Nitrile butadiene rubber (NBR) permeability testing is a critical quality control and materials characterization procedure that measures how gases and vapors pass through this high-barrier elastomer. This technical article explains the testing principles, specimen requirements, applicable standards such as ISO 2782 and ASTM D1434, and the key industries that depend on accurate NBR permeability data.
Technical Overview
Nitrile butadiene rubber (NBR), also known as nitrile rubber or Buna-N, is a copolymer of acrylonitrile and butadiene widely valued for its excellent oil resistance and outstanding gas barrier properties. The strong polarity of the nitrile (CN) groups restricts chain segment mobility and reduces the free volume available for gas diffusion, giving NBR significantly lower gas permeability than non-polar rubbers such as natural rubber, SBR, or EPDM. Permeability determination is therefore essential for verifying whether a specific NBR compound meets the barrier requirements of fuel hoses, seals, diaphragms, and protective gloves.
Permeability testing quantifies the steady-state transmission of a gas or vapor through a rubber membrane. The mass transport process is governed by three related mechanisms: sorption of the gas into the high-pressure surface, diffusion through the polymer matrix, and desorption from the low-pressure surface. Laboratory testing measures parameters such as the gas transmission rate (GTR), the permeability coefficient (P), the diffusion coefficient (D), and the solubility coefficient (S), where P equals the product of D and S. Because permeability is highly temperature-dependent and follows an Arrhenius-type relationship, tests must be conducted under strictly controlled temperature conditions.
The acrylonitrile (ACN) content of the NBR grade, typically ranging from 18% to 50%, has a decisive influence on the measured permeability. Higher ACN content increases polarity and chain stiffness, which lowers gas permeability; medium- and high-nitrile grades are consequently preferred for demanding barrier applications such as fuel system components and refrigerant seals.
Testing Samples
Vulcanized Flat Sheets
The standard specimen form is a flat, vulcanized NBR sheet with a smooth, defect-free surface. Sheets are compression-molded or calendar-cured at the customer-specified cure conditions, then die-cut into circular discs suitable for mounting in the permeation cell. The typical specimen thickness is between 0.5 mm and 2.0 mm, and the thickness must be uniform, with variation not exceeding 5% across the test area. Pinholes, bubbles, sink marks, or contaminated surfaces will invalidate the results.
Film and Membrane Specimens
Thin NBR films, latex-dipped films, and solution-cast membranes used in gloves, balloons, and diaphragms can be tested directly. These specimens require careful handling to avoid stretching, folding, or micro-cracking, and the effective test area must be free of any support fabric pattern that could create a leak path. For thin films below 0.1 mm, a supporting grid or porous backing plate may be used in the test cell to prevent bulging under differential pressure.
Specimens from Finished Products
Cut sections from finished goods such as fuel hose liners, O-rings, gaskets, and roll covers can be evaluated to verify product-level barrier performance. Curved samples must be flattened or sectioned to produce a sufficiently flat test area. Testing finished-product specimens accounts for the effects of actual processing, post-curing, and in-service aging on the permeability of the compound.
Specimen Conditioning
All specimens shall be conditioned in accordance with ISO 23529 or ASTM D618 prior to testing, typically at 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 16 hours. Thickness is measured at a minimum of five points using a dial gauge or micrometer with a flat anvil, and the mean value is used in the calculation of the permeability coefficient.
Testing Items
The scope of NBR permeability determination typically covers the following measurable parameters and gas media:
- Gas transmission rate (GTR) for oxygen, nitrogen, carbon dioxide, air, hydrogen, and helium
- Gas permeability coefficient (P), expressed in cm³·cm·cm⁻²·s⁻¹·Pa⁻¹ or barrer units
- Diffusion coefficient (D) and solubility coefficient (S), derived from time-lag or desorption data
- Water vapor transmission rate (WVTR) for moisture barrier evaluation
- Fuel vapor and hydrocarbon permeability for fuel-contact components, tested with reference fuels such as Fuel C or toluene vapor
- Temperature-dependence profiles, including activation energy of permeation determined at multiple test temperatures
- Comparative barrier ranking of NBR compounds with different acrylonitrile contents or filler loadings
Testing Methods
Differential Pressure Manometric Method
The manometric method, specified in ISO 2782-1 and ASTM D1434 (constant-volume variable-pressure technique), is the most widely used approach for NBR. The specimen is mounted as a sealed barrier between two chambers of a permeation cell. The downstream chamber is evacuated while test gas is admitted to the upstream side at a defined pressure. As gas permeates through the rubber disc, the pressure in the calibrated downstream volume increases, and the rate of pressure rise is recorded by a high-sensitivity pressure transducer. At steady state, the gas transmission rate and permeability coefficient are calculated from the slope of the pressure-time curve, the specimen thickness, the test area, and the test temperature. The time lag at the beginning of the transient region can additionally be used to compute the diffusion coefficient.
