Compression Set of Elastomeric Materials

Compression set test fixture - exploded viewCompression set testing measures the ability of rubber to return to its original thickness after prolonged compressive stresses at a given temperature and deflection. As a rubber material is compressed over time, it loses its ability to return to its original thickness. This loss of resiliency (memory) may reduce the capability of an elastomeric gasket, seal or cushioning pad to perform over a long period of time. The resulting permanent set that a gasket may take over time may cause a leak; or in the case of a shock isolation pad, the ability to protect an accidentally dropped unit may be compromised.

Compression set results for a material are expressed as a percentage. The lower the percentage, the better the material resists permanent deformation under a given deflection and temperature range. Compression set results can be confusing in that they are calculated different ways under different methods of testing.

Materials of Choice Where Compression Set Resistance is Critical

These are some of the leading materials utilized by Stockwell Elastomerics when compression set resistance is and stress relaxation are important consideration:

  • Silicone foam (HT-800, HT-820, HT-870, BF1000, F-12, BF-2000) – View data sheets
  • Silicone sponge (R10480 Soft, R10480 Medium) – View data sheets
  • PORON, urethane foam (4701-30, 4701-40, 4701-50, 4701-60, 4790-92) – View data sheets

Contact Stockwell Elastomerics for assistance with material selection or engineering questions.

Compression set test fixture

Measuring Compression Set of Solid Elastomers

Most compression set testing follows ASTM D395, Test Method B. Solid elastomer specimens 0.25″ thick x 0.52″ diameter are compressed to 0.177″ thick using a device as diagrammed, for 22 to 70 hour periods at elevated temperature in an air circulating oven. Compression set represents the percent of deflection that did not return. That means that if a sample neoprene disc, tested for 70 hours at 212°F, then allowed to cool for 30 minutes, measured .221″ thick, the Compression Set would be 40%. Since the disc was deflected .073″, yet returned .044″ (back up to .221″) indicating that the amount of the distance compressed that it did not return is 40%.

Solid Silicone

To run the math:

Compression Set (ASTM D395 Method B) = {(orig. thickness – final thickness) / (orig. thickness – space bar thickness)}*100

40% = {(0.25″ – 0.221″) / (0.25″ – 0.177″)}*100

Measuring Compression Set of Sponge Elastomers

A similar example can be shown for a cellular material. A closed cell neoprene sponge specimen 0.50″ thick is compressed 50% for 22 hours at 158°F, the load is released and a 24 hour conditioning period is allowed. If the final thickness is .438″ thick, the Compression Set would be 25%. Since the specimen was deflected .250″, yet returned .188″ (back up to .438″) indicating that the amount of the distance compressed that it did not return is 25%. For ASTM D1056, although sponge materials are compressed 50%, the Compression Set calculation for ASTM D395 Method B is still used.

Super Resilient Silicone Foam

To run the math on this example:

25% = {(0.500″ – 0.438″) / (0.500″ – 0.250″)}*100

Related: Stress Relaxation of Gasket Materials

Stress relaxation of gasket materials is the measure of how much the gasket reduces its “push back”. While there may be exceptions, it is fair to assume a material that takes a high compression set, will also relax significantly. This push back is critical to long term sealing since it is the pressure sealing out fluids and dust.

Most materials will relax to some degree but some are better than others. Silicone materials are very resistant to the influence of heat relative to other elastomers; this combined with its molecular structure give silicone very good stress relaxtion properties. Silicone will relax some but offer very good long term sealing pressure when the proper material is selected.

Contact Stockwell Elastomerics

Contact Us for assistance with material selection or engineering questions.