Compression Set vs Stress Relaxation: Why Seals Lose Sealing Force

Two measurements, two different questions

Elastomer seals work by being deformed in a gland so they press against mating surfaces. Over time, their viscoelastic structure rearranges and chemical aging may alter the polymer network. The result can be less contact force and leakage even though the seal still looks compressed. Compression set and stress relaxation describe different parts of this behavior.

Compression set measures how much deformation remains after a compressed specimen is released and allowed a defined recovery period. Stress relaxation measures how compressive force declines while the specimen remains at a fixed deformation. A compound may show acceptable dimensional recovery yet lose substantial force during service, or retain a visible set while still providing enough residual force for a forgiving static joint. Neither value alone predicts leak-free life.

What compression set tells you

In a typical compression-set test, a specimen of known thickness is compressed to a controlled deflection, aged for a specified time and temperature, released, conditioned, and measured again. The result expresses retained deformation relative to the imposed deflection. Lower set generally indicates better recovery under those test conditions.

The details matter: specimen geometry, compression level, exposure medium, aging time, temperature, cooling procedure, and recovery time can all change the result. Values from different methods or conditions should not be compared as if they were interchangeable. A data-sheet number generated in hot air may not represent a seal immersed in fluid, thermally cycled in a metal gland, or held for much longer.

What stress relaxation tells you

Stress relaxation begins with a fixed strain and tracks force over time. The initial force reflects material stiffness, geometry, and imposed squeeze. Physical rearrangement can reduce force relatively quickly, while oxidation, additional crosslinking, chain scission, or fluid interaction may dominate longer exposures. Because sealing depends on contact pressure, retained force can be more directly relevant than recovered thickness for many static joints.

Force can also change reversibly with temperature. A seal and its metal gland expand at different rates; fluid swelling may increase gland fill; cooling may stiffen the compound or open the joint. Measuring only at room temperature after exposure can hide what happened during the hot or cold portion of a cycle.

Why sealing force disappears

Time and temperature

Viscoelastic relaxation proceeds with time and generally accelerates as temperature rises. Short, high-temperature screening can compare candidates, but extrapolating directly to years of variable service is risky unless the aging mechanism remains the same. Thermal cycling adds changing squeeze and recovery demands that constant-temperature testing does not reproduce.

Fluid interaction

A process fluid may swell and soften a compound, extract ingredients and shrink it, or drive chemical degradation. Swelling can temporarily raise contact pressure while weakening extrusion resistance; subsequent drying can leave insufficient squeeze. Evaluate the complete media mixture, concentration, contamination, and cleaning sequence. The chemical compatibility guide is a useful screening starting point, not a substitute for exact-compound testing.

Gland and joint behavior

Initial squeeze that is too low leaves little reserve after relaxation. Excessive squeeze can create high assembly force, overfill the groove after thermal expansion or swelling, and accelerate damage. Surface waviness, flange rotation, fastener relaxation, pressure opening, and extrusion clearance can lower local contact pressure even when average compression appears correct. Use the O-ring design guide to review geometry as part of the system.

Compound formulation

Base polymer matters, but it does not determine performance by itself. Crosslink type and density, fillers, processing aids, plasticizers, post-cure, and manufacturing history influence relaxation and set. Hardness is not a direct measure of retained sealing force. The Shore hardness guide explains why a hardness value cannot stand in for modulus, compatibility, or long-term recovery.

How the failure appears in service

Loss of sealing force may appear as leakage after a hot dwell, seepage only after shutdown and cooling, or a seal removed from the gland with a pronounced flat. A flattened cross-section supports a set-related hypothesis but does not establish it. Check joint movement, fluid attack, surface condition, fastener load, pressure spikes, and extrusion. A seal can leak before it develops a dramatic visible flat if contact pressure falls below what the surface and pressure differential require.

Designing a useful qualification program

1. Define the service profile. Include actual media, pressure direction, minimum and maximum temperatures, dwell times, cycles, motion, shutdowns, and expected maintenance interval.
2. Use the real compound. Test the specified formulation, cure, hardness, and production condition rather than a generic polymer-family sample.
3. Represent the gland. Match squeeze, groove fill, hardware material, clearances, and surface condition. Include dimensional tolerances at adverse combinations.
4. Measure more than one property. Track sealing force or contact response alongside dimensions, hardness, mass or volume change, and visible damage.
5. Include combined exposures. Temperature, fluid, pressure, and cycling can interact; separate tests may miss the governing mechanism.
6. Set functional acceptance criteria. Define allowable leakage and minimum residual force or recovery based on the assembly, not an arbitrary universal percentage.

Practical ways to preserve contact

• Select a compound using long-duration data at representative temperature and media.
• Provide enough initial squeeze and surface conformity without overfilling the gland.
• Control extrusion gaps and use support elements where the design requires them.
• Account for flange and fastener behavior, thermal expansion, and pressure movement.
• Avoid treating a harder seal as an automatic cure; it may increase assembly force without improving long-term retention.
• Validate maintenance intervals with inspection and service evidence.

The central lesson is simple: dimensional recovery and retained force are related, but they are not the same property. Qualification should measure the behavior that controls leakage in the real assembly. For help defining representative conditions and candidate evidence, contact the sealing team with the compound, gland, media, and duty cycle.