Compression set analysis, also known as compression permanent deformation testing, measures the ability of rubber and elastomeric materials to recover their original thickness after prolonged compressive loading at controlled temperatures. This critical performance indicator determines whether seals, gaskets, and cushioning components will maintain effective sealing force throughout their service life. This article covers the testing principles, international standards such as ISO 815 and ASTM D395, specimen requirements, test procedures, calculation methods, and typical industrial applications.
Technical Overview
Compression permanent deformation, commonly referred to as compression set, is defined as the residual reduction in thickness of an elastomeric specimen after it has been compressed under specified conditions of deflection or load, temperature, and time, and subsequently released and allowed to recover. The result is expressed as a percentage of the original applied compression, and a lower percentage indicates superior elastic recovery and long-term sealing capability.
When an elastomer is held in a compressed state, particularly at elevated temperatures, molecular chains undergo rearrangement and relaxation. Over time, physical creep and chemical degradation mechanisms such as oxidative crosslinking, chain scission, and network breakdown convert elastic deformation into permanent deformation. Once the compressive load is removed, the material fails to return fully to its original dimensions, and this unrecovered portion is quantified as the compression set.
Compression set behavior is one of the most important failure criteria for sealing products. A seal exhibiting high compression set loses contact pressure against its mating surfaces, resulting in leakage, intrusion of contaminants, or loss of damping performance. For this reason, material specifications across the automotive, aerospace, medical, and industrial sectors almost always include compression set limits.
Testing Samples
Standard Cylindrical Buttons
The preferred specimen is a molded cylindrical button with smooth, parallel surfaces. Common dimensions include Type A specimens of 29 mm diameter and 12.5 mm height, and Type B specimens of 13 mm diameter and 6.3 mm height as specified in ISO 815-1. ASTM D395 specifies a standard disc of approximately 28.7 mm diameter and 12.5 mm thickness. Specimens may also be stacked to reach the required height, provided that the total thickness is measured accurately.
O-Rings and Actual Seal Components
Finished products such as O-rings, square rings, and custom-profile seals can be tested directly in dedicated fixtures. The axial cross-section thickness of the ring is measured before and after compression, typically at multiple circumferential positions to account for material non-uniformity.
Material Types
- Natural rubber (NR) and styrene-butadiene rubber (SBR) compounds
- Nitrile rubber (NBR) for fuel and oil-resistant sealing
- Ethylene propylene rubber (EPDM) for weather and hot-water applications
- Silicone rubber (VMQ) and fluorosilicone (FVMQ) for wide temperature ranges
- Fluoroelastomers (FKM) for aggressive chemical and high-temperature environments
- Thermoplastic elastomers (TPE, TPU) and cellular rubber materials
Testing Items
- Compression set at elevated temperatures under constant deflection
- Compression set at low temperatures, evaluating crystallization and glass transition effects per ISO 815-2
- Compression set under constant force (dead-load method) per ASTM D395 Method A
- Compression set after immersion in fluids, fuels, oils, or chemicals
- Compression set after thermal or oxidative aging preconditioning
- Complementary stress relaxation and compression stress decay measurements
Testing Methods
Constant Deflection Method (ASTM D395 Method B / ISO 815-1)
The specimen is compressed between two parallel plates to a fixed deflection, typically 25 percent of the original thickness, using rigid spacers or stops bolted into a compression fixture. The assembled fixture is placed in a circulating-air oven at the specified test temperature, commonly 70 °C, 100 °C, 125 °C, 150 °C, or 200 °C, for a defined period such as 22 h, 24 h, 70 h, 168 h, or 1000 h. After the exposure period, the specimen is removed, released, and allowed to recover at standard laboratory temperature for a defined interval, usually 30 minutes, before the final thickness is measured.
Calculation of Compression Set
The compression set under constant deflection is calculated as CB = (t0 − t2) / (t0 − t1) × 100 percent, where t0 is the original specimen thickness, t1 is the thickness of the spacer or the compressed height, and t2 is the final thickness after recovery. A result approaching 100 percent indicates essentially complete loss of elastic recovery, while values below approximately 20 to 30 percent are generally considered acceptable for many dynamic and static sealing applications, depending on the specification.
Constant Force Method (ASTM D395 Method A)
In Method A, a calibrated compression spring applies a constant force, typically 1.8 kN, to the specimen throughout the test. Because the applied stress is held constant rather than the deflection, this method better reflects service conditions such as static gaskets loaded by bolts, and the results are reported as the percentage decrease in thickness relative to the original specimen height.
Low-Temperature Method (ISO 815-2)
For low-temperature evaluation, the compressed specimen is cooled to the test temperature, released, and measured while still at low temperature using a special fixture with an integrated thickness gauge. The recovery percentage is recorded at defined intervals, such as 10 and 30 minutes after release, which reveals crystallization hardening or elastomeric recovery behavior below room temperature.
Equipment and Calibration
Testing requires precision compression fixtures with hardened parallel plates, calibrated spacers, a thickness gauge or micrometer accurate to 0.01 mm, a forced-convection aging oven with uniform temperature control, and, for low-temperature work, an environmental chamber. Accredited third-party laboratories performing these tests typically operate under ISO/IEC 17025 management system requirements to ensure traceable, reproducible results.
Applications
- Automotive sealing systems, including door seals, window channels, engine gaskets, and cooling system components
- Hydraulic and pneumatic O-rings, valve seals, and cylinder packings
- Aerospace seals and gaskets requiring long-term reliability under extreme thermal cycling
- Medical device components such as syringe stoppers, septa, and valve diaphragms
- Electronics enclosures, EMI shielding gaskets, and waterproofing seals
- Construction profiles, glazing seals, bridge bearings, and pipe gaskets
- Material qualification, incoming material inspection, batch release testing, and failure analysis
FAQ
What does a high compression set value indicate?
A high compression set percentage means the material has permanently deformed under load and cannot recover its original thickness. In practical terms, this signals a risk of seal leakage, loss of mounting pressure, or degradation of cushioning performance over time.
What is the difference between ASTM D395 Method A and Method B?
Method B compresses the specimen to a constant deflection held by fixed spacers, making it the most widely used method for material comparison. Method A applies a constant force through a calibrated spring, which better simulates bolted gasket loading conditions where the applied stress remains constant as the material creeps.
Why is the recovery time after release specified so precisely?
Elastomers continue to recover for hours after the load is removed. Early measurements show artificially high set values because the initial recovery is rapid, while the permanent component only becomes distinguishable after a standardized interval such as 30 minutes. Consistent recovery timing is therefore essential for comparable, reproducible results.
How is compression set different from stress relaxation?
Compression set measures residual geometric deformation after release, whereas stress relaxation measures the decay of sealing force while the material remains compressed. The two phenomena are related, and specifications often combine both to fully characterize long-term sealing performance.
How can compression set performance be improved?
Improvements are achieved by selecting inherently elastic polymer grades, optimizing the cure system to avoid over- or under-crosslinking, incorporating low-set filler systems, adding antioxidants and antiozonants, and ensuring proper post-curing, particularly for silicone and fluorocarbon elastomers.
Which standards govern compression set testing?
The primary standards include ISO 815-1 and ISO 815-2, ASTM D395, DIN ISO 815, JIS K 6262, and GB/T 7759. These standards define specimen geometry, deflection levels, temperature exposure, recovery conditions, and the calculation formulas used to report results.