An elastomeric seal earns its keep by pushing back. Squeeze it between two flanges and it should keep pressing outward for the life of the joint. Compression set measures how much of that push-back a material loses permanently, and it is often the property that decides when a seal starts to leak.

What Compression Set Is

Compression set is the permanent deformation left in a material after it has been held under a constant compressive load for a set time at a set temperature. Picture a rubber gasket clamped between two flanges for months. When the flanges come apart, if the gasket does not spring back to its original thickness, the deformation that remains is the compression set. It is reported as a percentage of the original deflection. A low value is good: it means the material kept its elasticity and will keep sealing. A value near 100 percent means the material has taken a permanent impression and no longer provides sealing force.

What Influences It

Compression set is not a fixed material constant. It is driven by several factors together:

• Polymer chemistry. Silicones and fluoroelastomers are known for very low compression set, which is why they dominate demanding sealing applications. General-purpose rubbers tend to sit higher.
• Temperature. Heat accelerates the chemical relaxation that causes permanent deformation, so a value measured at room temperature says little about behavior at 150 degrees C.
• Time under load. The longer the compression, the greater the set. Long-service seals need materials with very low set ratings.
• Cure and compound. A fully cured, well-formulated compound resists permanent deformation better than an undercured one. A post-cure step often improves the number measurably.
• Deflection level. Over-compressing a seal on assembly can worsen long-term set as much as heat does.

How It Is Measured

Compression set is tested to a recognized industry method, most commonly the ASTM standard for the property. A specimen is compressed to a defined deflection, typically 25 percent, and held for a specified time and temperature. The load is then released, the specimen recovers for a short fixed period, and the set is calculated as the ratio of remaining deformation to the original deflection. Zero percent is perfect recovery; 100 percent is total loss of elasticity. Always compare materials at the same test temperature and duration, since the ranking can change with conditions.

Practical Advice

• Specify by application, not by reputation. Choose a material on its compression-set data for the exact temperature and duration it will see in service, not on a generic “good for sealing” claim.
• Prioritize low set ratings. For seals and gaskets, a lower percentage means the material holds sealing force longer and resists leaks and premature failure.
• Use a proper post-cure. Where the process allows it, a post-cure raises crosslink density and improves set. Confirm the schedule with your material supplier.
• Control assembly deflection. Design the groove and the clamp load so the seal is compressed within its intended range, not crushed.
• Account for thermal cycling. A seal between dissimilar materials is also worked by differential expansion. Our guide on how CTE mismatch causes adhesive bond failure covers that added cyclic stress, which compounds compression set over time.

Where This Bites

Compression set shows up as slow fluid weeps around covers and housings, loss of environmental sealing on outdoor enclosures in renewable energy and transit equipment, and pressure decay in pneumatic and hydraulic joints. In each case the seal looks intact but no longer presses hard enough to close the surface profile. For joints where the seal also carries structural load, our comparison of which adhesive is stronger for heavy-duty repairs is a useful companion.

Email Us with your operating temperature, load duration, and fluid exposure, and Incure’s team can help match a material to the requirement.

Compression Set vs. Related Properties

Compression set is often confused with adjacent measurements, and specifying the wrong one leads to the wrong material:

• Compression stress relaxation measures how the sealing force decays over time while the seal stays compressed. It predicts leak onset more directly than compression set for a static seal, since a seal leaks when force drops, not when thickness is lost.
• Creep is continued deformation under constant load; compression set is the permanent part that remains after the load is removed.
• Tension set is the same idea under stretch rather than compression, relevant to elastic straps and boots.

For a long-life static gasket, ask the supplier for stress-relaxation data at your service temperature, not just a single compression-set number.

Reading a Data Sheet

A compression-set value is meaningless without its test conditions. “22 percent” could be an excellent result at 150 degrees C for 1000 hours or a poor one at 23 degrees C for 22 hours. Always record the temperature, duration, and deflection alongside the number, and compare candidate materials only at matched conditions. Where a data sheet lists a single value with no conditions, treat it as marketing rather than engineering data and request the full test report.

Working With Incure

Incure supplies high-performance elastomeric materials, including thermoplastic elastomers engineered for low compression set, along with technical support on material selection for seals, gaskets, and other components that must recover from sustained load.

Contact Our Team to discuss an elastomer selection question.

Visit www.incurelab.com for more information.