Compression set —
what it is, how it's measured
and why it matters
Compression set is the permanent deformation an elastomer retains after being held under compressive load. The O-ring or gasket has not split, cracked or visibly failed — it has simply flattened slightly and not recovered. That small change is enough to cause a leak. Understanding compression set is the key to understanding why material choice matters for long-service sealing applications.
What compression set actually is
When you compress an elastomeric seal — an O-ring, a rubber gasket, any elastomeric sealing element — you are deforming it. The material squeezes into the groove or joint and the elastic recovery force pushes back against the joint faces, maintaining contact pressure. That contact pressure is what creates the seal.
In an ideal elastic material, removing the compressive load returns the material to exactly its original dimensions. In real elastomers, some of that deformation is permanent. The polymer chains within the material rearrange under sustained load, particularly at elevated temperature, and do not fully return to their original configuration when the load is removed. The material stays slightly flatter than it started.
This permanent deformation is compression set. It is expressed as a percentage of the original deflection that has been permanently lost.
A compression set of 0% means the material fully recovered — it returned to its original thickness. A compression set of 100% means the material stayed permanently deformed to the compressed dimension — no recovery at all. In practice, all elastomers fall somewhere between these extremes. Lower is better.
What it looks like in practice
The critical point is in the third stage. The seal is still in position. It has not failed visibly. But its cross-section is smaller than it was, which means the contact force it exerts against the sealing surfaces is lower than it was. If that contact force drops below the minimum needed to maintain a seal — against system pressure, against thermal cycling, against vibration — the joint leaks.
Removed intact does not mean reusable. Any elastomeric seal or gasket that has been in service under compressive load has accumulated compression set. It will not seal as effectively the second time. This is why elastomeric seals are treated as single-use items.
How compression set is tested
The standard test method is ISO 815-1 (Method A) for elastomers, with ASTM D395 as the equivalent US standard. The procedure is straightforward:
- A test piece is compressed to a defined percentage of its original thickness — typically 25% compression — using a calibrated spacer
- The compressed assembly is held at a defined temperature for a defined time period
- The assembly is released and the test piece is allowed to recover at room temperature for 30 minutes
- Final thickness is measured and compression set is calculated from the formula above
The test conditions — temperature and duration — are the critical variables. A material might show 10% compression set at 22 hours and 100°C, 20% at 70 hours and 100°C, and 20% at 70 hours and 150°C. All are valid data points, but they tell you different things. The conditions must match the intended service environment for the data to be meaningful.
Reading a datasheet: when you see compression set data, always note the test temperature and duration alongside the percentage. A low percentage at mild conditions is not equivalent to a low percentage at severe conditions. The DVGW W534 water immersion tests at 1000h, 2000h and 3000h are specifically relevant because they reflect extended service in water supply conditions — not short-duration laboratory conditions.
Why temperature makes it worse
Temperature accelerates compression set because it accelerates the molecular rearrangement within the polymer network. At ambient temperature, polymer chains move slowly and the material retains most of its elastic character. At elevated temperature, chain mobility increases and permanent rearrangement happens faster.
This is why the same material that shows 10% compression set at 22 hours and 100°C can reach 20% at 70 hours and 100°C and 20% at 70 hours and 150°C. Both temperature and time compound the effect. A seal that performs adequately at 80°C for a year may show measurable degradation at 120°C over the same period.
For heating systems, hot water distribution and steam applications, this is the deciding factor in material selection. A material with good short-term compression set data but poor long-term high-temperature data will degrade in service even if it appears fine initially.
Why curing system matters — peroxide vs sulphur
The curing system used to vulcanise an elastomer determines the type and stability of the cross-links holding the polymer network together. This directly affects compression set behaviour.
Sulphur curing creates cross-links through sulphur bridges between polymer chains. Sulphur bridges are effective at moderate temperatures but begin to break down and reform in permanently deformed configurations at sustained elevated temperature. The result is higher compression set over time at hot service conditions.
Peroxide curing creates direct carbon-to-carbon cross-links between polymer chains. C–C bonds are more thermally stable than sulphur bridges. Under the same sustained temperature and load, the peroxide-cured network resists permanent rearrangement more effectively — resulting in lower compression set over time.
