Nitrile rubber (NBR) impact strength testing is essential for evaluating the toughness, crack resistance, and durability of this oil-resistant elastomer under high strain-rate loading conditions. This technical guide covers sample preparation, recognized standards such as ASTM D256 and ISO 179, laboratory test methods, and typical industrial applications. Whether you are qualifying NBR compounds for automotive seals, oilfield components, or low-temperature service, reliable impact testing provides the data needed to ensure safe and consistent material performance.
Technical Overview
Nitrile rubber, designated NBR or Buna-N, is a synthetic elastomer produced by the free-radical copolymerization of acrylonitrile (ACN) and butadiene. The acrylonitrile content, typically ranging from 18% to 50%, governs the balance between oil and fuel resistance on one hand and low-temperature flexibility on the other. Because NBR combines good mechanical strength, abrasion resistance, and resistance to hydrocarbons, it is widely used in seals, hoses, gaskets, diaphragms, and molded components that may experience sudden mechanical shock during service.
Impact strength testing quantifies the amount of energy a material absorbs before fracturing under rapid, dynamic loading. For elastomeric materials such as NBR, the test result reflects the compound's ability to resist crack initiation and propagation, and it is particularly informative for highly filled compounds, semi-rigid formulations, NBR/PVC blends, and rubber-to-metal bonded assemblies. Soft, unfilled vulcanizates often deform without fracture at room temperature, so impact data for NBR are most meaningful when evaluated together with hardness, tensile properties, and low-temperature brittleness behavior.
Several formulation and processing factors influence the measured impact strength of nitrile rubber, including:
- Acrylonitrile content, which raises the glass transition temperature and reduces low-temperature impact performance
- Type and loading of reinforcing fillers such as carbon black or silica
- Curing system and degree of vulcanization, including under-cure or over-cure conditions
- Plasticizer content, which improves flexibility but may reduce strength
- Test temperature, strain rate, and specimen geometry, especially notch sharpness
- Presence of processing defects such as porosity, poor dispersion, or contamination
Testing Samples
Molded Sheets and Plaques
Standard specimens are preferably prepared by compression or injection molding directly from the cured compound. Sheets with a thickness of 2 mm to 4 mm are produced under controlled temperature, pressure, and cure time, verified by a moving-die rheometer to confirm optimum cure state. Molded plaques provide uniform structure and eliminate surface damage that can occur when specimens are cut from finished goods, ensuring that measured impact energy reflects the intrinsic properties of the compound.
Notched and Unnotched Bars
Pendulum impact specimens are machined or molded as rectangular bars. For Charpy testing to ISO 179-1, the typical specimen measures 80 mm × 10 mm × 4 mm, with optional type A notches of 45 degrees and a 0.25 mm root radius. For Izod testing to ASTM D256, specimens commonly measure 63.5 mm × 12.7 mm with thickness between 3.2 mm and 12.7 mm, and a standard notch depth of 2.54 mm. Notches are cut with a precision cutter and fresh blade, because notch quality strongly affects the measured energy to break.
Specimens Cut from Finished Products
When testing production parts such as seals, bushings, or hose covers, specimens may be buffed or die-cut from the article. In such cases, the report must state the sampling location and orientation, since anisotropy from milling, calendering, or extrusion can cause significant scatter. Specimens must be free of surface defects, and thickness is measured at multiple points before testing.
Conditioning Requirements
All specimens are conditioned prior to testing in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for a minimum of 24 hours, in accordance with ISO 291 or ASTM D618. For low-temperature impact programs, specimens are additionally conditioned at the specified test temperature using a controlled environmental chamber or liquid bath to ensure thermal equilibrium before striking.
Testing Items
A complete NBR impact evaluation program typically includes the following measured parameters:
- Notched Izod impact strength, expressed in J/m or ft·lb/in of notch
- Unnotched Izod impact strength for overall toughness comparison
- Charpy impact strength, expressed in kJ/m² of cross-sectional area
- Tensile-impact energy to break, determined per ASTM D1822
- Falling-weight or drop-tower failure energy and puncture force for sheet materials
- Ductile-to-brittle transition behavior across a defined temperature range
- Low-temperature brittleness point, determined per ASTM D2137 or ISO 812
Results are reported as the mean of at least five valid specimens, together with standard deviation and the observed failure mode, classified as complete break, partial break, hinge break, or non-break. Recording the failure type is essential because energy values for non-breaking elastomeric specimens are not directly comparable to true fracture energies.
Testing Methods
Pendulum Impact Testing (Izod and Charpy)
The pendulum method is the most widely used procedure for determining the impact strength of semi-rigid NBR compounds and NBR blends. A calibrated hammer of suitable energy capacity, commonly 1 J to 22 J, is released from a fixed height and strikes the clamped specimen. The energy absorbed by the specimen is calculated from the difference between the initial and residual hammer heights, after correction for friction and windage losses. Izod tests strike the vertically clamped, notched face of the specimen per ASTM D256 or ISO 180, while Charpy tests strike the horizontally supported specimen at mid-span per ISO 179-1. The standard impact velocity is approximately 3.5 m/s for Izod and 2.9 m/s or 3.8 m/s for Charpy.
