Volume Reduction Rate Determination Process: Testing Workflow, Standards and Applications

The volume reduction rate determination process is a core laboratory procedure for evaluating how much of a material's original volume is permanently lost after compression, cyclic loading, or environmental aging. Widely applied to flexible cellular polymers, thermoplastic foams, and fiber-based cushioning materials, this test supports quality control and product qualification in accordance with recognized standards such as ISO 1856, ASTM D3574, and GB/T 6669. This article explains the full testing workflow, from specimen preparation and conditioning to calculation and reporting.

Technical Overview

Volume reduction rate is defined as the percentage decrease in the volume of a test specimen relative to its original volume, measured after the specimen has been subjected to a defined compression treatment, cyclic loading regime, or environmental aging condition followed by a controlled recovery period. The parameter quantifies the permanent volumetric loss of porous, cushioning, and cellular materials and is a key durability indicator for products whose comfort, energy absorption, or insulating performance depends on maintaining thickness and loft.

Although related to the compression set value, which is typically calculated from thickness measurements alone, the volume reduction rate considers the specimen as a three-dimensional body. This is important because compressed foam and fiber materials may exhibit lateral spreading, buckling, or edge distortion, meaning that a thickness-based calculation alone can underestimate or overestimate the true volumetric change. Accredited third-party laboratories therefore determine volume reduction rate by measuring the full geometry of the specimen before treatment and after recovery.

Typical testing references include ISO 1856 for flexible cellular polymeric materials, ASTM D3574 for slab, bonded, and molded urethane foams, ASTM D395 for compression set of rubber-like materials, and GB/T 6669 for soft foam polymeric materials. The selection of deflection level, test duration, temperature, and recovery time is defined by the applicable product standard or by agreement between the purchaser and the laboratory.

Testing Samples

Flexible Cellular Polymeric Materials

Slabstock and molded flexible polyurethane foams, high-resilience (HR) foams, viscoelastic (memory) foams, and latex foams are the most frequently tested material categories. Specimens are usually cut to rectangular blocks, for example 50 mm × 50 mm × 25 mm or 100 mm × 100 mm × 50 mm, with parallel faces and clean, undamaged edges. Specimens should be taken at least the specified minimum distance from the skin of molded parts and from the outer surface of slabstock to avoid structural anomalies.

Thermoplastic Foams

Cross-linked and non-cross-linked polyethylene (PE), ethylene-vinyl acetate (EVA), and polypropylene (PP) foams in sheet, roll, or profiled form are tested to assess volumetric stability after compression during storage, lamination, or packaging. Because thin sheets may require stacking or special fixtures to reach the minimum test thickness, specimen preparation follows the thickness requirements of the governing standard.

Fiber-Based and Nonwoven Cushioning Materials

Polyester and polypropylene fiber pads, thermal-bonded wadding, needled nonwovens, and blended fill materials are evaluated for volume reduction rate to quantify loft loss after compression, which directly affects thermal resistance, resilience, and perceived quality in bedding, apparel padding, and furniture applications.

Finished Components

Where product standards permit, complete components such as seat inserts, cushion assemblies, packaging pads, and insulation panels may be tested at component level, with sampling positions and orientation documented so that results are representative of the manufactured article.

Testing Items

The volume reduction rate determination process typically covers the following test items, individually or in combination:

• Volume reduction rate after constant-deflection static compression at 50%, 75%, or 90% of original thickness
• Volume reduction rate after constant-load compression at a specified applied force or pressure
• Volume reduction rate after cyclic compression fatigue loading (for example, 20,000 to 80,000 cycles)
• Volume reduction rate after thermal aging, typically 22 h, 72 h, or 168 h at 70 °C, 80 °C, or another agreed temperature
• Volume reduction rate after humid aging, for example 95% relative humidity at 40 °C
• Volume reduction rate after combined compression-plus-aging treatment, in which the specimen is compressed and simultaneously exposed to elevated temperature
• Supplementary thickness-based compression set and dimensional stability measurements for correlation purposes

Testing Methods

Specimen Preparation and Conditioning

Specimens are die-cut or sawn to the dimensions required by the selected method, ensuring flat, parallel upper and lower faces free of cells ruptured by cutting. Materials must be aged after production for the period specified in the standard before specimens are cut, and freshly cut specimens are normally rested for a defined interval to allow entrained air and cutting stresses to stabilize. Prior to testing, specimens are conditioned in a standard laboratory atmosphere, typically 23 °C ± 2 °C and 50% ± 5% relative humidity, for at least 16 h and up to 72 h depending on the material and standard.