At water-supply and heating temperatures, curing system can become a practical selection factor. The following example values from compound M534 (peroxide-cured EPDM, 70 Shore A) illustrate how compression-set data should be read against time, temperature and test medium:
| Test condition | Standard | Compression set |
|---|---|---|
| 22h at 100°C | ISO 815-1 Met. A | 10% |
| 70h at 100°C | ISO 815-1 Met. A | 20% |
| 70h at 150°C | ISO 815-1 Met. A | 20% |
| 1000h at 110°C in water | DVGW W534 | 10% |
| 2000h at 110°C in water | DVGW W534 | 15% |
| 3000h at 110°C in water | DVGW W534 | 19.5% |
In this example, after 3000 hours at 110°C in water, compression set is 19.5%. The DVGW W534 rows are especially relevant for water-supply selection because they test compression set after extended hot-water exposure, rather than only short-duration laboratory conditions.
Peroxide-cured EPDM is generally selected when better retention at sustained elevated temperature is required, because the curing network is more thermally stable than sulphur-cured systems. Long-duration hot-water data is therefore especially useful for water supply and heating selection.
What the numbers mean in a sealing context
Compression set percentage alone does not tell you whether a seal will fail. What matters is whether the remaining elastic recovery force is sufficient to maintain contact pressure against the joint faces under operating conditions.
Low compression set — good
The O-ring retains most of its original cross-section. Contact force against sealing faces remains high. The seal continues to function effectively over the service interval. Suitable for long-service applications, elevated temperature, sustained pressure.
High compression set — problematic
The O-ring has flattened significantly and not recovered. Contact force has dropped. The system may still hold pressure at steady state, but loses sealing effectiveness under thermal cycling, vibration or pressure fluctuation. Leak path opens progressively.
The threshold where compression set becomes a sealing problem depends on the groove design, the system pressure and the operating conditions. As a general reference, compression set values above 40–50% in service conditions typically indicate the seal is approaching the end of its effective service life — though this varies by application and gland geometry.
Compression set vs other failure modes
Compression set is one of several ways an elastomeric seal degrades over time. It is worth distinguishing it from related failure modes:
- Compression set — permanent deformation from sustained compressive load. The material is chemically intact but geometrically changed. Progressive, often invisible until the joint leaks.
- Chemical degradation — the polymer network breaks down from contact with incompatible media. Changes in hardness, swelling, surface cracking. Can accelerate compression set.
- Thermal degradation — sustained high temperature causes oxidation and polymer chain scission. Material becomes harder and more brittle. Different mechanism from compression set but often co-occurs.
- Ozone cracking — surface cracking from ozone attack, particularly on unsaturated rubbers like NBR. EPDM is ozone-resistant. Visible damage, distinct from compression set.
- Extrusion — under high pressure, the elastomer is forced into the gap between mating surfaces. A hardware issue (gap too large) as much as a material issue.
Compression set is insidious because it produces no visible damage. The seal looks fine. The joint looks assembled. The leak develops slowly as contact pressure drops below the sealing threshold, and the cause is not obvious without disassembling and measuring the seal cross-section against its original specification.
Practical implications — what to look for when specifying
When evaluating a sealing material for a temperature or long-service application, compression set data is one of the most informative values in the datasheet. Hardness and tensile strength are useful but relatively easy to match between compounds. Long-term compression set at elevated temperature is harder to optimise — it reflects the quality and stability of the cross-link network directly.
- Look for data at temperatures that reflect your actual service conditions, not just ambient or moderate temperature tests
- Look for data at extended durations — 70h, 1000h or longer — not just 22h which is the minimum standard test
- For water supply and heating applications, DVGW W534 water immersion compression set data at 1000h and beyond is the most relevant benchmark
- Compare peroxide-cured and sulphur-cured options at the same temperature and duration if both are available — the difference becomes visible at sustained elevated temperature
- A material with good short-term data but no long-duration data at service temperature has not been characterised for long-service applications — that is a gap, not a positive indicator
Compression set is one of the most important long-term performance indicators for an elastomeric seal — and the one most often overlooked in standard material selection.
Hardness gets specified. Tensile strength gets checked. Compression set at service temperature and duration gets skipped because the number is not on the front page of the datasheet. But it is the number that determines whether a seal that looks fine after one year still seals effectively after five. Low compression set under sustained heat and load is the difference between a seal you replace on schedule and one you replace because it failed.