Compression Set: What It Is and Why It Affects Seal Life
An elastomeric seal may appear dimensionally correct, be manufactured from the specified material and provide effective sealing during the initial stages of operation. However, this does not necessarily mean that it will maintain the same performance over time.
When a seal remains compressed inside its groove for prolonged periods, the material may progressively lose part of its ability to recover its original shape. This phenomenon is commonly known as compression set, one of the most important parameters for evaluating the long-term behaviour of elastomers and sealing components.
The issue becomes particularly relevant when compression is combined with temperature, fluids, operating cycles and long periods of service. Under these conditions, a seal that appears intact may no longer be able to maintain sufficient contact pressure against the surfaces that need to be sealed.
What is compression set?
Compression set indicates the permanent residual deformation of an elastomeric material after it has been kept under compression for a defined period and subsequently released from the load.
Elastomers are used in seals precisely because they are capable of deforming under compression and, within certain limits, recovering their original geometry when the load is removed.
However, this recovery is not always complete.
During prolonged periods of compression, physical changes may occur and, particularly as temperature and exposure time increase, chemical changes may also reduce the material’s ability to return to its initial dimensions. ISO 815-1 uses this principle to determine the compression set characteristics of vulcanised or thermoplastic rubbers subjected to constant deformation.
A material with a low compression set retains a greater ability to recover elastically under the specified test conditions. A high value, on the other hand, indicates greater permanent deformation.
This parameter should not, however, be interpreted in isolation: time, temperature, applied deformation, specimen geometry, compound and test environment all have a direct influence on the result.
How is compression set measured?
The principle of the test consists of initially measuring the thickness of the specimen, compressing it under defined conditions and keeping it in that state for a specified period and at a specified temperature.
At the end of the test, the material is released from compression and, according to the prescribed method, measured again after a defined recovery period.
Compression set is then expressed as a percentage of the initially applied deformation:
Compression set (%) = (initial height − height after recovery) / (initial height − height under compression) × 100
In theoretical terms, a value of 0% corresponds to complete recovery from the applied deformation. As the value approaches 100%, the material retains an increasingly large proportion of the deformation and progressively loses its ability to recover.
It is important to note that a compression set value is meaningful only when the conditions under which it was obtained are also considered.
Comparing, for example, two values measured at different temperatures, exposure times or compression percentages may lead to unreliable conclusions. Test duration also affects the result: a short test and a prolonged test do not necessarily describe the same material behaviour.
Why elastic recovery is essential for sealing
A static seal works because it is deformed inside its groove and develops a contact force against the surfaces that need to be sealed.
It is therefore not enough for the component simply to occupy the available space. It must continue to maintain effective contact even after hours, months or years of operation.
If the elastomer accumulates permanent deformation, its ability to recover decreases.
As a result, the seal may become progressively less capable of adapting to dimensional variations in the groove, thermal expansion, vibrations or changes in operating conditions.
The result may be a gradual reduction in sealing effectiveness.
This phenomenon is particularly critical because it does not always cause immediate failure. A seal may work correctly during system start-up and begin to show micro-leakage only after prolonged operation.
For this reason, when a leak appears after a certain period of service, simply replacing the component with an identical seal may not address the actual cause of the problem.
Temperature and time change elastomer behaviour
Compression set is strongly dependent on temperature.
At elevated temperatures, ageing phenomena increase and chemical changes within the elastomer structure may become more significant. ISO 815-1 highlights how, at higher temperatures, chemical transformations increasingly contribute to permanent deformation.
This means that a material that performs adequately at room temperature may behave very differently when used close to its thermal limit or when exposed to heat for prolonged periods.
Time is equally important.
An assessment carried out after only a few hours does not automatically predict behaviour after thousands of hours in service. For this reason, in critical applications, material data must be interpreted by considering the expected service life, continuous operating temperature, possible temperature peaks and actual operating conditions.
The maximum temperature stated for a family of elastomers is therefore not, by itself, sufficient to determine seal life.
Material, compound and compression set
NBR, HNBR, EPDM, FKM, VMQ, FFKM and other elastomer families behave differently, but two compounds belonging to the same material family may also show different performance.
Behaviour depends not only on the base polymer, but also on compound formulation, curing system, additives, hardness, manufacturing process and the conditions in which the material is used.
For this reason, it is not correct to associate a specific compression set value with an entire material family in absolute terms.
An FKM compound, for example, may be considered for applications involving high temperatures or specific chemical requirements, while NBR may be fully suitable under less severe conditions. The selection must nevertheless always take into account the specific compound and the real operating profile of the application.
Using a material that appears more highly performing on paper does not automatically guarantee longer service life if geometry, compression, fluid or installation conditions are not consistent with the selected solution.
The fluid and temperature can also affect the result
A seal does not normally operate in air.
Oil, fuels, gases, water, steam, detergents, process fluids and chemical substances can alter the behaviour of the elastomer over time.
Interaction with the fluid may cause swelling, shrinkage, changes in hardness or modifications to the material’s mechanical properties. These effects can become even more significant as temperature increases. For this reason, values obtained from a test in air do not necessarily provide a complete description of how the seal will behave in the actual application.
Temperature also has a direct influence on the material’s response. At elevated temperatures, the elastomer may become softer and more deformable, which can initially improve its ability to conform to the surfaces but may also increase the risk of extrusion, relaxation, accelerated ageing and loss of mechanical properties. Heat can also intensify reactions with the fluid, accelerating swelling, degradation or changes in hardness.
