Determination of Compression Resilience Rate of Cellular Plastics: Standards, Methods and Applicatio

The compression resilience rate of cellular plastics quantifies how completely a foam material recovers its original thickness after a compressive load is removed, making it a critical quality indicator for cushioning, packaging, seating, and sealing applications. This article provides a comprehensive technical guide covering the definition of compression resilience, applicable international and national standards, sample requirements, test procedures, calculation methods, and typical industrial applications for both flexible and rigid foams.

Technical Overview

Cellular plastics, commonly known as foam plastics, are polymeric materials with a gas-phase cellular structure that provides low density, energy absorption, thermal insulation, and mechanical cushioning. Among their most important functional properties is the ability to recover their original shape and thickness after being compressed, which is quantified as the compression resilience rate (also referred to as compression recovery rate or rebound rate).

The compression resilience rate is defined as the ratio of the recovered thickness of a foam specimen to its original thickness after a specified compressive deformation has been applied and released, expressed as a percentage. It is mathematically complementary to compression set: a foam with a low compression set exhibits a high compression resilience rate. Together, these parameters describe the dimensional stability and long-term cushioning performance of foam materials under static or repeated compressive loading.

Testing of compression resilience is governed by several internationally recognized standards, including GB/T 6669-2008 (flexible cellular polymeric materials — determination of compression set), ISO 1856 (flexible cellular polymeric materials — determination of compression set), ASTM D3574 Test D (constant deflection compression set for flexible polyurethane foams), ISO 7214 (cellular plastics — polyethylene — methods of test), GB/T 8813-2008 and ISO 844 (determination of compression behaviour of rigid cellular plastics), and ASTM D3575 (flexible cellular materials made from olefin polymers). For dynamic rebound behavior, ball rebound resilience methods such as GB/T 6670-2008, ISO 8307, and ASTM D3574 Test H are also widely applied and are frequently evaluated alongside static compression recovery.

Testing Samples

Flexible Polyurethane Foams

Slabstock and molded flexible polyurethane foams used in mattresses, upholstered furniture, and automotive seating are the most common test subjects. These materials are typically evaluated at 50% or 75% constant deflection to simulate service loading in cushioning applications.

Polyolefin and Elastomeric Foams

Cross-linked and non-cross-linked polyethylene (PE) foams, ethylene-vinyl acetate (EVA) foams, EPDM and rubber foams used in protective packaging, sports equipment, footwear midsoles, and expansion joints are tested according to ISO 7214 or ASTM D3575 procedures, often combining compression set and short-term recovery measurements.

Polystyrene and Rigid Insulation Foams

Expandable polystyrene (EPS), extruded polystyrene (XPS), polyurethane rigid foam, and phenolic foam boards used in construction and transport packaging are evaluated for compressive behavior and recovery after deflection to 10% strain or 50% thickness, referencing GB/T 8813, ISO 844, or ASTM D1621.

Specimen Preparation and Conditioning

Standard specimens for flexible foam compression tests are typically 50 mm × 50 mm × 25 mm (thickness), with the original thickness measured under a light standardized load using a thickness gauge. Larger finished articles may be cut to representative sections. All specimens must be free of skins, voids, and visible defects unless the skin is integral to the product design. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5% relative humidity for at least 16 hours.

Testing Items

Typical test items evaluated in a comprehensive compression resilience characterization program include:

• Compression resilience rate (recovery rate) at 50%, 75%, or 90% constant deflection
• Compression set under constant deflection, at room temperature, elevated temperature (e.g., 70 °C), or with humid aging
• Compressive stress at 10% deformation for rigid foams (compressive strength)
• Ball rebound resilience (dynamic resilience) as a complementary indicator
• Compression fatigue behavior under repeated or cyclic loading
• Recovery behavior after prolonged static loading (aging simulation)
• Thickness and dimensional stability of specimens before and after compression

Testing Methods

Static Compression Recovery (Constant Deflection) Method

The specimen is placed between two flat parallel plates of a compression device, and the original thickness t0 is recorded. The specimen is then compressed to a specified fraction of its original thickness, most commonly 50%, and held at this constant deflection for a defined period, typically 22 hours at 23 °C (or at an elevated temperature such as 70 °C for accelerated aging studies). After the compression period, the load is removed, the specimen is allowed to recover freely for a specified recovery time, commonly 30 minutes and, in some standards, up to 24 hours, and the final recovered thickness tr is measured.

