Determination of Rebound Resilience of Nitrile Butadiene Rubber (NBR)

Rebound resilience determination is one of the most important quality control tests for nitrile butadiene rubber (NBR), quantifying the ability of a vulcanized compound to return elastic energy after impact deformation. This article provides a comprehensive technical guide to NBR rebound resilience testing, covering international standards such as ISO 4662, ASTM D2632, and GB/T 1681, specimen preparation, test procedures, calculations, and practical applications in the rubber industry.

Technical Overview

Nitrile butadiene rubber (NBR) is a synthetic elastomer produced by the copolymerization of acrylonitrile and butadiene. It is widely valued for its excellent resistance to oils, fuels, and many non-polar solvents, which makes it the material of choice for seals, gaskets, hoses, and rollers in demanding industrial environments. However, because the acrylonitrile content introduces additional polarity and steric hindrance into the polymer chain, NBR generally exhibits lower rebound resilience than natural rubber, and this property must be carefully measured and controlled.

Rebound resilience, also known as rebound elasticity, is defined as the ratio of the mechanical energy recovered after an impact deformation to the energy applied to the test piece, expressed as a percentage. In practical terms, it measures how much of the impact energy a rubber specimen returns when struck by a falling mass or a pendulum hammer. A higher resilience value indicates a more elastic, lower-hysteresis compound, while a lower value indicates greater energy absorption and damping.

The determination of NBR rebound resilience is governed by several internationally recognized standards, including ISO 4662 (pendulum method), ASTM D2632 (vertical rebound method), GB/T 1681 (the Chinese national standard for rebound resilience of vulcanized rubber), and DIN 53512 (Schob pendulum). These tests are essential for compound development, incoming material verification, batch-to-batch quality control, and the qualification of finished rubber products for dynamic service conditions.

Testing Samples

Standard Cylindrical Test Pieces

The preferred specimen for pendulum-type testing is a molded cylindrical disc. For the Schob-type pendulum specified in ISO 4662 and GB/T 1681, the standard test piece is typically 29 mm in diameter and 12.5 mm in thickness. The two end faces must be flat, parallel, and free from pores, blisters, and surface defects, because any irregularity at the impact point will distort the energy transfer and produce unreliable readings. Specimens are usually compression-molded under controlled temperature, pressure, and cure time to ensure a uniform degree of vulcanization throughout the cross-section.

Molded Sheet Specimens

When testing is performed on a vertical rebound tester, specimens are commonly cut or punched from standard vulcanized sheets, typically with a thickness of approximately 6 mm to 12.5 mm depending on the apparatus. Sheets should be molded with a consistent cure state, and test pieces must be taken from areas free of flow lines, knit marks, or thickness variation. Multiple specimens are normally prepared so that replicate measurements can be averaged.

Product-Derived Specimens

In many quality control programs, resilience is measured directly on samples taken from finished products such as O-rings, roller coverings, or molded seals. Where the product geometry does not permit a standard specimen, sections may be cut and ground to a suitable thickness, provided that the reported results clearly state the deviation from the standard specimen dimensions. Comparative testing is only valid when all specimens are prepared in the same manner.

Specimen Conditioning

All test pieces must be conditioned before testing in accordance with ISO 23529 or the equivalent clause of the applicable test standard. Typical conditioning requires a minimum of 16 hours at a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity. Specimens must also be protected from light, ozone, and heat, and at least 30 minutes should elapse between molding and testing to allow the rubber to stabilize. For tests at elevated or sub-ambient temperatures, specimens must be held at the test temperature for a sufficient equilibration period, typically 15 to 30 minutes, immediately before measurement.

Testing Items

Rebound resilience testing of NBR typically encompasses the following measurement items, which can be selected according to the purpose of the evaluation:

• Rebound resilience at standard laboratory temperature (23 °C), the fundamental acceptance parameter for compound qualification.
• Rebound resilience as a function of temperature, commonly between -40 °C and +100 °C, to characterize the glass transition behavior and low-temperature serviceability of the compound.
• Resilience retention after heat aging, oil immersion, or fluid exposure, to assess durability and compatibility in service environments.
• Comparative resilience evaluation of different filler loadings, plasticizer systems, or cure packages during compound development.
• Batch conformity testing against customer specifications or internal control limits for production release.

Testing Methods

Vertical Rebound Method (ASTM D2632)

In the vertical rebound test, a weighted plunger is allowed to fall freely from a fixed height onto the flat surface of the horizontally supported test piece. The rebound height of the plunger is measured electronically or photoelectrically, and the resilience is calculated as the ratio of the rebound height to the drop height. The instrument is calibrated against a reference material of known resilience, and typically three to five impacts are performed on each specimen, with the mean value reported. This method is rapid, requires a compact specimen, and is widely used for routine quality control in North American practice.

