Nitrile rubber (NBR) mechanical properties testing evaluates the tensile strength, elongation, hardness, tear resistance, and compression set of acrylonitrile-butadiene rubber compounds used in seals, gaskets, hoses, and protective gloves. Testing is performed in accordance with internationally recognized standards such as ASTM D412, ISO 37, ASTM D2240, and ISO 815 to verify material quality, support formulation development, and confirm compliance with industry specifications. This technical guide outlines specimen types, test items, test methods, typical results, and frequently asked questions related to NBR mechanical characterization.
Technical Overview
Nitrile rubber, also known as NBR or acrylonitrile-butadiene rubber, is a synthetic elastomer produced by the copolymerization of acrylonitrile and butadiene. The acrylonitrile content, which typically ranges from 18% to 50%, governs the balance between oil resistance and low-temperature flexibility, while the formulation of fillers, plasticizers, and curing agents determines the final mechanical performance of the vulcanizate.
Mechanical properties testing provides objective evidence that an NBR compound meets the requirements of its intended application. Because NBR is widely used in dynamic and static sealing systems, fuel handling components, and protective equipment, even small deviations in tensile strength, hardness, or compression set can lead to premature leakage, cracking, or product failure. Accredited laboratories therefore test raw compounds, cured sheets, and finished parts against standardized procedures to ensure batch-to-batch consistency.
Testing is generally conducted under controlled laboratory conditions of 23 ± 2 °C and 50 ± 5% relative humidity, following conditioning requirements defined in ISO 23529 or ASTM D618. Relevant test methods are published by ASTM International, the International Organization for Standardization (ISO), and national bodies such as GB/T in China, many of which are technically equivalent to the international standards.
Testing Samples
Standard Vulcanized Sheets
Most mechanical tests begin with compression- or injection-molded sheets of NBR compound, typically 2 mm thick, cured under production-equivalent time, temperature, and pressure conditions. Molding sheets under controlled conditions ensures that the measured properties represent the true state of cure of the compound rather than artifacts of finished part geometry.
Dumbbell Specimens
Tensile and tear tests use die-cut dumbbell specimens. The most common geometries are ISO Type 1 and Type 2 dumbbells and ASTM Die C specimens, cut with a sharp steel die to avoid edge defects that could cause premature failure. Miniature dumbbells may be used when material availability is limited, such as during compound development studies.
Ring Specimens
Ring specimens are used for tensile testing when the product geometry is cylindrical, for example O-rings and rotary shaft seals. Ring testing captures the effect of trimming and surface quality on failure initiation, which is often the critical factor in sealing applications.
Specimens from Finished Products
Finished parts such as O-rings, grommets, diaphragms, and glove cuffs can be tested directly to verify conformance of a production lot. Hardness, compression set, and aging tests are commonly performed on whole articles, while tensile strips may be cut from flat regions of larger components.
Conditioned and Aged Specimens
To assess durability, replicate specimens are exposed to hot air, standard reference oils, or ozone before mechanical testing. Comparing tensile strength, elongation at break, and hardness before and after exposure quantifies the degradation behavior of the compound under service-representative conditions.
Testing Items
The core mechanical properties measured for NBR compounds and products include:
- Tensile strength (MPa), the maximum engineering stress sustained before rupture
- Elongation at break (%), the strain achieved at the point of rupture
- Tensile stress at 100% and 300% elongation (modulus), indicating stiffness and cure state
- Hardness, expressed in Shore A units or International Rubber Hardness Degrees (IRHD)
- Tear strength (kN/m), measured on trouser, angle, or crescent specimens
- Compression set (%), the permanent deformation retained after prolonged compression
- Abrasion resistance, reported as relative volume loss in cubic millimeters
- Rebound resilience (%), a measure of elastic energy return
- Retention of tensile and hardness properties after heat, oil, or ozone aging
- Adhesion strength for bonded or fabric-reinforced NBR articles
Typical sulfur-cured NBR compounds exhibit tensile strength of 10 to 25 MPa, elongation at break of 300% to 600%, and hardness of 40 to 90 Shore A, although values vary substantially with acrylonitrile content, carbon black loading, and the curing system used.
