Nitrile rubber (NBR) solvent resistance testing is a critical quality control procedure used to evaluate how nitrile butadiene rubber materials perform when exposed to fuels, oils, and chemical liquids. This technical article explains the testing standards, sample preparation, test items, immersion methods, and evaluation criteria for NBR solvent resistance in accordance with ASTM D471, ISO 1817, and GB/T 1690.
Technical Overview
Nitrile rubber, commonly designated as NBR or Buna-N, is a synthetic rubber copolymer produced from acrylonitrile (ACN) and butadiene. Its outstanding resistance to oils, fuels, and many solvents is derived from the strongly polar nitrile (–CN) groups incorporated in the polymer chain. These polar groups resist penetration by non-polar hydrocarbon media such as mineral oils, diesel fuel, gasoline, and hydraulic fluids, following the general principle that chemically dissimilar substances do not dissolve each other.
The acrylonitrile content of commercial NBR grades typically ranges from 18% to 50%. Higher ACN content improves fuel and oil resistance and increases hardness and tensile strength, but reduces low-temperature flexibility and elasticity. Lower ACN grades offer better cold flexibility but swell more in aggressive fuels. Solvent resistance testing is therefore essential for grade selection and for verifying that a cured NBR compound meets the swelling, mechanical, and aging requirements of a specific sealing or fluid-handling application.
While NBR performs well against non-polar and weakly polar media, it has poor resistance to strongly polar solvents such as ketones (acetone, MEK), esters, chlorinated hydrocarbons, and aromatic hydrocarbons at high concentrations. Quantitative immersion testing reveals these limitations and defines the safe operating envelope of the material.
Testing Samples
Standard Laboratory Specimens
For material qualification, standard test pieces are prepared from vulcanized sheets of 2 mm ± 0.2 mm thickness. Dumbbell-shaped specimens cut in accordance with ASTM D412 or ISO 37 are used for tensile property evaluation, while rectangular strips (commonly 25 mm × 50 mm) or small cut squares are used for mass and volume change determination. Specimens must be free of surface defects, bubbles, and mold imperfections that could distort fluid uptake.
Finished Product Samples
When testing actual components such as O-rings, oil seals, gaskets, fuel hose liners, diaphragms, or glove material, the parts may be immersed whole or cut into representative sections. Testing finished products is important because cure state, post-curing, surface treatment, and part geometry all influence real-world solvent performance.
Sample Conditioning
All specimens are conditioned before testing in a controlled atmosphere of 23 °C ± 2 °C and 50% ± 5% relative humidity for a minimum of 16 hours after vulcanization, in accordance with ISO 23529 or ASTM D1349. Initial mass, dimensions, hardness, and thickness are measured and recorded before immersion to serve as baseline values.
Testing Items
A complete solvent resistance evaluation of NBR includes the following measurable parameters:
- Volume change (swelling or shrinkage) after immersion, expressed as a percentage
- Mass (weight) change after immersion, reflecting absorbed liquid or extracted plasticizers and soluble constituents
- Change in tensile strength and retention of strength after fluid exposure
- Change in elongation at break after fluid exposure
- Change in hardness, typically measured on the Shore A durometer scale
- Dimensional changes in length, width, and thickness
- Compression set after fluid aging, relevant for sealing performance
- Visual surface assessment for cracking, blistering, tackiness, discoloration, or embrittlement
Testing Methods
Liquid Immersion Test Procedure
The core method is the immersion test described in ASTM D471, ISO 1817, and GB/T 1690. Specimens are fully submerged in the test liquid in closed containers, with the liquid volume maintained at no less than 15 times the specimen volume (typically 100 mL minimum per specimen group) to ensure the fluid composition is not depleted. The container is placed in a temperature-controlled oven or bath at standard test temperatures such as 23 °C, 40 °C, 70 °C, 100 °C, 125 °C, or 150 °C, depending on the service condition being simulated. Common immersion durations are 22 h, 70 h, 168 h, or multi-week extended exposure for service-life studies.
