ASTM D395

Standard Test Methods for Rubber Property — Compression Set — the critical test for evaluating how well an elastomer recovers its original thickness after being held in compression at elevated temperature, directly predicting sealing performance. Two methods: Method A (constant force — a fixed load compresses the specimen) and Method B (constant deflection — specimen compressed 25% of original thickness, most commonly specified). Procedure: compress specimens between parallel steel plates with spacers, place in oven at test temperature (typically 70, 100, 125, or 150°C per ASTM D2000 line call-out) for 22, 70, or 168 hours, remove from oven, cool in fixture, release, and measure recovered thickness after 30 minutes at room temperature. Calculation: CS(%) = (original - recovered)/(original - spacer) × 100%. Lower compression set = better recovery = better seal performance. Typical values at 100°C/22h: EPDM 15-30%, NBR 20-40%, FKM 15-25%, silicone 25-40%, NR 10-25%. Compression set is the #1 property determining long-term sealing reliability. Per ASTM D395-18. Equivalent: ISO 815. For FKM and silicone seals, post-curing dramatically reduces compression set (by 30-50%).

What you need to know

Two methods: Method A (constant force — a fixed load compresses the specimen) and Method B (constant deflection — specimen compressed 25% of original thickness, most commonly specified).

Procedure: compress specimens between parallel steel plates with spacers, place in oven at test temperature (typically 70, 100, 125, or 150°C per ASTM D2000 line call-out) for 22, 70, or 168 hours, remove from oven, cool in fixture, release, and measure recovered thickness after 30 minutes at room temperature.

Calculation: CS(%) = (original - recovered)/(original - spacer) × 100%.

Lower compression set = better recovery = better seal performance.

Full definition

ASTM D395 defines the standard test methods for evaluating the compression set of elastomers, an essential property that indicates how well a rubber material can recover its original thickness after being subjected to compression at elevated temperatures. This test is crucial in determining the long-term sealing performance of rubber materials in various industrial applications, particularly in environments where high temperatures and pressures are present. The standard outlines two primary methods for measuring compression set: Method A, which applies a constant force to the specimen, and Method B, which compresses the specimen to a constant deflection, typically to 25% of its original thickness. Method B is the more commonly specified method due to its replicable and relevant results in practical applications.

To perform the test, rubber specimens are placed between parallel steel plates with spacers to ensure uniform compression. The assembly is then subjected to a specified temperature, often 70°C, 100°C, 125°C, or 150°C, for a defined duration, which can range from 22 hours to 168 hours, depending on the material and application requirements. After the exposure period, the specimens are allowed to cool in the fixture before being released and measured for their recovered thickness after a 30-minute cooling period at room temperature. The compression set percentage is calculated using the formula: CS(%) = (original - recovered)/(original - spacer) × 100%, where a lower percentage indicates better recovery and, hence, superior sealing performance.

Typical results for various elastomers at 100°C for 22 hours include EPDM with a compression set of 15-30%, NBR ranging from 20-40%, FKM at 15-25%, silicone between 25-40%, and natural rubber (NR) achieving values of 10-25%. The compression set is recognized as the most critical property influencing the durability and reliability of seals in dynamic and static applications. Notably, post-curing processes can significantly enhance the compression set performance of FKM and silicone seals, reducing values by 30-50% and thereby improving their sealing capabilities in demanding environments. This standard is aligned with ISO 815, further emphasizing its global relevance in material testing and quality assurance.

What you need to know

What you need to know: ASTM D395 determines the compression set of elastomers, critical for sealing applications.

Two methods exist: Method A (constant force) and Method B (constant deflection, typically 25% compression).

Testing temperatures are usually set at 70°C, 100°C, 125°C, or 150°C for periods of 22 to 168 hours.

Typical compression set values for elastomers at 100°C/22h include EPDM 15-30%, NBR 20-40%, and silicone 25-40%.

Compression set is the primary factor influencing long-term sealing reliability in industrial applications.

Industrial applications

1Sealing applications in automotive engines, where high temperatures and pressures require reliable materials.

2Industrial machinery seals that must maintain integrity over prolonged exposure to heat.

3Aerospace components where elastomer reliability is critical under variable thermal conditions.

4Electrical insulation materials that need to withstand heat without compromising performance.

5Hydraulic systems where rubber seals must perform under high pressure and elevated temperatures.

Common mistakes

✕Not following the specified compression rates or times, leading to inaccurate results.

✕Using non-standard temperatures outside of ASTM D395 recommendations, which can skew performance data.

✕Failing to allow proper cooling time before measuring recovery, affecting the accuracy of test results.

ASTM D395

What you need to know

Full definition

What you need to know

Formula

Industrial applications

Common mistakes

Pro tip

Technical standards

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Applicable standards