Volumetric Method
The constant-pressure variable-volume technique measures the volume of gas passing through the specimen by means of a calibrated capillary or a soap-film burette on the downstream side, which is maintained at atmospheric pressure. ISO 2782-1 also covers this approach. The volumetric method is straightforward and robust, particularly suitable for relatively high transmission rates and for quality control screening of production compounds, although its sensitivity is generally lower than that of the manometric technique.
Coulometric Sensor Method
For oxygen permeability of thin NBR films, the coulometric method per ASTM D3985 or ISO 15105-2 offers very high sensitivity. Oxygen permeating the specimen is swept to the downstream sensor by a carrier gas (typically nitrogen) and quantified by a coulometric oxygen sensor. This equal-pressure method avoids mechanical stress on thin films and is ideal for glove films and barrier laminates. Water vapor transmission can be measured analogously with an infrared sensor according to ASTM F1249.
Test Conditions and Procedure
Standard test conditions are 23 °C with a relative humidity of 50% where applicable, or 23 °C in the dry state for the differential pressure methods. Elevated-temperature testing at 40 °C, 70 °C, or 100 °C is performed to simulate service conditions or to construct Arrhenius plots for lifetime prediction. The test gas should be dry and of at least 99.9% purity. Testing continues until the pressure or volume increase reaches a steady linear state for a defined period, and at least three replicate specimens are normally tested to assess variability. Helium leak checks or blank-cell runs are used to verify system integrity before each measurement series.
Applications
Accurate NBR permeability data support design and qualification decisions across many industries:
- Automotive fuel systems: fuel hoses, filler necks, injector seals, and fuel tank gaskets where low hydrocarbon and alcohol-blend vapor permeability is mandated by evaporative emission regulations
- Sealing technology: O-rings, valve seats, and hydraulic seals where gas tightness and resistance to gas explosion decompression depend on low gas solubility and diffusion
- Protective equipment: chemical-resistant and medical gloves requiring low vapor transmission of solvents, disinfectants, and cytotoxic drugs
- Pumps and instrumentation: diaphragms and bladders for gas meters, air pumps, and damping elements
- Refrigeration and air conditioning: seals exposed to refrigerants such as HFC and HFO blends, where refrigerant permeability determines long-term charge retention
- Material development: ranking of NBR blends, NBR/PVC alloys, and nanocomposites with layered fillers designed to further reduce gas permeation
FAQ
How does acrylonitrile content affect the permeability of NBR?
Increasing the acrylonitrile content from approximately 18% to 50% substantially reduces gas permeability because the polar nitrile groups increase cohesive energy and restrict segmental motion. High-nitrile NBR grades can achieve air permeability values several times lower than low-nitrile grades, making ACN content a primary selection criterion for barrier applications.
Which test gases are most commonly used for NBR permeability determination?
Air, oxygen, nitrogen, and carbon dioxide are the standard gases. Carbon dioxide permeates NBR faster than oxygen or nitrogen due to its higher solubility in the polymer, while helium is used for high-sensitivity leak and integrity studies. Fuel vapor testing with reference fuel C is common for automotive fuel-line compounds.
Why must permeability tests be temperature-controlled?
Gas permeability in rubber follows an Arrhenius relationship and can double or triple with a temperature rise of only 20 to 30 °C. Tight temperature control, typically within ±0.5 °C of the set point, is therefore required to obtain reproducible and comparable data, and multi-temperature testing is recommended when service conditions deviate from room temperature.
What specimen defects can distort the results?
Pinholes, trapped air bubbles, under-cure, surface contamination, thickness non-uniformity, and poor sealing at the cell gasket are the most common sources of error, usually causing falsely high transmission values. Careful visual inspection, thickness mapping, thickness measurement, and system leak verification are essential quality steps before testing.
How does NBR compare with other elastomers in gas barrier performance?
NBR offers significantly better gas barrier properties than natural rubber, SBR, and EPDM due to its polar nitrile groups. It is broadly comparable to chloroprene rubber, while butyl rubber remains the benchmark for air impermeability. NBR/PVC blends and nanoclay-filled NBR compounds can approach butyl-level barrier performance while retaining superior oil resistance.