Hardness is one of the most critical quality parameters for nitrile butadiene rubber (NBR), directly influencing sealing performance, compression behavior, and part durability. This technical article explains the standard NBR hardness testing methods, including Shore A durometer and IRHD procedures, sample preparation requirements, applicable standards, and practical guidance for accurate measurement. It serves as a practical reference for rubber manufacturers, quality engineers, and testing laboratories.
Technical Overview
Nitrile butadiene rubber (NBR), commonly known as nitrile rubber or Buna-N, is a copolymer of acrylonitrile and butadiene valued for its excellent resistance to oils, fuels, and hydraulic fluids. In practical compounding, NBR materials are formulated across a hardness range of approximately 40 to 90 Shore A, achieved by adjusting carbon black loading, plasticizer content, and crosslink density.
Hardness is defined as the resistance of a material to indentation under specified conditions. For NBR, it is a primary specification parameter in nearly every purchase agreement, material data sheet, and quality control plan because it correlates strongly with mechanical stiffness, sealing force, compression set behavior, and overall part performance. Hardness measurement is also a sensitive indicator of cure state, degree of over-cure, and post-aging degradation.
The internationally recognized methods for determining the hardness of NBR vulcanizates are based on the following standards:
- ASTM D2240 — Standard Test Method for Rubber Property — Durometer Hardness
- ISO 7619-1 — Rubber, vulcanized or thermoplastic — Determination of indentation hardness — Part 1: Durometer method (Shore hardness)
- ISO 48 — Rubber, vulcanized or thermoplastic — Determination of hardness (hardness in IRHD, international rubber hardness degrees)
- ASTM D1415 — Standard Test Method for Rubber Property — International Hardness
- ASTM D1349 / ISO 23529 — Standard practices for conditioning and basic test parameters for rubber test pieces
These methods fall into two families: the portable durometer (Shore) method and the dead-load IRHD method. Selecting the correct method depends on specimen geometry, hardness range, and the required agreement between laboratories.
Testing Samples
Standard Laboratory Test Slabs
The preferred specimen for precision hardness measurement is a flat molded slab with smooth, parallel surfaces. For Shore A testing, the minimum thickness is 6 mm; for the standard IRHD method, a thickness of 8 mm to 10 mm is required. Where thinner sheets must be used, layers may be stacked for Shore testing, up to a maximum of three plies, provided the plies are flat, free of gaps, and cannot slip against one another. Stacking is not permitted for IRHD testing.
The test area must be large enough that no indentation point is closer than 9 mm (IRHD) or 12 mm (Shore A where practical, with a minimum of 6 mm from any edge) to the specimen edge, and consecutive indentations must be separated to avoid mutual influence.
Finished Products and Small Parts
NBR products such as O-rings, small seals, gaskets, and thin diaphragms frequently cannot meet the thickness requirements of standard methods. For these parts, micro-hardness testing is applied:
- Micro IRHD method (ISO 48, micro procedure) using a 0.395 mm ball indenter for specimens 1.5 mm to 2.5 mm thick, including O-ring cord sections
- Shore AM (micro durometer) for small, curved, or thin specimens
- Where the product radius or thickness is below the limits of micro methods, hardness may be reported only as an indicative reference value
Sample Preparation and Conditioning
Correct preparation and conditioning are essential for reproducible results:
- Vulcanization must be complete; a minimum interval of 16 hours after vulcanization, and typically not more than 4 weeks, should elapse before testing
- Condition specimens at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 3 hours (large specimens require proportionally longer conditioning)
- Specimens must be free of surface defects, bloom, release agents, fabric impressions, and machining marks at the test points
- Report the specimen type, thickness, method of preparation (molded, cut, stacked), and any deviation from standard geometry
Testing Items
A typical NBR hardness evaluation program includes the following test items:
- Shore A hardness — instantaneous (1 s) reading and/or timed reading after 3 s or 15 s dwell, as specified
- Hardness in International Rubber Hardness Degrees (IRHD) — standard (dead-load) or micro procedure
- Hardness change after heat aging (Δhardness) — determined per ASTM D573 or ISO 188 followed by re-measurement, typically within ±10 IRHD/Shore A as a common specification limit
- Hardness change after fluid immersion — relevant for NBR used in fuel and oil service, tested per ASTM D471 followed by hardness re-measurement
- Durometer calibration verification — using reference blocks traceable to national standards
For NBR compounds, the median of at least five readings is reported for Shore hardness; for IRHD testing, three to five measurements are taken depending on the procedure, and the median value is reported.
Testing Methods
Shore A Durometer Method (ASTM D2240 / ISO 7619-1)
The Shore A durometer uses a spring-loaded indenter with a truncated 35° cone terminating in a 0.79 mm to 0.84 mm flat tip. The instrument is pressed firmly against the specimen until the presser foot contacts the surface, and hardness is read from the scale (0 = no resistance, 100 = no measurable penetration).
