How Are Silicone Rubber Seals Tested for Durability?

Silicone rubber seals are tested for durability by measuring how well they retain their shape, flexibility, strength and sealing performance after exposure to compression, temperature changes, repeated movement, weathering and contact with relevant substances.

There is no single test that can prove a seal will perform reliably in every application. A static gasket fitted to an indoor enclosure experiences different stresses from a door seal that is compressed thousands of times or an external profile exposed to sunlight, rain and frost.

Durability testing therefore needs to reflect the function of the seal and the environment in which it will operate.

Common assessments include compression set testing, tensile strength testing, elongation testing, hardness measurement, accelerated heat ageing and resistance checks for weather, ozone, ultraviolet light or chemicals. Finished parts may also be subjected to dimensional inspection, leak testing, repeated compression cycles and practical trials within the actual assembly.

The most useful test programme begins with a clear understanding of the expected service conditions.

Which properties determine the durability of a silicone rubber seal?

A durable silicone rubber seal must do more than remain physically intact. It must continue performing its intended sealing function.

A seal may still look acceptable while no longer applying enough pressure to close a gap. It may become harder after heat exposure, stretch out of position or develop small cracks at corners and fixing points.

The properties most commonly associated with durability include:

• Compression recovery
• Resistance to permanent deformation
• Tensile strength
• Elongation at break
• Tear resistance
• Hardness stability
• Resistance to temperature and weathering
• Dimensional consistency
• Compatibility with fluids and cleaning agents

Not every application requires the same emphasis. Compression recovery may be critical for a cabinet gasket, while tear resistance may matter more for a seal that is repeatedly stretched during installation.

Durability depends on the application

A seal that lasts for years in one assembly may fail quickly in another.

For example, a silicone strip used as a static seal between two bolted plates may remain under a consistent load with little movement. A similar strip fitted to an access door may be compressed and released every day.

The second seal experiences repeated mechanical cycling, possible friction and a greater chance of contamination or accidental damage.

The surrounding design also matters. Sharp metal edges, uneven clamping, excessive compression and poor retention can shorten the life of an otherwise suitable material.

Testing should therefore evaluate both the silicone compound and the way the finished seal behaves within the application.

How is compression set tested?

Compression set is one of the most important measures for seals and gaskets.

It describes how much permanent deformation remains after rubber has been compressed for a defined time and then released. In simple terms, the test indicates how well the material recovers towards its original thickness.

A seal with poor compression recovery may gradually flatten and stop pressing firmly against the mating surface. This can allow water, air, dust or process material to pass through the joint.

What happens during a compression set test?

A sample of silicone rubber is measured and then compressed by a specified amount between rigid plates or within a test fixture.

The compressed sample is held for a set period, often at an elevated temperature. It is then released and allowed to recover for a defined time before being measured again.

The difference between its original thickness and recovered thickness is used to calculate the compression set.

The exact specimen dimensions, compression level, temperature and test duration depend on the relevant test method and application requirements.

The result is usually expressed as a percentage. A lower figure generally indicates better recovery under the stated test conditions.

However, the number should not be interpreted without context. A result achieved at room temperature cannot automatically be compared with one produced after prolonged heat exposure.

Why compression set matters in sealing applications

A seal relies on stored elastic force. When compressed, it pushes against the adjoining surfaces and closes the path through which contaminants could travel.

If the material remains permanently flattened, that force reduces.

This can create several problems:

• Reduced contact pressure
• Leakage after temperature cycling
• Gaps caused by movement or tolerance variation
• Greater dependence on fastener load
• More frequent replacement

Compression set testing is particularly relevant for enclosure gaskets, flange seals, door strips and other components that remain under load for extended periods.

Static tests do not show everything

A standard compression set test provides useful material data, but it does not fully reproduce real service conditions.

Actual seals may experience fluctuating compression, vibration, uneven loading and repeated opening. The mating surfaces may also expand, contract or distort.

For critical applications, static material tests should be combined with testing of the finished seal in a representative assembly.

What do tensile strength and elongation tests reveal?

Tensile testing measures how a material behaves when stretched.

A shaped test specimen is pulled apart at a controlled rate until it breaks. The equipment records the force required and how much the sample stretches.

Two common results are tensile strength and elongation at break.

Tensile strength indicates the maximum pulling stress the rubber can withstand. Elongation at break shows how far it can stretch before rupture.

Why tensile strength matters

Silicone seals are not always exposed to direct pulling forces during service, but tensile strength still provides valuable information about the compound’s mechanical integrity.

A seal may be stretched during installation, pulled around a corner or placed under tension when retained in a groove. Weak material may split at joints or narrow sections.

Tensile testing can also reveal changes caused by ageing. A sample may be tested before and after heat, chemical or weather exposure.