Instrumented Drop Tower Testing
Instrumented falling-weight testing provides a full force–deflection curve during impact, yielding peak force, energy to peak force, and total failure energy. A striker with a hemispherical tip, typically 20 mm in diameter, impacts a clamped disk or plaque at a controlled velocity. This method is especially valuable for NBR sheet goods, membranes, and NBR/PVC blends because it distinguishes between ductile puncture and brittle fracture, and it supports statistical determination of failure energy at a 50% failure probability using the staircase method.
Tensile Impact Testing
In tensile-impact testing per ASTM D1822, one end of a small dumbbell specimen is attached to the pendulum cross-clutch and the other end is rigidly fixed. When the pendulum is released, the specimen is fractured in tension at high strain rate. This method is useful for compounds that are too flexible to fracture meaningfully in flexural impact, and it provides a comparative measure of high-rate tensile toughness for quality control of NBR batches.
Low-Temperature Brittleness Testing
Because impact performance of NBR degrades sharply near the glass transition, brittleness testing is a critical companion procedure. Specimens are immersed in a chilled medium at a specified temperature for a defined period and then struck by a spring-loaded striker. The lowest temperature at which specimens do not crack or fracture is reported as the brittleness temperature. Higher acrylonitrile grades exhibit brittleness temperatures closer to minus 20 °C, while low-ACN, plasticized compounds can remain flexible below minus 40 °C.
Quality Assurance of Test Data
Pendulum machines must be verified before each test series using reference specimens and zero-energy checks. Hammer capacity should be selected so that the absorbed energy is between 10% and 80% of the available energy. Specimen dimensions are measured to 0.02 mm accuracy, and any specimen that slips in the clamp or fractures outside the notch region is rejected and replaced.
Applications
Impact strength data for nitrile rubber directly support material selection, quality control, and failure analysis across many industries. Key application areas include:
- Automotive fuel system components, including filler neck hoses, injector seals, and diaphragms subject to vibration and shock
- Oil and gas equipment, such as blowout preventer seals, packer elements, and valve seats exposed to impact during installation
- Industrial rubber rolls, gyration-resistant gaskets, and machinery mounts that must absorb dynamic loads
- NBR/PVC blends used in conveyor belt skirting, cable jacketing, and protective padding where both toughness and oil resistance are required
- Footwear components, tool grips, and anti-vibration pads requiring durable energy absorption
- Rubber-to-metal bonded parts, where impact testing validates both the compound and the adhesive interface
By correlating impact strength with formulation variables, manufacturers can optimize filler loading, cure systems, and plasticizer levels to meet specific toughness targets while retaining the oil resistance for which NBR is selected.
FAQ
What is the typical impact strength of nitrile rubber?
Impact strength values vary widely with formulation. Soft, well-vulcanized NBR compounds usually do not fracture in pendulum tests at room temperature and are reported as non-break. Highly filled or semi-rigid compounds and NBR/PVC blends typically show Charpy values in the range of a few kJ/m² up to tens of kJ/m², while brittleness at low temperature causes a sharp drop in absorbed energy. Testing the actual compound at the service temperature is therefore essential.
Which standards apply to NBR impact strength testing?
The most commonly applied standards are ASTM D256 and ISO 180 for Izod impact, ISO 179-1 for Charpy impact, ASTM D1822 for tensile impact, ASTM D2137 and ISO 812 for low-temperature brittleness, and instrumented falling-weight methods based on ISO 6603-2. Chinese national standards GB/T 1843 and GB/T 1043 are equivalent pendulum methods frequently referenced in regional specifications.
Why are notched specimens used in impact testing?
The notch concentrates stress at a known point and creates a triaxial stress state that promotes brittle fracture, making the test sensitive to differences in crack initiation resistance. Notched results reveal the material's sensitivity to stress raisers such as mold defects or scratches, while unnotched results reflect overall toughness.
How does acrylonitrile content influence impact performance?
Increasing acrylonitrile content improves oil and fuel resistance but raises the glass transition temperature, which reduces flexibility and low-temperature impact resistance. Lower ACN grades retain impact toughness at colder temperatures but offer reduced solvent resistance, so the ACN level is selected as a compromise between chemical resistance and dynamic durability.
At what temperature does NBR become brittle?
The brittleness temperature depends on ACN content and plasticizer level. High-ACN compounds typically become brittle between minus 10 °C and minus 25 °C, whereas low-ACN, plasticized grades can pass impact tests at temperatures below minus 40 °C. The brittleness temperature should always be verified by testing rather than assumed from datasheet values.
How should samples be prepared for accurate results?
Specimens should be molded to the final dimensions whenever possible, cured to the optimum state determined by rheometry, conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours, and notched with a sharp, freshly prepared cutter immediately before testing. Consistent specimen preparation and careful control of notch geometry are the most important factors for achieving repeatable impact data.