Testing Methods
Tensile Properties — ASTM D412 / ISO 37
Tensile testing is performed on a universal testing machine fitted with pneumatic or mechanical grips and an extensometer. Standard dumbbell specimens are pulled at a crosshead speed of 500 mm/min until rupture while force and extension data are recorded continuously. Tensile strength, elongation at break, and stress at specified elongation are calculated from the load-extension curve and the original cross-sectional area.
Hardness — ASTM D2240 / ISO 48
Shore A hardness is measured with a spring-loaded durometer pressed against a flat specimen at least 6 mm thick, while IRHD is determined with a dead-load instrument that provides improved repeatability. Micro-IRHD testing is preferred for thin-walled O-rings and small seals, and a minimum of five readings are averaged for the reported result.
Tear Strength — ASTM D624 / ISO 34
Tear strength is determined by pulling notched trouser, angle, or crescent specimens to rupture. The maximum force divided by the specimen thickness yields tear strength in kN/m. Because tear results depend strongly on die geometry and notch quality, the specimen type must always be reported together with the data.
Compression Set — ASTM D395 / ISO 815-1
Compression set is measured using the constant-deflection method, in which standard pellets or O-rings are compressed by 25% between steel plates for a defined period, commonly 22 or 70 hours at 100 °C or 125 °C. After release and a 30-minute recovery period, the retained deformation is calculated as a percentage of the applied deflection. Low compression set values indicate good long-term sealing capability.
Heat and Fluid Aging — ASTM D573 / ISO 188 / ASTM D471
Accelerated aging exposes specimens to hot air in a circulating oven or to reference fluids such as ASTM No. 1 and IRM 903 oils at elevated temperature. Tensile strength, elongation, hardness, volume swell, and mass change are retested after exposure, and property retention percentages are compared against specification limits.
Abrasion and Resilience — ISO 4649 / ASTM D5963 / ISO 4662
Abrasion resistance is measured by moving a specimen across a rotating cylindrical drum covered with abrasive cloth under a defined load, with the relative volume loss reported. Rebound resilience is determined with a vertical rebound tester in which a pendulum or falling mass strikes the specimen and the rebound height is recorded.
Applications
- Automotive sealing systems: O-rings, valve stem seals, and fuel system gaskets rely on verified tensile strength and compression set data
- Oil and gas equipment: blowout preventer seals, packers, and hose liners require oil-aged property retention testing
- Industrial hoses and belting: hydraulic and fuel hoses depend on tensile, burst, and adhesion verification
- Medical and laboratory gloves: NBR examination gloves are tested for tensile strength and elongation before and after accelerated aging
- Aerospace and defense components: fuel bladders, diaphragms, and gaskets are qualified against AMS and MIL specification limits
- Consumer products: non-slip soles, grips, and grommets are evaluated for hardness and abrasion performance
FAQ
What is the most common tensile test standard for NBR?
ASTM D412 and ISO 37 are the two most widely used tensile standards for NBR. Both specify dumbbell or ring specimens, a test speed of 500 mm/min for standard geometries, and identical definitions of tensile strength and elongation at break, so results are generally comparable when specimen geometry is matched.
Why is compression set important for NBR seals?
Compression set quantifies the permanent deformation a seal retains after being squeezed between mating surfaces. A high compression set means the seal loses contact pressure over time, leading to leakage. Most sealing specifications limit compression set to approximately 15% to 25% after 70 hours at the service temperature.
How does acrylonitrile content affect mechanical test results?
Higher acrylonitrile content increases hardness, tensile strength, and oil resistance but reduces elongation and low-temperature flexibility. Mechanical test data must therefore always be interpreted together with the compound formulation and the intended service environment.
Do NBR parts need conditioning before testing?
Yes. Specimens should be conditioned for at least 16 hours at 23 ± 2 °C and 50 ± 5% relative humidity after vulcanization and before mechanical testing. A minimum rest period after cure is also required so that post-vulcanization effects do not bias the results.
Can finished NBR products be tested instead of molded sheets?
Finished products can be tested directly for hardness, compression set, and aging behavior. For tensile testing, results from product cuttings are acceptable when the geometry allows, but the sampling location and specimen type must be documented because properties can differ from those of molded sheets.