A large reduction in tensile strength may indicate that the material has become more vulnerable to mechanical damage.

Why elongation matters

Elongation shows how flexible and extensible the material is before failure.

High elongation can be useful where a seal must bend, stretch or accommodate movement. However, a high elongation figure alone does not guarantee good long-term performance.

A seal must also recover after deformation. If it stretches but does not return adequately, it may become loose or move out of position.

Elongation results should therefore be considered alongside hardness, tensile strength and compression recovery.

Changes after ageing

Testing the same material before and after accelerated ageing can show whether it becomes brittle, weaker or less flexible.

For example, heat-aged specimens may be compared with unaged specimens. The change in tensile strength and elongation helps indicate how the material may respond to prolonged temperature exposure.

This does not provide an exact service-life prediction, but it supports material comparison and specification.

How is rubber hardness tested?

Hardness testing measures the resistance of the rubber surface to indentation.

For silicone seals, hardness affects how easily the material compresses, conforms to surfaces and resists deformation.

A softer seal may close around irregularities with relatively low force. A harder seal may provide more resistance to compression but require stronger fasteners, catches or clamping systems.

Measuring hardness consistently

A hardness tester uses a spring-loaded indenting point that is pressed against the rubber surface.

The reading depends on the depth of indentation. Different hardness scales and instruments may be used depending on the material and specimen thickness.

The sample should be thick and flat enough to produce a reliable reading. Testing too close to an edge or on a curved finished seal may give inconsistent results.

For this reason, hardness is often measured on a standard test sheet produced from the same compound as the finished product.

Why hardness stability matters

Hardness can change after prolonged exposure to heat, weather or chemicals.

If a seal becomes harder, it may no longer conform to the mating surface. Closing force can increase, and small surface irregularities may no longer be filled.

If it becomes softer, it may extrude from the joint, compress excessively or lose dimensional stability.

Comparing hardness before and after ageing tests can help assess whether the material is likely to retain suitable flexibility.

How are seals tested for heat resistance?

Heat ageing tests expose silicone rubber to a controlled elevated temperature for a specified period.

After exposure, the material is inspected and tested again. Changes in hardness, tensile strength, elongation, dimensions and appearance can then be measured.

Silicone rubber is often selected for elevated-temperature applications, but performance still varies between grades and compounds.

Accelerated heat ageing

Accelerated ageing uses higher temperatures to produce material changes within a shorter testing period.

This allows manufacturers and engineers to compare compounds without waiting years for normal service exposure.

However, accelerated testing has limitations. Increasing temperature may change the type of degradation rather than simply making the same process happen faster.

The results should therefore be used as comparative evidence rather than an exact promise that a seal will last for a specific number of years.

Continuous and intermittent temperatures

A seal may tolerate brief temperature peaks that would not be appropriate for continuous exposure.

Testing should reflect whether the application involves a steady temperature, repeated cycles or occasional short-term extremes.

Thermal cycling may be particularly important. Repeated heating and cooling can cause surrounding metal or plastic components to expand and contract.

The seal must continue compensating for these dimensional changes without cracking, becoming detached or losing compression.

Testing the finished assembly

Where possible, the complete sealed assembly can be subjected to temperature cycling.

The enclosure, panel or joint is heated and cooled while being inspected for leakage, movement or loss of contact.

This approach can identify problems that material samples alone may not reveal, such as uneven clamping or different expansion rates between components.

How is weather resistance assessed?

External silicone rubber seals may be exposed to sunlight, ozone, rainfall, moisture, frost and atmospheric pollution.

Weather resistance testing evaluates how the material responds to these conditions.

Some tests focus on a single factor, such as ozone exposure. Others use accelerated weathering equipment that combines light, heat and moisture.

Ultraviolet and light exposure

Ultraviolet light can cause many materials to fade, harden, crack or lose mechanical strength.

Accelerated light exposure uses controlled lamps to simulate aspects of sunlight. Samples may be inspected for colour change, surface cracking and changes in mechanical properties.

Silicone generally offers useful resistance to ultraviolet exposure, but pigmentation, compound formulation and operating conditions can affect the result.

A coloured seal may show visible change before its mechanical properties become unsuitable. In some applications, appearance matters as well as sealing performance.

Ozone resistance

Ozone can attack some elastomeric materials, particularly when they are stretched.

Ozone testing places specimens in a controlled atmosphere, sometimes while they are held under strain. The surface is then inspected for cracking.

Even small cracks can become significant in a seal because they provide a route for water or air and may grow under repeated movement.

Moisture and water exposure

Water resistance may be assessed by immersing samples, spraying finished assemblies or exposing seals to cycles of condensation and drying.

The test may measure changes in mass, hardness, dimensions and strength.

For an enclosure seal, a practical water-ingress test can be more relevant than immersion of a material specimen. Water may enter through corners, joints, fixing holes or areas of uneven compression.