Initial Volume Measurement

Initial dimensions are measured with a calibrated instrument at multiple positions on each face. Length and width are recorded at a minimum of three positions each, and thickness is measured under a low specified contact pressure at a minimum of five defined points. The initial volume V0 is calculated as the product of the mean length, mean width, and mean thickness, or, where the standard requires it, by a geometric measurement procedure that accounts for the specimen's actual shape. Each specimen is numbered and its orientation relative to the rise direction is recorded.

Compression and Aging Treatment

For constant-deflection testing, the specimen is placed between two flat, parallel plates and compressed to the specified fraction of its original thickness, with spacer bars or stops maintaining the exact deflection throughout the test. For constant-load testing, a calibrated mass or pneumatic loading system applies the specified force to the specimen. The compressed assembly is then held under the specified conditions: room temperature for short-duration screening, or an environmental chamber for heat and humidity exposure at the agreed temperature and duration. At the end of the exposure period the load is released within a standardized short time.

Recovery Period

After release, the specimen is placed freely on a flat surface in the standard atmosphere and allowed to recover. Common recovery intervals are 30 min, 24 h, and 48 h; the selection depends on the test method and the material family. Viscoelastic foams and fiber-based materials generally require the longer recovery intervals specified for their class, and the recovery interval used must be stated in the report.

Final Measurement and Calculation

At the end of the recovery period, the same measurement positions and contact pressures used for the initial measurement are repeated, and the final volume V1 is calculated in the identical manner. The volume reduction rate is then determined as follows:

Volume reduction rate (%) = (V0 − V1) / V0 × 100

where V0 is the initial specimen volume before treatment and V1 is the specimen volume after treatment and recovery. A lower value indicates better volumetric recovery and durability. Where the governing standard also requires the thickness-based compression set value, it is calculated from the initial thickness, the final thickness, and the compressed thickness, and reported alongside the volume reduction rate for correlation.

Reporting

A compliant test report states the material identification and specimen source, specimen dimensions and orientation, the test method and standard applied, the deflection or load level, the exposure temperature, humidity, and duration, the recovery interval, the calculated volume reduction rate for each specimen, and the arithmetic mean. Any visible structural changes such as cell collapse, cracking, delamination, or lateral distortion are recorded as observations.

Applications

Volume reduction rate determination is applied across industries in which long-term loft, thickness retention, and energy absorption are performance-critical:

• Automotive interiors and seating, for validating foam durability over the vehicle service life and meeting OEM material specifications
• Mattresses, pillows, upholstered furniture, and bedding fills, for comfort retention and quality grading
• Protective packaging design, for confirming that cushioning materials retain thickness after sustained compression in storage and transit
• Medical and mobility cushioning, including wheelchair and pressure-redistribution cushions where volumetric loss affects clinical performance
• Building and appliance insulation foams, for assessing dimensional stability of porous materials after compressive and thermal exposure
• Footwear midsoles and apparel padding, for resilience and loft retention after repeated compression
• Research and development, incoming material inspection, supplier benchmarking, and failure analysis of returned components

FAQ

What is the difference between volume reduction rate and compression set?

Compression set is calculated from thickness measurements taken at defined points, while volume reduction rate reflects the change in the full three-dimensional volume of the specimen, including lateral spreading and edge distortion. For materials that deform non-uniformly under compression, the volume reduction rate gives a more complete picture of permanent volumetric loss, and the two values are frequently reported together.

Which deflection level should be selected for testing?

Common constant-deflection levels are 50%, 75%, and 90% of original thickness. The appropriate level is defined by the product standard or material specification and should reflect the severity of the intended service condition; 75% and 90% deflections are typical for screening severe compression, while 50% is used for milder, long-duration assessments.

How long should the specimen recover before the final measurement?

Recovery intervals of 30 min, 24 h, and 48 h are the most common options, depending on the standard and the material. Viscoelastic and fiber-based materials generally require 24 h or longer to complete measurable recovery, and the chosen interval must be documented because it directly influences the calculated result.

Can thin foam sheets be tested for volume reduction rate?

Yes, provided the specimen meets the minimum thickness requirements of the selected method. Where a single sheet is too thin, standards may permit lamination of plies or the use of alternative fixture arrangements, and the preparation method must be described in the report so that results remain comparable between laboratories.

How are the results interpreted?

A lower volume reduction rate indicates better retention of original volume and, generally, better long-term durability and comfort performance. Acceptance limits are set by the applicable product standard, OEM specification, or purchase agreement; typical requirements for seating-grade flexible foams limit permanent volumetric loss to a small percentage after the specified aging treatment.

Which standards govern the determination process?

ISO 1856 and GB/T 6669 cover compression-set-related volumetric testing of flexible cellular polymeric materials, ASTM D3574 defines the corresponding procedures for urethane foams, and ASTM D395 addresses compression set of rubber-like materials under constant deflection or constant load. The laboratory selects the method based on the material family, the product specification, and the destination market.