Rubber Compression Set Explained: The Critical Seal Performance Metric

Rubber Compression Set Explained

Published: 2026-05-25 | Reading time: 7 minutes

What Is Compression Set?

Compression Set (CS) is the percentage of a rubber specimen's original deflection that is not recovered after a specified compression period at a specified temperature. In simple terms: you compress a rubber sample to a fixed thickness, hold it at a specified temperature for a specified time, release it, and measure how much of the compression it "remembers." A low CS (approaching 0%) means the rubber springs back almost completely -- excellent for seals. A high CS (approaching 100%) means the rubber stays permanently deformed after compression -- catastrophic for seals.

The standard calculation:

CS (%) = [(t₀ - tᵣ) / (t₀ - tₛ)] x 100

Where:

• • t₀ = original thickness of the specimen before compression
• • tᵣ = recovered thickness 30 minutes after releasing compression
• • tₛ = spacer thickness (the thickness to which the specimen is compressed, typically 75% of t₀ for 25% compression)

Example calculation: A 10.00 mm specimen compressed to 7.50 mm (25% compression) that recovers to 9.50 mm: CS = [(10.00 - 9.50) / (10.00 - 7.50)] x 100 = (0.50 / 2.50) x 100 = 20% -- a fairly good result for a general-purpose seal.

Why Compression Set Matters -- The Seal Failure Mechanism

An O-ring or gasket works by exerting a continuous sealing force against the mating surfaces. This force comes from the rubber's elastic recovery -- its tendency to push back against the compression it was installed under. When compression set occurs, the rubber permanently deforms, and the sealing force decays.

The failure sequence:

1. Seal installed at 20-25% compression -- initial sealing force established

1. Over time at service temperature, physical and chemical relaxation processes reduce the elastic recovery

1. Sealing force progressively decreases as compression set increases

1. When compression set reaches approximately 80-90% (sealing force approaches zero), the seal leaks

1. Failure may occur even before leakage -- a seal with >80% CS may not survive a system pressure surge or temperature cycle

Physical Mechanism -- Two Simultaneous Processes

Two distinct processes occur simultaneously when rubber is held under compression at elevated temperature:

1. Physical Relaxation (Partially Recoverable)

Polymer chains under strain undergo conformational rearrangement -- they reptate (snake) through entanglements to adopt lower-energy configurations that relieve the imposed stress. This is a physical process involving chain movement through entanglements, not breaking or forming chemical bonds. Over time (minutes to hours), the stress decays even in the absence of any chemical change.

• • At room temperature: physical relaxation is slow (hours to days) because chain mobility is limited
• • At elevated temperature (>Tg + 30°C): chain mobility increases exponentially, and physical relaxation accelerates
• • This component is partially recoverable -- if the rubber is heated unconstrained (e.g., during post-curing), some physical relaxation may recover
• • Contribution to total CS: roughly 30-50% at moderate temperatures

2. Chemical Relaxation (Irrecoverable)

Two competing chemical processes change the network structure:

Chain scission: Thermal energy or oxidative attack breaks polymer backbone bonds or crosslinks. Fewer crosslinks = less elastic recovery force. Broken chains have no memory of their pre-deformation configuration.

New crosslink formation: Radical recombination creates new crosslinks between chains in their compressed (deformed) configuration. These new crosslinks "lock in" the deformed state -- the rubber has chemically adapted to being compressed. When released, these new crosslinks resist expansion back to the original shape.

The balance between chain scission and new crosslinking determines the net effect:

• • New crosslinking dominates in NR, SBR, NBR, EPDM, CR -- compression set from network rearrangement
• • Chain scission dominates in IIR (butyl) -- compression set from molecular weight reduction
• • Both contribute in HNBR and FKM at their upper temperature limits

Chemical relaxation is irrecoverable -- no post-treatment can reverse chemically locked-in deformation.