The complete design should therefore be considered.

How are seals tested for chemical compatibility?

Silicone rubber is suitable for many applications, but it is not resistant to every oil, solvent, fuel, cleaning chemical or process fluid.

Chemical compatibility testing exposes samples to the relevant substance for a controlled time and temperature.

The material is then assessed for swelling, shrinkage, softening, hardening, discolouration and loss of strength.

Volume and mass change

A rubber sample may absorb a liquid and increase in size or weight.

Swelling can alter the fit of the seal, increase friction and reduce mechanical strength. In a confined groove, an enlarged seal may be damaged when the assembly moves.

Shrinkage can be equally problematic. A seal that becomes smaller may pull away from corners or lose contact with the mating surfaces.

The degree of change is often measured after immersion and compared with the original dimensions or mass.

Mechanical property changes

A seal may show limited visible swelling but still become weaker or softer.

Tensile strength, elongation and hardness can be tested after chemical exposure to reveal these changes.

The test conditions should match the real application as closely as practical. Concentration, temperature and exposure duration can all influence compatibility.

A material that withstands occasional splashes may not be suitable for continuous immersion.

Cleaning and sterilisation chemicals

Seals used in food, pharmaceutical or laboratory equipment may be exposed repeatedly to cleaning and sterilisation processes.

A single exposure test may not represent the effect of hundreds of cycles.

Repeated cleaning trials can help determine whether the seal gradually hardens, swells, loses colour or develops surface damage.

The complete cleaning process should be considered, including chemical concentration, temperature, contact time and rinsing.

How is tear resistance tested?

Tear resistance measures how easily an existing cut or notch can grow through the rubber.

This property is important where seals contain sharp internal corners, bolt holes, narrow sections or areas that are stretched during installation.

A test specimen with a defined shape or notch is pulled until the tear propagates. The required force is measured.

Why seal geometry affects tearing

A thick, simple strip may resist damage well, while a more complex profile made from the same compound could tear at a thin lip or sharp corner.

This is why material test results must be considered alongside component design.

Rounded transitions can reduce stress concentration. Installation tools and groove edges should also be checked for sharp points that could nick the seal.

A small installation cut may later grow during compression or movement.

Inspecting fabricated gaskets

Cut gaskets should be inspected around bolt holes, internal openings and joints.

Rough cutting, incomplete corners or narrow material sections can create weak points.

Consistent fabrication helps reduce variation between parts and prevents premature tearing during fitting.

How are seals assessed for repeated movement?

Some seals are compressed once and remain static. Others move during every operating cycle.

Dynamic testing may repeatedly compress, flex, stretch or rub the seal to simulate service.

The number of cycles, movement speed, temperature and applied load should reflect the intended application.

Repeated compression tests

A seal can be placed in a fixture that compresses and releases it for a defined number of cycles.

Its thickness, hardness and sealing force may be measured at intervals.

This can reveal gradual loss of recovery that would not appear in a single compression set test.

Repeated compression is particularly relevant for access doors, lids, valves and equipment that undergoes frequent servicing.

Friction and abrasion tests

A seal that slides across another surface may experience abrasion.

Testing can reproduce rubbing contact and measure material loss or visible damage.

Surface finish, lubrication, contact pressure and movement direction all influence wear.

In some cases, changing the seal geometry or installation position can reduce abrasion more effectively than selecting a harder material.

Flex fatigue testing

Thin lips, bellows and flexible seal sections may be repeatedly bent to assess fatigue resistance.

Cracks often begin at stress concentrations or where the material is constrained.

Testing the finished profile is valuable because the exact shape determines how strain is distributed.

How are finished seals checked before supply?

Material test data confirms the properties of the silicone compound, but finished products also require dimensional and visual inspection.

A seal made from a suitable material can still fail if its dimensions are incorrect or its surface contains defects.

Dimensional inspection

Critical dimensions may include width, thickness, internal diameter, external diameter, profile features, hole positions and overall length.

Tolerances should be agreed according to the manufacturing process and functional requirements.

Overly tight tolerances can add unnecessary cost, while loose tolerances may lead to inconsistent sealing.

The most important dimensions are those that control fit, compression and retention.

Visual inspection

Finished seals can be checked for cuts, tears, surface contamination, incomplete joins, air pockets, distortion and colour variation.

Not every visual mark affects performance, but defects in sealing surfaces or thin sections can create weak points.

Inspection criteria should distinguish between cosmetic variation and functional defects.

Join and corner testing

Fabricated gaskets may contain bonded or joined corners.

These areas can be tested through pulling, bending or compression to confirm adequate integrity.

The joint should remain aligned and should not create a raised section that changes local compression.

