Determination of Rebound Resilience in Rubber Formulations: Test Methods, Standards and Applications

Rebound resilience testing is a fundamental measurement in rubber formulation development, quantifying the percentage of impact energy a cured compound returns after deformation. This article explains the testing principles, applicable international standards such as ISO 4662, ASTM D2632 and GB/T 1681, specimen requirements, test procedures, and the industrial applications of rebound resilience determination for rubber compounds.

Technical Overview

Rebound resilience is one of the most widely used single-value indicators of the dynamic behavior of a rubber formulation. It is defined as the ratio, expressed as a percentage, of the mechanical energy returned by a rubber test piece after impact to the mechanical energy supplied to it during that impact. In practice, a calibrated mass is allowed to strike the flat, conditioned surface of a cured test piece, and the height or angle of its rebound is compared with the height or angle from which it was released.

The rebound resilience value R is calculated as:

R = (h2 / h1) × 100 %, where h1 is the drop height, or equivalent supplied energy, of the striking body and h2 is the rebound height, or recovered energy.

Because the measurement integrates the viscoelastic response of the compound over a rapid deformation cycle, rebound resilience is highly sensitive to formulation variables. It is therefore routinely used in compounding studies, quality control and specification compliance testing. The most influential factors include:

• Crosslink density and vulcanization system (conventional sulfur, efficient vulcanization, peroxide cure)
• Filler type, particle size, structure and loading level (carbon black, precipitated silica, clays)
• Polymer type and glass transition temperature relative to the test temperature
• Plasticizer and process oil content, and their compatibility with the elastomer phase
• Test temperature, strain rate and the conditioning history of the specimen

Testing Samples

Standard Vulcanized Test Pieces

The preferred specimen is a molded cylindrical button cured to a state representative of full commercial cure. For pendulum methods in accordance with ISO 4662 and GB/T 1681, the standard test piece is typically 29 mm in diameter and 12.5 mm thick. For the vertical rebound method of ASTM D2632, a cylindrical button of approximately 19 mm diameter and 25 mm height is customary. Specimens shall be free of bubbles, voids and surface defects, with parallel faces, and the thickness shall be verified at the point of impact.

Thermoplastic Elastomers (TPE, TPV, TPU)

Thermoplastic elastomers may be evaluated by the same pendulum procedures, since ISO 4662 explicitly covers both vulcanized and thermoplastic rubbers. The material is injection or compression molded into flat-faced buttons and conditioned before testing, ensuring comparability with thermoset compounds.

Formulation Trial Slabs and Finished Articles

Laboratory trial batches are normally cured as flat slabs from which buttons are cut or punched, enabling direct side-by-side comparison of resilience across candidate formulations. Finished components such as pads, bushings or soles may also be tested directly on a flat, uniform area, although results obtained on non-standard geometries should be treated as indicative rather than specification-grade.

Testing Items

The principal measurand is the rebound resilience value determined under standard laboratory conditions. Accredited third-party laboratories commonly offer the following related determinations as part of a complete formulation characterization package:

• Rebound resilience at 23 °C as a single-point screening test for compound variants
• Temperature-dependent resilience profiles across service ranges, for example −40 °C to +100 °C
• Resilience retention after heat aging, fluid immersion or ozone exposure
• Correlation of resilience with Shore hardness, tensile properties, tear strength and compression set to assess overall formulation balance

Testing Methods

Pendulum Method (Schob and Lüpke Apparatus) — ISO 4662 / GB/T 1681 / JIS K 6255

A pendulum of defined geometry and mass is released from a set angle and strikes the test piece with an impact velocity in the region of 2 m/s. The rebound of the pendulum is read from an angular scale or electronic transducer, and resilience is computed as the ratio of recovered to supplied energy. The specimen is conditioned at 23 ± 2 °C, subjected to a few preliminary conditioning impacts, and the median of at least three valid readings is reported. The Chinese standard GB/T 1681 is technically aligned with the pendulum principle of ISO 4662.

Vertical Rebound (Falling Plunger) Method — ASTM D2632

A weighted plunger with a spherical striking surface falls freely from a fixed height onto the specimen, and the maximum rebound height is indicated on a calibrated graduated scale. Resilience is reported as the percentage ratio of rebound height to drop height. The method is simple, fast and well suited to production quality control of small molded buttons.

Temperature-Controlled Determination

For cold- or heat-flexibility screening, the test piece is conditioned in an environmental chamber attached to the tester until thermal equilibrium is reached, and the impact is performed rapidly at the target temperature so that the measured resilience reflects the true viscoelastic state at that temperature. This approach is particularly valuable for tire, anti-vibration and outdoor service applications.

General Test Conditions

• Conditioning and testing at 23 ± 2 °C and 50 ± 5 % relative humidity in line with ISO 23529 unless otherwise specified
• Minimum of three test pieces or three valid readings, with the median value reported
• Impacts applied only to the flat, molded or machined face, with relocation of the impact point between readings
• Full documentation of specimen dimensions, cure state, apparatus type and test temperature in the test report

Applications

Rebound resilience data guide formulation decisions across nearly every sector of the rubber industry:

• Tire compounding: balancing low hysteresis, high resilience and reduced rolling resistance against wet grip and abrasion performance
• Anti-vibration mounts and bushings: selecting compounds with deliberately moderate or low resilience where energy dissipation is desired
• Footwear midsoles and outsoles: maximizing energy return and cushioning liveliness
• Sporting goods such as ball bladders, grips and synthetic turf shock pads
• Conveyor belting, rollers and drive belts: managing heat build-up under cyclic deformation
• Seals, gaskets and diaphragms: verifying that aging or fluid exposure has not degraded elastic recovery
• Incoming compound inspection and batch-to-batch statistical process control

FAQ

What is rebound resilience and in what units is it reported?

Rebound resilience is the percentage of impact energy recovered by a rubber test piece after a defined impact. It is a dimensionless percentage, typically ranging from below 10 % for highly damped compounds at low temperature to above 70 % for natural rubber formulations at room temperature.

Which standards govern resilience determination?

The principal methods are ISO 4662 and GB/T 1681 for pendulum testing, JIS K 6255 for the Japanese market, and ASTM D2632 for vertical rebound. Each standard defines the apparatus, specimen, conditioning and calculation requirements that accredited third-party laboratories follow when issuing test reports.

What specimen size is required?

Pendulum methods generally use a button of about 29 mm diameter and 12.5 mm thickness, while the vertical rebound method uses a button of approximately 19 mm diameter and 25 mm height. Where a standard button cannot be prepared, laboratories may test a flat cut specimen of comparable thickness and note the deviation from the standard.

How does temperature affect the result?

Resilience falls sharply as the test temperature approaches the glass transition region of the polymer, because molecular segmental mobility becomes restricted and more energy is dissipated as heat. Formulations are therefore frequently screened at several temperatures representative of their intended service environment.

How do formulation changes influence resilience?

In general, increasing carbon black or fine silica loading lowers resilience and raises hysteresis; higher crosslink density within the optimum cure range tends to raise resilience; and plasticizers may raise or lower resilience depending on their compatibility and their effect on the glass transition temperature.

What is the difference between rebound resilience and damping?

The two properties are complementary: resilience measures the energy returned by the compound, while damping, or hysteresis, measures the energy dissipated as heat. A high-resilience compound dissipates little energy, whereas a high-damping compound is deliberately formulated to convert vibration energy into heat.