Pendulum Rebound Method (ISO 4662 / GB/T 1681)

The pendulum method uses a hammer attached to the arm of a pendulum, which is raised to a defined starting angle and released so that it strikes the test piece at a controlled impact velocity of approximately 2 m/s in the Schob configuration. The extent of the rebound swing is recorded, either as a rebound angle or as an equivalent rebound height. The test piece is held firmly against a rigid anvil, usually by vacuum clamping, so that no energy is lost through specimen movement. ISO 4662 permits several pendulum designs, provided the impact velocity and geometry comply with the standard, and results from different devices may show small systematic differences.

Calculation and Expression of Results

Rebound resilience is calculated from the measured heights or angles using the relationship R (%) = h2 / h1 × 100, where h1 is the initial drop height of the impacting body and h2 is the maximum rebound height. For pendulum instruments based on angle measurement, the corresponding angular values are converted to heights before the ratio is computed. The median or mean of the valid impacts is reported as the rebound resilience of the specimen, together with the test temperature, the standard used, and the specimen dimensions. Repeatability within a laboratory is typically better than 2 percentage points when specimen preparation and conditioning are properly controlled.

Factors Affecting Test Results

Numerous material and procedural factors influence the measured resilience of NBR and must be controlled for meaningful comparisons:

• Acrylonitrile content: resilience decreases as the acrylonitrile level increases, with glass transition temperatures ranging from approximately -55 °C for low-ACN grades to around -25 °C for high-ACN grades.
• Filler type and loading: increasing carbon black loading, particularly fine-particle reinforcement grades, lowers resilience, while soft thermal blacks such as N990 have a smaller damping penalty.
• Cure system: efficiently vulcanized sulfur systems generally give higher resilience than conventional high-sulfur systems, and peroxide cures can further improve elastic recovery in many NBR formulations.
• Plasticizers: ester plasticizers can improve resilience at low temperatures but may reduce it at room temperature depending on loading and compatibility.
• Test temperature: resilience falls sharply as the material approaches its glass transition, making temperature control during testing essential.
• Specimen condition: under-cure, porosity, surface contamination, or inadequate conditioning can all depress the measured value.

Applications

Rebound resilience data for NBR are used across the full product lifecycle, from research and development through production control to field failure analysis. Typical application areas include:

• Dynamic seals, O-rings, and hydraulic gaskets, where adequate resilience ensures proper sealing contact recovery after compression.
• Fuel hoses, diaphragms, and flexible couplings, where resilience and oil resistance must be balanced for long-term flexibility.
• Vibration isolation mounts and damping elements, where low resilience combined with oil resistance provides controlled energy absorption.
• Industrial and printing rollers, where resilience affects nip behavior, ink transfer, and roll covering durability.
• Footwear components, gaskets, and molded rubber goods requiring specified elastic performance.
• Compound certification and supplier qualification programs in the automotive, aerospace, petrochemical, and machinery industries.

By correlating resilience values with field performance, engineers can select the optimal acrylonitrile content, filler system, and cure architecture for each application, and quality teams can set statistically justified acceptance limits that protect both performance and manufacturability.

FAQ

What is rebound resilience in NBR testing?

Rebound resilience is the percentage of impact energy that a vulcanized NBR specimen returns after being struck by a falling mass or pendulum hammer. It is a direct indicator of the elastic, low-hysteresis character of the compound and complements hardness, tensile, and compression set measurements in a complete material specification.

Which standards are used for NBR rebound resilience determination?

The most widely applied standards are ISO 4662 (pendulum methods, including the Schob device), ASTM D2632 (vertical rebound), GB/T 1681 (Chinese national standard using the Schob pendulum), and DIN 53512. Although the measured value is physically equivalent, results from different instruments and standards should not be compared without confirming their correlation.

What are typical rebound resilience values for NBR compounds?

Depending on acrylonitrile content, filler loading, and cure system, NBR compounds typically show rebound resilience in the range of approximately 20% to 50% at room temperature. Low-ACN, lightly filled, efficiently vulcanized compounds reach the upper end of this range, while high-ACN, highly reinforced compounds exhibit significantly greater damping.

How does temperature affect the test result?

Resilience is strongly temperature dependent. As the test temperature approaches the glass transition region of the NBR grade, resilience drops sharply because the rubber transitions from a rubbery to a leathery, energy-absorbing state. For this reason, low-temperature resilience testing is a key qualification requirement for seals and hoses used in cold climates or cryogenic-adjacent service.

How can variability in resilience test results be minimized?

Variability is minimized by using defect-free specimens with the specified dimensions and cure state, conditioning all test pieces for at least 16 hours at 23 ± 2 °C, performing replicate impacts and reporting the mean, calibrating the instrument regularly with certified reference materials, and strictly controlling the test temperature. Documenting these conditions alongside the results ensures traceability and comparability between laboratories.