Batch consistency

Manufacturers may test samples from production batches to confirm that hardness, dimensions and other properties remain within specification.

Traceability can help identify the compound, production date and inspection records associated with a supplied part.

This can be important where seals are used in regulated, safety-related or repeat manufacturing applications.

Why should seals be tested within the real assembly?

Laboratory tests provide controlled and repeatable data, but a seal functions as part of a wider system.

The housing, groove, fasteners, hinges, surface finish and closing force all affect performance.

A practical assembly test can identify issues such as:

• Uneven compression around corners
• Panel distortion between fasteners
• Seal movement during closure
• Excessive closing force
• Leakage through joints or fixing points
• Abrasion against sharp edges
• Changes after repeated opening

For critical applications, prototypes can be installed and tested under realistic temperature, pressure, movement and environmental conditions.

Leak and ingress testing

Finished assemblies may be checked using air pressure, vacuum, water spray or immersion methods, depending on the application.

The test should reflect the type of ingress being controlled.

A seal that prevents dust entry may not be designed to resist water pressure. A weather seal may manage rain but not full immersion.

The required performance level should be defined before testing.

Pressure testing

Where the seal contains internal pressure, the joint may be tested at the normal operating pressure and an appropriate higher test pressure.

Safety procedures are essential, particularly with compressed gases.

Pressure testing should consider temperature because seal properties and internal pressure can change as the assembly heats.

Field trials

A field trial places the component in actual operating conditions for a defined period.

This can reveal unexpected factors such as cleaning practices, installation variation, operator handling or vibration patterns.

Field testing is especially useful when replacing an existing seal that has failed repeatedly. The new design can be monitored to confirm that it addresses the true cause.

How should durability test results be interpreted?

Test results should be compared with agreed acceptance criteria rather than viewed in isolation.

A tensile strength value may be technically high, but tensile strength may not be the main requirement for a static gasket. A low compression set result may be more relevant.

The test method, specimen type, temperature, duration and conditioning should always be recorded.

Results produced using different methods are not necessarily directly comparable.

Test data is not a service-life guarantee

Accelerated tests can indicate relative performance and identify unsuitable materials. They cannot account for every variable in real operation.

Service life may be affected by assembly tolerances, contamination, maintenance, pressure spikes and accidental damage.

Durability should therefore be managed through a combination of correct material selection, suitable design, testing and planned inspection.

Testing should match the risk

A simple non-critical dust seal may need basic material confirmation and dimensional inspection.

A seal used in temperature-sensitive processing equipment may require extensive ageing, chemical compatibility and assembly testing.

The cost and depth of testing should reflect the consequences of failure.

Silco Products supplies silicone rubber gaskets and other sealing components for industrial and specialist applications. Clear information about operating conditions can help determine which material properties and checks are most relevant.

Frequently Asked Questions

What is the most important durability test for a silicone seal?

There is no single test that applies to every seal. Compression set is particularly important for many static gaskets, while dynamic seals may require repeated movement, abrasion or fatigue testing. The test programme should reflect the actual application.

Does a low compression set mean a seal will last longer?

A low compression set suggests that the material recovers well after controlled compression. This is beneficial for sealing, but service life also depends on temperature, chemical exposure, movement, dimensions and installation.

Can accelerated ageing predict the exact life of a silicone seal?

Accelerated ageing helps compare materials and assess changes after heat or environmental exposure. It does not provide an exact lifespan because real operating conditions include variables that may not be reproduced in the laboratory.

Should finished seals be tested as well as the raw material?

Yes, particularly in critical applications. Material tests confirm compound properties, while finished-part testing can identify dimensional problems, weak joints, uneven compression and issues created by the surrounding assembly.

How often should silicone seals be inspected in service?

Inspection frequency depends on the application, environment and consequences of failure. Seals exposed to frequent movement, chemicals, weather or high temperatures may require more regular checks than static indoor components.

Silicone rubber seals are tested for durability through a combination of material assessments and finished-product trials.

Compression set testing measures recovery after prolonged loading. Tensile strength, elongation and tear tests assess mechanical integrity. Heat ageing, weather exposure and chemical immersion show how properties may change in demanding environments.

Finished seals should also be checked for dimensional accuracy, surface defects and joint quality. Where performance is critical, testing the seal inside the actual assembly provides the most realistic indication of whether it will maintain contact and control leakage.

The appropriate testing programme depends on how the seal will be used. Temperature, movement, compression, fluids and environmental exposure should be defined before the material and test requirements are selected.

Silco Products supplies silicone rubber seals, gaskets and custom components for industrial, commercial and specialist requirements. To discuss dimensions, material properties or an application experiencing premature seal failure, contact the team for practical support.

Phone: 01903 732088
Email: sales@silcoproducts.co.uk
Find out more: https://siliconerubberproducts.co.uk/
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