Calculation of Resilience Rate and Compression Set

The compression resilience rate R is calculated as R = (tr / t0) × 100%, where t0 is the original thickness and tr is the recovered thickness. The corresponding compression set Ca is calculated as Ca = (t0 − tr) / t0 × 100%, so that a material exhibiting a resilience rate of 92% shows a compression set of 8%. Standards such as GB/T 6669 and ISO 1856 also permit calculation relative to the compressed thickness, and the calculation basis must always be stated in the test report. A minimum of three to five specimens are tested and the arithmetic mean is reported together with the individual values.

Short-Term Recovery Method for Polyolefin Foams

For PE and EVA foams, ISO 7214 and related specifications describe a shorter procedure in which the specimen is compressed to 50% of its thickness for a defined duration, released, and the thickness is remeasured after a specified recovery interval, such as 24 hours. This method is well suited to process control for packaging and midsole materials where rapid elastic recovery is required.

Ball Rebound Resilience Method (Related Dynamic Method)

To characterize dynamic resilience, a standardized steel ball is dropped from a fixed height (typically 500 mm) onto a horizontally supported foam specimen, and the maximum rebound height is recorded. The rebound resilience is calculated as the ratio of rebound height to drop height, expressed as a percentage, following GB/T 6670, ISO 8307, or ASTM D3574 Test H. While not a substitute for static compression recovery, ball rebound data provide valuable insight into the energy-return behavior of cushioning foams.

Equipment and Test Conditions

Testing is performed using a compression set apparatus or universal testing machine with parallel compression plates, a calibrated thickness gauge or dial indicator, a timer, and, where required, a thermostatically controlled oven for elevated-temperature exposure. Optional low-temperature chambers allow evaluation of recovery behavior at subzero conditions, which is relevant for packaging, outdoor, and automotive applications.

Applications

Compression resilience testing supports quality control, material qualification, and product design across many industries:

• Furniture and bedding: verification of mattress and upholstery foam durability and shape retention over long-term use
• Automotive: qualification of seat cushions, headrests, and interior padding against sagging and permanent deformation
• Protective packaging: design of EPS, EPE, and molded PU cushioning that must recover between repeated transport shocks
• Footwear: evaluation of EVA and PU midsole resilience and fatigue resistance
• Sports and leisure: performance assessment of gym mats, helmets, and protective padding
• Construction: confirmation of recovery and compressive performance for XPS and PU insulation boards and expansion joint fillers
• Research and development: comparison of foam formulations, blowing agents, cross-linking degrees, and aging resistance in material development programs
• Regulatory compliance: documentation of performance data for product certifications and customer specifications through accredited third-party laboratories

FAQ

What is the difference between compression resilience rate and ball rebound resilience?

Compression resilience rate is a static parameter describing how completely a foam recovers its thickness after sustained compression, whereas ball rebound resilience is a dynamic parameter describing the instantaneous energy return when a ball strikes the foam. Both are relevant to cushioning performance but measure different aspects of viscoelastic behavior and are specified by different test methods.

Which deflection level should be used for testing?

A 50% constant deflection is the most widely specified condition for flexible foams because it approximates severe service loading. A 75% or 90% deflection may be applied for high-load applications such as heavy-duty packaging, while rigid foams are commonly characterized at 10% deformation to determine compressive strength.

How long must the specimen recover before the final thickness is measured?

Most standards specify a recovery period of 30 minutes after load release for the primary measurement, and some procedures add a 24-hour recovery measurement to distinguish short-term viscoelastic recovery from true permanent set. The chosen recovery time must be clearly reported.

How do temperature and humidity affect the results?

Elevated temperature accelerates creep and typically increases compression set, lowering the measured resilience rate. High humidity can plasticize hydrophilic polymer systems. For this reason, tests intended for comparison must be conducted under identical conditioning atmospheres, typically 23 ± 2 °C and 50 ± 5% relative humidity.

How many specimens are required for a valid test?

A minimum of three specimens is generally required by the standards, although five specimens provide better statistical reliability for heterogeneous or anisotropic foams. The mean value, individual values, deflection level, exposure time, temperature, and calculation basis must all be documented in the test report.