compressionset

Compression set is a material property that describes the permanent deformation that remains after a material—typically an elastomer or polymer—has been compressed for a specified period under a defined load and temperature, and then allowed to recover. It is usually expressed as a percentage of the original thickness, calculated as 100 × (t0 − tR)/t0, where t0 is the original thickness and tR is the recovered thickness after the test. A higher compression set indicates greater permanent deformation and weaker ability to regain shape after compression.

Compression set is especially important for seals and gaskets, where the material must recover after assembly or thermal cycling to maintain a reliable seal. Different materials such as NBR (nitrile), FKM (fluoroelastomer), and EPDM show distinct compression-set behavior, influenced by formulation, crosslink density, fillers, and aging. Temperature, duration of compression, degree of deflection, and environmental exposure (chemicals, oil, ozone) tend to increase compression set.

Testing for compression set is standardized in methods such as ASTM D395 and ISO 815, which outline specimen geometry, deflection, loading and recovery conditions, and the calculation of percent compression set after a defined recovery period. These measurements help engineers select materials and estimate service life for static or quasi-static sealing applications.

Compression set is distinct from creep: creep refers to continued deformation under load, whereas compression set is the permanent deformation remaining after the load is removed and recovery occurs.