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Compression Set

The percentage of permanent deformation remaining in an elastomer after it has been compressed under controlled conditions and then released — the single most critical property for sealing applications. Measured per ASTM D395: Method B (constant deflection, most common — specimen compressed 25% for 22-70 hours at test temperature, then released and measured after 30-minute recovery). Formula: CS = (t₀ - tᵢ) / (t₀ - tₛ) × 100%, where t₀ = original thickness, tᵢ = recovered thickness, tₛ = spacer thickness. Lower values = better recovery = better seal performance. Typical values at 100°C/22h: NR 15-30%, EPDM 15-35%, NBR 20-40%, FKM 15-30%, silicone 20-40%. A seal with high compression set loses its sealing force over time and eventually leaks. Post-cure dramatically reduces compression set in FKM and silicone (by 30-50%). Temperature and time are the main enemies — compression set always increases with higher temperature and longer duration. Per ASTM D395 and ISO 815. This is the first property to specify when selecting O-ring and gasket materials.

What you need to know

  • The percentage of permanent deformation remaining in an elastomer after it has been compressed under controlled conditions and then released — the single most critical property for sealing applications.
  • Measured per ASTM D395: Method B (constant deflection, most common — specimen compressed 25% for 22-70 hours at test temperature, then released and measured after 30-minute recovery).
  • Formula: CS = (t₀ - tᵢ) / (t₀ - tₛ) × 100%, where t₀ = original thickness, tᵢ = recovered thickness, tₛ = spacer thickness.
  • Lower values = better recovery = better seal performance.
  • Typical values at 100°C/22h: NR 15-30%, EPDM 15-35%, NBR 20-40%, FKM 15-30%, silicone 20-40%.

Full definition

Compression set is a crucial property of elastomers, particularly in sealing applications, where the ability to maintain a seal over time is vital. It refers to the percentage of permanent deformation that remains in an elastomer after it has been compressed under controlled conditions and subsequently released. This property is quantified through standardized testing methods, most commonly ASTM D395: Method B, where a specimen is compressed to 25% of its original thickness and held for a period ranging from 22 to 70 hours at a specified temperature. After this compression period, the specimen is allowed to recover, and its thickness is measured after a 30-minute recovery period. The formula used to calculate the compression set is CS = (t₀ - tᵢ) / (t₀ - tₛ) × 100%, where t₀ represents the original thickness, tᵢ is the recovered thickness, and tₛ is the spacer thickness. Lower compression set values indicate better recovery characteristics, which translates to improved sealing performance.

In practice, different elastomers exhibit varying compression set values at elevated temperatures, such as 100°C over 22 hours. For instance, natural rubber (NR) typically exhibits a compression set of 15-30%, while EPDM ranges from 15-35%. Nitrile rubber (NBR) can have a compression set of 20-40%, and fluorocarbon elastomers (FKM) often fall between 15-30%. Silicone elastomers usually show a compression set of 20-40%. A seal with a high compression set will gradually lose its sealing force, leading to potential leaks as the material fails to return to its original shape.

Post-curing treatments can significantly enhance the performance of elastomers such as FKM and silicone, reducing their compression set by 30-50%. It is crucial to note that temperature and time are significant factors affecting compression set; higher temperatures and prolonged exposure lead to increased compression set values. Therefore, when selecting O-ring and gasket materials, compression set should be the first property to specify to ensure optimal sealing performance over the intended service life.

What you need to know

  • What you need to know: Compression set is critical for sealing applications, indicating how well an elastomer can return to its original shape after compression.
  • Measured per ASTM D395, Method B, specimens are compressed to 25% for 22-70 hours at a specified temperature.
  • Typical compression set values at 100°C/22h: NR 15-30%, EPDM 15-35%, NBR 20-40%, FKM 15-30%, silicone 20-40%.
  • A high compression set can lead to seal failure due to loss of sealing force over time.
  • Post-curing can improve compression set values by 30-50%, enhancing performance in applications.

Formula

CS = (t₀ - tᵢ) / (t₀ - tₛ) × 100%

Industrial applications

  • 1Used in O-rings and gaskets for automotive and aerospace applications where sealing integrity is critical.
  • 2Employed in industrial machinery seals to prevent fluid leaks in hydraulic systems.
  • 3Utilized in HVAC systems for sealing ducts and preventing air leakage.
  • 4Applied in consumer products such as seals in household appliances to maintain efficiency.

Common mistakes

  • ✕Failing to account for temperature effects, which can lead to incorrect material selection for sealing applications.
  • ✕Neglecting post-curing processes, which can significantly enhance the compression set performance of elastomers.
  • ✕Using elastomers with high compression set values in critical sealing applications, risking seal failure.
  • ✕Overlooking the recovery time during testing, which can lead to inaccurate compression set measurements.
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Pro tip

Always specify compression set values when selecting elastomers for sealing applications to ensure longevity and performance.

Technical standards

  • ASTM D395: Standard Test Methods for Rubber Property—Compression Set
  • ISO 815: Rubber, vulcanized or thermoplastic—Determination of compression set

Suppliers of industrial rubber in Mexico

AguascalientesVer Hule Industrial
Ciudad JuárezVer Hule Industrial
Ciudad de MéxicoVer Hule Industrial
GuadalajaraVer Hule Industrial
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Applicable standards

ASTM D395ISO 815.
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