusion; significant negative volume change indicates plasticizer extraction, which usually accompanies hardening, reduced elongation, and eventual cracking. A comparison of volume, hardness, and tensile data distinguishes reversible physical plasticization from irreversible chemical degradation such as additional crosslinking or chain scission.
Applications
Oil resistance swelling test data directly support the design and qualification of NBR components across many industries:
- Automotive fuel systems: fuel injector O-rings, fuel sender seals, filler neck gaskets, and fuel hose inner liners tested against reference fuels
- Engine and powertrain sealing: crankshaft seals, valve stem seals, oil pan gaskets, and transmission seals tested against engine and gear oils
- Hydraulic systems: piston and rod seals, wipers, and backup rings qualified in hydraulic oils and fire-resistant fluids
- Petrochemical and oilfield equipment: blowout preventer seals, pump valves, and downhole tooling elastomers
- Industrial machinery: diaphragms, expansion joints, rubber rollers, and chemical-resistant linings
- Consumer and safety products: oil-resistant gloves, protective apparel, and fuel container components
Manufacturers use the test results to rank compounds, set acceptance limits in material specifications, validate formulation changes, and investigate field failures by comparing the swelling behavior of returned parts against control samples.
FAQ
What volume swell value is acceptable for NBR?
Acceptance limits depend on the specification, but a common requirement for NBR in ASTM No. 3 oil at 100 °C for 70 hours is a volume change between −5% and +25%, with premium high-nitrile compounds achieving +10% or less. Tighter limits apply for critical fuel-handling seals, often specified against Reference Fuel C or D.
How does acrylonitrile content influence the test result?
Higher acrylonitrile content increases the polarity of the rubber, reducing its compatibility with non-polar mineral oils and fuels. As a result, 50% ACN grades swell markedly less than 18% to 22% ACN grades, though at some cost in low-temperature flexibility.
Why did the specimen shrink or lose volume during the test?
Negative volume or mass change indicates that soluble ingredients, most commonly plasticizers and processing oils, were extracted from the compound faster than the test fluid was absorbed. This typically produces a harder, less extensible rubber and suggests the need for more fluid-resistant plasticizers or a higher crosslink density.
Can finished parts be tested instead of lab coupons?
Yes. Standards such as ASTM D471 and ISO 1817 allow whole articles, including O-rings and small gaskets, to be immersed provided that the fluid volume, specimen mass, and measurement timing requirements are met. Testing actual parts is the most reliable way to confirm the performance of the finished, post-cured component.
Which test liquid best simulates engine oil exposure?
ASTM No. 3 oil (IRM 903) is the standard high-severity simulator for petroleum lubricants and is the default choice in most oil resistance specifications. When qualification against a specific product is required, the actual service engine oil should be used as an additional immersion medium.
How long does an NBR oil resistance test take?
Standard short-term tests at 70 hours or 168 hours are the most common, requiring a few days of laboratory time including conditioning and measurement. Extended qualification tests of 1000 hours or more are used for long-life sealing applications and can take six weeks or longer to complete.