Compression set is the permanent deformation that remains in a flexible material after a compressive force is removed. When you squeeze a rubber gasket, foam pad, or elastomer seal for an extended period and then release it, the material may not fully return to its original shape. The percentage of that unrecovered thickness is the compression set. A value of 0% means the material bounced back completely; 100% means it stayed fully compressed and didn’t recover at all.
How Compression Set Works
Every elastic material has some ability to spring back after being squeezed. Over time, though, the internal structure of the material begins to change under sustained pressure. Molecular chains slip, chemical bonds rearrange, and the material gradually “learns” its compressed shape. When you finally release the pressure, part of that deformation is locked in permanently.
The standard test method, ASTM D395, measures the ability of rubber compounds to retain their elastic properties after prolonged compressive stress. In practice, a sample disc of known thickness is compressed by a specific amount (typically 25%), held at a set temperature for a defined period (commonly 22 or 70 hours), then released and allowed to recover before being measured again.
The Calculation
There are two common ways to express compression set, referred to as Method A and Method B.
Method A compares the original height to the height after compression and recovery. It’s a straightforward ratio: how much thickness did the material lose compared to where it started?
Method B is more widely used for sealing applications. It compares the unrecovered thickness to the amount the sample was actually compressed during the test. The formula is: (original height minus recovered height) divided by (original height minus compressed height), multiplied by 100. This gives you a percentage that directly reflects how much of the applied squeeze was permanently lost.
For example, if you compress a 10 mm disc down to 7.5 mm (a 25% squeeze) and it only recovers to 9 mm after release, the Method B compression set is (10 – 9) / (10 – 7.5) x 100 = 40%. That means the material permanently lost 40% of the deflection that was applied to it.
What Affects Compression Set
Two factors dominate: temperature and time. Higher temperatures accelerate the chemical and physical changes inside the material that cause permanent deformation. Research on hydrogenated nitrile rubber aged in simulated oil and gas environments at 130°C and 150°C for up to nine months found that temperature imposed a greater effect on compression set than aging time alone. Both factors are interchangeable to some degree, meaning a short exposure at high temperature can produce similar results to a longer exposure at moderate temperature.
The internal structure of the material matters just as much. Materials with a denser network of molecular cross-links resist compression set more effectively. Studies on EPDM rubber have demonstrated that the higher the cross-linking density, the lower the compression set. This is why the curing process during manufacturing plays such a critical role in final performance. An under-cured rubber compound will show significantly worse compression set than the same compound fully cured.
Chemical exposure, humidity, and the presence of oils or solvents can also degrade a material’s recovery ability over time by attacking those cross-links or softening the polymer chains.
Typical Values for Common Materials
Compression set values vary widely depending on the rubber compound, hardness, and test conditions. Data from standardized O-ring testing at 25% squeeze gives a useful comparison across common sealing materials:
- Nitrile (NBR), 70 hardness: max 20% compression set. At 100°C, a standard NBR compound showed 12.8% after 22 hours and 22.7% after 70 hours, illustrating how time under load increases the set.
- EPDM, 70 hardness: max 25-30% depending on the compound grade and temperature rating.
- Silicone, 70 hardness: max 40% at 200°C. Silicone tolerates a much wider temperature range (down to -60°C and up to 220°C) but tends to show higher compression set values than nitrile or EPDM at comparable conditions.
Harder compounds generally show higher compression set. A 90-hardness nitrile rubber maxes out at 30%, compared to 20% for the same family at 70 hardness. This tradeoff between stiffness and recovery is one of the key decisions in material selection for seals and gaskets.
Why It Matters for Seals and Gaskets
Compression set is one of the most important performance metrics for any application where a flexible material must maintain a seal over time. An O-ring in a hydraulic fitting, a gasket between pipe flanges, or a door seal on an oven all rely on the material pushing back against the surfaces it contacts. As the material takes a permanent set, it loses that push-back force, and the seal weakens.
This is especially critical in applications with temperature cycling. If a seal compresses at high operating temperature and then the system cools, the material may have lost enough thickness that it no longer fills the gap, leading to leaks. Choosing a material with low compression set at the expected service temperature is the primary way to prevent this kind of failure.
Compression Set vs. Stress Relaxation
These two terms describe related but distinct phenomena, and confusing them is common. Compression set measures the permanent change in shape (strain) after the compressive force is removed. Stress relaxation measures the gradual loss of internal push-back force (stress) while the material remains compressed.
A seal could have low compression set, meaning it recovers its shape well, but still suffer from high stress relaxation, meaning it lost most of its sealing force while it was compressed. Both matter for long-term sealing performance. Compression set tells you whether the material will recover its dimensions. Stress relaxation tells you whether it was still doing its job while it was squeezed. For static seals that stay compressed for years, stress relaxation is often the more relevant measurement, but compression set remains the more commonly tested and reported value.