Compression set is a critical performance property in elastomer engineering, especially for rubber components used in seals and gaskets. It directly determines whether a material can maintain sealing force over time. This property affects the performance of parts in various applications, such as automotive seals, industrial equipment, and medical devices. When selecting materials for custom parts, the compression set must be considered to ensure reliability over time.
What is Compression Set?
Compression set is the measure of a material’s permanent deformation after it has been compressed for a defined period under specified conditions. It quantifies how much of the original thickness is not recovered once the compressive load is removed. It is calculated based on the difference between the original thickness and the recovered thickness, divided by the amount of compression applied. In rubber materials, compression set occurs due to changes in the polymer structure during compression.
For example, in a hydraulic system, an O-ring is compressed to form a seal. If the material exhibits high compression set after long-term service, it may not recover its original shape, resulting in leakage.
How to Measure Compression Set?
Measuring compression set involves standardized procedures to ensure consistent results across different materials and applications. The most common standard is ASTM D395, particularly Method B (constant deflection), which is the most commonly used procedure for elastomers.
The sample is compressed to a fixed percentage of its original thickness, often 25%. The sample is placed in a fixture and held at a specified temperature for a set duration. After this period, the compression is released, and the sample is allowed to recover at room temperature for 30 minutes. The final thickness is measured, and the compression set is calculated using the formula: Compression Set (%) = [(Original Thickness – Recovered Thickness) / (Original Thickness – Compressed Thickness)] × 100%.
For example, if a rubber sample starts at 10 mm thick, is compressed to 7.5 mm, and recovers to 9 mm, the compression set is [(10 – 9) / (10 – 7.5)] × 100 = 40%. This indicates 40% permanent deformation.
A compression set in rubber can also be assessed using ISO 815, which is similar to ASTM D395 but may specify different conditions. Some tests incorporate aging, where samples are exposed to heat before compression to evaluate long-term effects.
Factors That Influence Compression Set
Several factors affect the compression set in rubber materials.
Polymer type
Different elastomers have different backbone chemistries and molecular mobility. Materials with greater thermal stability and stronger intermolecular forces typically resist permanent deformation more effectively.
Crosslink density
Crosslinking creates a three-dimensional network that resists flow and structural rearrangement. Higher crosslink density generally improves recovery, although excessive crosslinking can increase stiffness and brittleness.
Formulation additives
Fillers, plasticizers, and curing agents influence elasticity, thermal resistance, and long-term stability. These formulation variables directly affect compression set performance.
Compression level
Higher compressive strain increases stored internal energy. When molecular chains cannot fully recover, permanent deformation results.
Duration of load
The compression set is time-dependent. Even materials with good short-term recovery may gradually deform under sustained load.
Stress relaxation
Under constant deformation, internal stress gradually decreases due to molecular rearrangement. This relaxation contributes to reduced recovery.
Temperature Effects
Elevated temperature increases molecular motion and accelerates chemical degradation such as oxidation or chain scission, reducing elastic recovery and increasing permanent deformation.
Environmental Exposure
External media can alter material structure over time. Oxygen and ozone can degrade polymer chains. Oils and solvents may cause swelling or plasticization. Moisture and aggressive chemicals may also alter the crosslink structure and reduce elasticity.
Compression Set by Material
The table below summarizes typical compression set ranges, advantages, and recommended applications for common elastomers under standard test conditions.
| Material | Typical Compression Set Range | Key Advantages | Best Applications |
|---|---|---|---|
| Silicone (VMQ) | Moderate to high | Excellent temperature stability | High-temperature sealing, electronics |
| EPDM | Low to moderate | Good weather and aging resistance | Outdoor seals, water systems |
| NBR (Nitrile) | Moderate | Cost effective, oil resistant | Hydraulic seals, automotive components |
| FKM (Fluoroelastomer) | Low | Excellent chemical and heat resistance | High-performance industrial sealing |
| Neoprene (CR) | Moderate | Balanced mechanical properties | General industrial sealing |
Prevention and Mitigation Strategies
A compression set cannot be eliminated, but it can be controlled through appropriate engineering decisions.
- Choose a seal material that resists both operating and friction-generated heat.
- Select elastomer materials with a low compression set.
- Verify material compatibility with intended application environments.
- Use a more elastic polymer structure.
- Confirm the seal is not over-squeezed.
- Reduce excessive frictional heat build-up at the seal interface.
- Lower system operating temperature.
Conclusion
A compression set is a key factor governing the long-term sealing performance of elastomer components. Although permanent deformation cannot be completely eliminated, it can be significantly reduced by selecting appropriate materials, optimizing the design, ensuring consistent manufacturing processes (e.g., compression molding), and maintaining controlled operating conditions.