At low temperatures, on the other hand, the material generally tends to become stiffer and lose elasticity. The seal may therefore have greater difficulty following movements, surface irregularities or dimensional variations. Under particularly cold conditions,
reduced elastic recovery can compromise contact and promote leakage, especially during start-up or after periods of inactivity. Repeated cycles between low and high temperatures can also generate additional stresses due to the different thermal expansion rates of the seal, housing and mating components.
In particular, the combination of loss of elastic recovery and material shrinkage can become critical for sealing continuity, because both phenomena may contribute to reduced contact with the surfaces. Likewise, excessive swelling can increase friction, hinder movement or promote damage during installation and operation.
In this case too, laboratory data must therefore be interpreted within the context of the complete system, taking into account the fluid, operating temperature, thermal peaks, exposure time and the actual pressure and movement conditions.
Seal compression and groove geometry
Compression set is not only a material selection issue.
Groove geometry determines how much the seal is compressed during installation. Inappropriate compression may alter component behaviour and create operating conditions that differ from those considered during the initial selection phase.
Insufficient compression may fail to generate the required contact pressure. Excessive deformation, on the other hand, may increase stress on the material and interfere with correct sealing behaviour.
For this reason, cross-section diameter, groove depth and width, tolerances, surface conditions and compression percentage must be evaluated together.
Changing the compound without checking these elements may simply shift the problem rather than solve it.
Compression set and stress relaxation are not the same thing
In advanced seal analysis, it is useful to distinguish compression set from compression stress relaxation.
Compression set primarily evaluates how much the material is able to recover dimensionally after being released from compression.
Stress relaxation, on the other hand, considers the reduction in force exerted by a compressed elastomer over time while deformation remains constant. ISO 3384-1 defines specific procedures for measuring the decrease in counterforce developed by a material maintained under compression.
The two phenomena are related, but they are not equivalent.
A seal must not only recover dimensionally, but also continue to exert sufficient force against the sealing surfaces.
For particularly critical applications, relying on a single parameter may therefore not be sufficient to predict the complete behaviour of the system.
When compression set becomes a service-life issue
Compression set becomes particularly important in applications where the seal remains compressed for very long periods, especially in the presence of elevated temperatures or severe operating conditions.
Certain signs may indicate the need to investigate this aspect more closely:
the seal works correctly after installation but begins to leak after a certain period;
periodic replacements reveal flattened components or limited recovery;
the problem becomes more evident as temperature increases;
after disassembly, the O-Ring clearly retains the shape of the groove;
replacing the seal with the same material reproduces the problem after a similar interval;
leakage appears mainly after thermal cycles, shutdowns or restarts.
The presence of one of these conditions does not automatically prove that compression set is the cause of the failure. It does, however, indicate that long-term elastic behaviour should be analysed together with material, groove geometry, chemical compatibility, pressure and operating conditions.
Problem solving must start from the real application
When a seal shows a progressive loss of performance, the question should not simply be: “Which material has the lowest compression set?”
The correct question is:
“Which combination of material, compound, geometry and operating conditions allows the required sealing performance to be maintained for the expected service life?”
The difference is substantial.
A laboratory value is a useful tool for comparing materials and compounds, but by itself it does not represent an automatic prediction of seal life.
Actual temperature, exposure time, fluid, pressure, groove geometry, compression, thermal cycles, tolerances and installation conditions must all be evaluated together.
This is the step that turns technical data into an application decision.
Tecnonext’s approach to seal evaluation
Tecnonext supports companies, technical departments and purchasing managers in the selection of seals and materials for industrial applications, including cases involving progressive leakage, reduced service life or unstable behaviour over time.
The analysis begins with the actual operating conditions: material currently in use, fluid, minimum and maximum temperature, pressure, application type, groove geometry, compression, operating cycles, expected service life and the way in which the problem occurs.
The objective is not simply to identify a compound with better nominal values, but to understand which factors are limiting sealing performance and evaluate a solution that is consistent with the complete system.
Compression set is therefore an important parameter, but it must be included within a broader assessment that considers material, geometry and operating conditions together.
For a technical discussion, an application assessment or an analysis of issues related to seal life, Tecnonext can be contacted at:
035 059 0010
Via Rudello, 11, 24067 Sarnico BG
FAQ – Compression set and seals
What does compression set mean in a seal?
Compression set indicates the permanent deformation that an elastomeric material retains after being compressed for a defined period and subsequently released from the load. A high value generally indicates a lower ability to recover from the deformation.
Does a low compression set always mean that the seal will last longer?
Not necessarily. Compression set is an important parameter, but service life also depends on temperature, fluid, pressure, groove geometry, compression, compound, operating cycles and installation conditions.
Why can a seal start leaking only after several months?
One possible cause is the progressive reduction in the material’s elastic recovery. Over time, contact pressure may decrease and make the seal less effective. Compression set, ageing, chemical compatibility and operating conditions should be analysed together.
Are compression set and stress relaxation the same thing?
No. Compression set concerns the dimensional recovery of the material after compression, while stress relaxation concerns the reduction over time of the force exerted by the material while it remains under deformation. Both may be relevant when evaluating seal behaviour.
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