Compression set is a measure of the deformation that remains in a rubber material after it has been compressed for a specified period under a defined set of conditions (e.g., temperature, pressure). It quantifies how much a rubber material loses its ability to return to its original shape after being subjected to compressive stress.

How Compression Set Is Measured

Compression set is typically determined by subjecting a rubber specimen to a specified amount of compression (usually around 25% or more of its original thickness) and maintaining that compression for a certain duration at a given temperature. After the period of compression is over, the specimen is released, and the permanent deformation is measured as the percentage of the original thickness that the rubber did not recover.

The formula is:

Compression Set (%)=Original Thickness/Deformed Thickness​×100

For example, if a rubber sample is compressed by 25% of its thickness and, after release, it recovers 90% of its original thickness, the compression set would be 10%.

Factors Affecting Compression Set in Rubber

1. Rubber Type – Different rubbers have different inherent resistance to compression set. For instance:

   • Silicone rubber typically exhibits excellent recovery, with low compression set.

   • Nitrile rubber may show higher compression set, especially when exposed to oils.

   • EPDM rubber tends to have moderate compression set resistance.

2. Temperature – Higher temperatures generally increase the compression set. Rubber materials may soften under heat, making it more difficult for them to return to their original shape after compression.

3. Compression Duration – The longer the rubber is compressed, the higher the likelihood of permanent deformation, leading to an increased compression set.

4. Pressure – Higher compressive forces typically lead to greater permanent deformation and a higher compression set.

5. Aging and Environmental Exposure – Exposure to environmental factors like UV radiation, ozone, and oxidative aging can degrade the rubber, leading to an increase in compression set over time.

Importance of Compression Set in Rubber Materials

1. Seal Integrity – In sealing applications, low compression set is crucial because it ensures that the rubber maintains its ability to form a reliable seal after being compressed, such as in gaskets, O-rings, and automotive seals. If the rubber fails to recover fully, the seal can become ineffective, leading to leaks.

2. Durability – A high compression set means the rubber will experience permanent deformation over time, reducing its lifespan. This is important in applications like engine mounts and vibration dampers, where materials need to maintain their shape and function over long periods under compression.

3. Performance in Dynamic Applications – For products like tire sidewalls or rubber bushings, a low compression set is important for maintaining performance characteristics, such as flexibility, over time.

4. Energy Efficiency – In sealings or gaskets, if the material doesn’t recover to its original shape, it can lead to higher energy losses due to leaks or loss of sealing force, especially in critical systems like compressors or pumps.

Applications Sensitive to Compression Set

• O-Rings & Gaskets – For seals in engines, industrial equipment, and pipelines, low compression set is critical for maintaining sealing integrity under pressure.

• Automotive Components – Rubber components like bushings, grommets, and engine mounts benefit from low compression set to ensure they continue to perform effectively over time.

• Electrical Insulation – Compression set can affect the performance of cable seals and insulating materials.

• Footwear – Rubber soles need to retain their shape and flexibility, especially in terms of their shock-absorbing properties, so compression set plays a role in ensuring long-lasting comfort and durability.

Improving Compression Set Resistance

To reduce compression set, rubber materials can be formulated with:

• Plasticizers or softeners to enhance flexibility and recovery.

• Vulcanization or crosslinking agents that help increase rubber's elasticity.

• Fillers like carbon black to provide additional strength and improve dimensional stability.

In summary, compression set is an essential property to consider when selecting rubber for sealing, durability, and performance in various applications. Rubber that exhibits a low compression set is better at maintaining its function over time, while high compression set indicates a material that may lose its performance capacity under sustained compression.