Key procedural requirements:
- Apply the durometer vertically with sufficient, smooth pressure; avoid shock loading
- Take readings at points at least 6 mm apart and at least 6 mm from the specimen edge
- Support the specimen on a rigid, flat surface; specimen deflection must be negligible
- Take a minimum of five readings at different points and report the median
- Record the reading time (instantaneous, 3 s, or 15 s) since NBR exhibits stress relaxation that can lower later readings by several points
- The Shore A scale is most accurate between 20 and 90; for harder NBR compounds above 90 Shore A, a Shore D durometer is recommended
IRHD Dead-Load Method (ISO 48 / ASTM D1415)
The IRHD method measures the difference in penetration depth of a 2.5 mm diameter spherical indenter between a small preliminary force (0.30 N) and a total force (5.70 N) applied for 30 seconds. The penetration difference is converted to International Rubber Hardness Degrees through a standard scale, giving results numerically similar to Shore A in the mid-range.
Procedure outline:
- Place the conditioned specimen on a rigid horizontal anvil
- Apply the preliminary force, then the additional main force, ensuring no shock or vibration
- Maintain the total force for 30 seconds and record the indentation depth
- Take three to five measurements at points at least 6 mm (ISO 48 requires 10 mm for standard specimens) apart and report the median
- Apply foot corrections for thin specimens where thickness is below 4 mm, or preferably test to the micro procedure
The IRHD method provides superior repeatability and reproducibility compared with the durometer method because the loads are dead weights rather than spring forces, making it the preferred referee method for interlaboratory comparisons of NBR materials.
Micro-Hardness Testing (ISO 48 Micro Procedure / Shore AM)
The micro IRHD procedure scales down the standard method using a 0.395 mm diameter ball, a preliminary force of 0.0083 N, and a total force of 0.1532 N. It is designed for specimens 1.5 mm to 2.5 mm thick and is the standard method for NBR O-rings and precision seals. Measurements on O-rings are taken along the cord cross-section, with the specimen supported in a suitable V-groove fixture to prevent distortion.
General Test Conditions and Reporting
Regardless of the method selected, the test report should include:
- Identification of the NBR material and specimen type (standard slab or product)
- Test method and standard reference (Shore A, Shore AM, IRHD, or micro IRHD)
- Specimen thickness and number of layers
- Conditioning atmosphere, test temperature, and reading time
- Median hardness value with individual readings and the direction of any deviation from standard conditions
Applications
Hardness testing of NBR materials underpins quality control and engineering decisions across many industries:
- Sealing products: O-rings, hydraulic seals, rotary shaft seals, and gaskets are almost universally specified at 70 or 90 Shore A (equivalent IRHD values), and hardness verification confirms compound correctness before assembly
- Automotive and fuel systems: hoses, fuel line components, and diaphragms rely on hardness checks to validate oil and fuel resistance after fluid immersion aging
- Industrial gloves: NBR examination and chemical gloves are hardness-tested to balance tactile sensitivity with puncture and chemical resistance
- Rollers and belting: NBR-covered rollers for printing and steel processing are measured to verify grinding quality and compound uniformity
- Incoming inspection and material qualification: hardness is a rapid screening test for verifying supplier lot consistency against material data sheets
- Failure analysis: abnormal hardness, whether increases from post-curing and oxidation or decreases from plasticizer extraction and reversion, provides diagnostic evidence of service degradation
FAQ
What hardness range is typical for NBR rubber? Commercial NBR compounds generally range from 40 to 90 Shore A. The most common specifications are 70 Shore A for dynamic and static seals and 90 Shore A for backup rings, wipers, and harder extrusion-resistant parts.
Are Shore A and IRHD values interchangeable? They are numerically close between approximately 30 and 90 hardness units, but they are not identical measurement systems. Differences become significant at the extremes of the scale and on curved or thin specimens. Contract documents should state which method and standard apply; conversions should only be used as approximations.
What is the minimum thickness for a valid hardness test? Shore A testing requires at least 6 mm thickness (up to three plies may be stacked), the standard IRHD method requires 8 mm to 10 mm, and micro IRHD applies to specimens 1.5 mm to 2.5 mm thick. Testing below these thresholds yields falsely low or erratic readings.
Why do hardness readings change over time during a test? NBR is a viscoelastic material that undergoes stress relaxation under the indenter load. Immediate readings are higher than readings taken after 3 or 15 seconds. Always record the dwell time and use consistent timing when comparing results.
How often should hardness instruments be calibrated? Durometers and IRHD testers should be verified against certified reference blocks at the start of each testing session or at defined intervals per the laboratory quality system, and fully calibrated at least annually by a traceable calibration service.
How does heat aging affect NBR hardness? Thermal oxidation during aging typically increases NBR hardness due to additional crosslinking. A hardness increase after aging is therefore a key acceptance criterion in specifications such as those based on ASTM D573 or ISO 188, with common limits of ±10 units or as agreed between purchaser and supplier.