Standard Reference Liquids
To ensure comparability between laboratories, standardized reference fluids are used:
- ASTM Reference Fuel A — 100% isooctane, representing a low-swell aromatic-free fuel
- ASTM Reference Fuel B — 70% isooctane / 30% toluene blend, a moderate swellant
- ASTM Reference Fuel C — 50% isooctane / 50% toluene blend, an aggressive swellant widely used for fuel-system qualification
- ASTM Reference Fuel D and Fuel E/F series — blends containing ethanol and diisobutylene that simulate oxygenated and alcohol-containing fuels
- IRM 901, IRM 902, and IRM 903 reference oils — standard petroleum oils of increasing aniline point (decreasing swelling power)
- Service liquids — actual hydraulic oils, brake fluid, diesel, biodiesel blends, or process chemicals specified by the customer
Measurement and Calculation of Volume and Mass Change
Upon removal from the liquid, specimens are quickly blotted with lint-free cloth or filter paper to remove surface liquid, and weighed immediately. Volume is determined gravimetrically by hydrostatic weighing in water (with corrections for water absorption). The key calculations are:
- Mass change: Δm (%) = (m2 – m1) / m1 × 100, where m1 is the initial mass and m2 is the mass after immersion
- Volume change: ΔV (%) = (V2 – V1) / V1 × 100, where V1 and V2 are volumes before and after immersion
- Tensile retention: Change (%) = (value after immersion – original value) / original value × 100
Positive values indicate swelling from liquid absorption; negative values indicate shrinkage, usually caused by extraction of plasticizers, process oils, or other soluble compounding ingredients.
Post-Immersion Mechanical Testing
Tensile testing is performed on immersed dumbbell specimens in accordance with ASTM D412 or ISO 37, generally within 60 minutes of removal from the liquid to prevent evaporation losses that would understate the plasticizing effect. Hardness is measured per ASTM D2240 or ISO 7619-1 on flat stacked specimens after the same conditioning window. For compression set after fluid immersion, stacked disc specimens are compressed and aged in the test liquid following ASTM D395 or ISO 815 procedures.
Applications
Solvent resistance testing of NBR directly supports material qualification in numerous industries:
- Automotive fuel systems — fuel hoses, filler necks, injector seals, carburetor and fuel-pump components tested against Fuel C and oxygenated fuels
- Hydraulic and pneumatic sealing — O-rings, rod and piston seals, and wipers qualified against ASTM reference oils and hydraulic fluids
- Oil and gas industry — blowout preventer seals, valve seats, and packer elements exposed to drilling muds, diesel-based fluids, and crude oil fractions
- Chemical protective gloves — NBR glove material evaluated for degradation and permeation when contacting solvents and cleaning agents
- Aerospace and marine fuel handling — gaskets and diaphragms requiring validated low swell over long service intervals
- Printing and industrial rollers — NBR roll covers in contact with ink oils and aromatic wash solutions
Test data allow engineers to specify the correct ACN content, filler loading, cure system, and compound formulation so that seals and components maintain dimensional stability, mechanical strength, and leak-tight performance throughout their service life.
FAQ
What volume change is considered acceptable for NBR in fuel immersion?
Acceptance limits depend on the application specification, but a volume swell of no more than +25% to +30% after 70 h immersion in Reference Fuel C at room temperature is a common requirement for general fuel-contact seals. Critical sealing applications often demand tighter limits, and any shrinkage or significant loss of tensile strength is normally cause for rejection.
Why does NBR perform poorly against ketones and chlorinated solvents?
The polar nitrile groups in NBR are chemically similar in polarity to ketones, esters, and chlorinated hydrocarbons. Following the principle of like dissolves like, these strongly polar solvents penetrate and plasticize the rubber rapidly, causing excessive swelling and loss of mechanical properties. For such media, fluoroelastomers (FKM) or other specialty polymers are usually recommended.
How does acrylonitrile content affect test results?
Higher ACN content (for example, 40–50%) reduces swelling in fuels and oils, producing lower volume change values in immersion tests. However, it raises the glass transition temperature, degrading low-temperature flexibility. Standard 33–36% ACN medium-nitrile grades provide a balanced compromise and are the most widely tested compounds.
Which standards govern NBR solvent resistance testing?
The three principal standards are ASTM D471 (Rubber Property — Effect of Liquids), ISO 1817 (Vulcanized or thermoplastic rubber — Determination of the effect of liquids), and GB/T 1690, the Chinese national standard for rubber resistance to liquids. They define specimen preparation, immersion conditions, reference liquids, measurement procedures, and calculation methods, ensuring consistent and comparable results across laboratories.