Test Methods

Test Procedure Detail (ASTM D395 Method B)

1. Measure original thickness (t₀) of each specimen (standard: 12.5 mm or 6.3 mm thick disc, 29 mm or 13 mm diameter)

1. Assemble specimens between parallel steel plates with spacer bars (tₛ) that enforce the specified compression (typically 75% of original thickness)

1. Place the compressed assembly in an oven at the specified test temperature for the specified duration

1. Remove from oven, disassemble, and allow specimens to recover for 30 minutes at room temperature (23±2°C) on a thermally non-conductive surface

1. Measure recovered thickness (tᵣ)

1. Calculate CS

Critical Test Variables

Material CS Values Comparison

What Determines CS Performance?

Cure System -- The Single Largest Factor

Filler Type and Loading

Antioxidant System

Heat-resistant antioxidants (TMQ, ZMTI, MBI, diphenylamine derivatives) protect against oxidative chain scission and crosslinking during the CS test. Without adequate antioxidant protection, oxidative hardening (new crosslinks formed in the compressed state) dramatically increases CS.

Post-Curing

Post-curing (a secondary bake after molding) provides three benefits for CS:

1. Completes residual cure: Certain crosslinking reactions complete slowly. Post-curing drives them to completion, stabilizing the network before it enters service.

1. Removes volatiles: Low-molecular-weight species (unreacted curatives, decomposition byproducts) act as internal plasticizers that increase physical relaxation. Post-curing drives them out.

1. Relaxes molded-in stresses: The physical relaxation component that would occur in service (and contribute to CS) is partially induced during post-curing in the unconstrained state.

Four Core Strategies for Low CS

1. Peroxide Cure (When Compatible with Polymer)

C-C crosslinks (bond energy ~350 kJ/mol) are far more thermally stable than C-Sx-C polysulfidic crosslinks (~150 kJ/mol). Switching from sulfur to peroxide cure typically reduces CS by 30-50% at the same test temperature. This is the single most effective formulation change.

Limitations: Peroxide cure is not optimal for NR and SBR (inefficient due to few allylic hydrogens; chain scission competes with crosslinking). EPDM, HNBR, and Silicone are ideally suited for peroxide cure.

2. High-Structure Carbon Black

Using higher-structure grades (N330 instead of N550, N550 instead of N774) reduces CS by increasing the bound rubber content. More polymer chains are physically immobilized on the carbon black surface and cannot undergo the conformational rearrangement that contributes to physical relaxation.

3. Post-Curing -- Always for Critical Seals

A secondary bake (e.g., 150°C x 4h for EPDM, 200°C x 16-24h for FKM) completes residual cure, removes volatiles, and stabilizes the network. This can reduce CS by an additional 10-30%, depending on material. For FKM, post-curing is not optional -- uncured FKM has very high CS; post-curing is essential to achieve specified CS values.

4. Heat-Resistant Antioxidant Package

For service temperatures above 100°C, standard antioxidants (TMQ, 6PPD) provide inadequate protection. Add ZMTI (zinc 2-mercaptotoluimidazole) or MBI (2-mercaptobenzimidazole) at 0.5-1.5 phr for superior high-temperature oxidation resistance. ZMTI is particularly effective in NBR and EPDM at 120-150°C.

CS Acceptance Criteria by Application

Low-Temperature Compression Set

Standard CS testing is performed at elevated temperature. However, for seals operating at low temperatures (-20°C to -60°C), low-temperature CS (ISO 815-2) is equally important. At low temperatures:

• • Chain mobility is drastically reduced
• • Physical relaxation is slow (which is good for CS)
• • BUT: if the temperature approaches Tg, the material is glassy and may not seal at all
• • Low-temperature CS testing typically shows lower numerical values than high-temperature CS -- the dominant concern is glass transition, not permanent set

Inquiry & Technical Support

Nanjing Yuhang Rubber has extensive experience in low-compression-set compound design for critical sealing applications. For material recommendations, CS testing, and compound development: Products | Materials | Contact