by Sara McCaslin Sara McCaslin No Comments

From Datasheet to Service Life: A Better Framework for Seal Qualification

In our previous article, we discussed the problems that can arise in high-performance seals when engineers focus too much on compressive stress when choosing a sealing solution. That leads to the question: How does one achieve a better characterization of seal behavior? Consider the following methods.

Compression Stress Relaxation (CSR) testing (ASTM D6147 / ISO 3384)

This approach measures retained sealing force over time under constant strain. CSR testing directly models what happens in a real gland rather than just measuring recovery after the fact. ISO 3384-1:2024 covers constant-strain conditions; ISO 3384-2:2019 extends this to alternating compression cycles, useful for simulating thermal cycling effects on effective gland compression. 

CSR is better aligned with force retention, but standard test results alone cannot predict installed life. A standard specimen compressed between parallel plates does not reproduce:

  • O-ring curvature and nonuniform strain
  • Actual gland squeeze and volume fill
  • Surface texture and groove corner effects
  • Internal pressure loading and pressure direction
  • Clearances, backup rings, and extrusion gaps
  • Media exposure, permeation, or fluid aging
  • Thermal gradients, dwell periods, and transient cycles
  • Dynamic motion or system vibration

CSR data can be an important input to a calibrated service-life assessment, but it is not a standalone life prediction. Correlation requires representative temperature, media, deformation, hardware compliance, pressure, and validation testing. 

Percent Retained Sealing Force (%RSF) 

This particular approach serves as a more directly actionable output than a compression set percentage. It answers the question engineers actually care about: how much sealing force is left after time and temperature have done their work. 

However, be aware that a normalized percentage can hide an inadequate initial sealing load. For example:

  • Material A retains 80% of an initially inadequate force.
  • Material B retains 65% of a substantially higher initial force.

Material B may retain more absolute force and provide the better seal. The design-relevant question is whether the absolute residual contact pressure/force exceeds the minimum required sealing threshold by a margin.

When using %RSF, report force decay or force retention together with the initial counterforce and a defined minimum acceptable residual force or contact-pressure threshold. A percentage alone can be misleading if the starting force differs between materials or geometries.

Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis (DMA) evaluates an elastomer’s viscoelastic behavior by applying a small oscillating deformation while changing temperature, frequency, strain amplitude, or time. Its primary outputs are storage modulus, loss modulus, and tan δ\deltaδ. Here is what they tell us:

  • Storage modulus indicates the material’s elastic stiffness
  • Loss modulus reflects energy dissipation through internal molecular motion
  • tan δ\deltaδ, the ratio of loss modulus to storage modulus, indicates damping behavior

For sealing applications, DMA is especially useful for identifying temperature ranges where stiffness changes rapidly. A decrease in storage modulus at elevated temperature can reduce the contact force generated at a given gland squeeze and may reduce extrusion resistance under pressure. At low temperature, approaching a glass-transition or secondary-transition region can make the material less conformable, reducing its ability to accommodate surface roughness, dimensional variation, and thermal contraction. DMA can therefore help compare compounds with similar room-temperature hardness but that behave differently across the operating temperature range.

Keep in mind that DMA is a material-screening and diagnostic tool, not a direct seal-performance or service-life test. Test reports should state the test mode, frequency, strain amplitude, temperature ramp, specimen geometry, and the convention used to identify transitions, since reported transition temperatures can vary with method. Use DMA alongside compression stress relaxation, media aging, extrusion resistance, pressure cycling, and functional leakage testing on representative gland hardware.

Media-Representative Testing

Media-representative testing evaluates the seal while exposed to the actual service fluid or gas (or a technically justified simulant) rather than just relying solely on dry compression testing. The exposure should reproduce the relevant fluid composition, concentration, contaminants, temperature, pressure, dwell time, and fluid-aging or replenishment conditions. This is actually key because a material rated broadly as “compatible” may behave differently when it comes to real mixtures containing additives, water, cleaning agents, dissolved gases, oxidation products, or contaminants.`

Fluid exposure can cause issues such as shrinkage, swelling, plasticization and softening, extraction of compounding ingredients, hardening, cracking, embrittlement, and loss of mechanical properties. Such changes may alter both the installed squeeze and the force the seal retains over time. 

Standard liquid-exposure methods such as ASTM D471 evaluate changes in volume, mass, hardness, tensile strength, and elongation after immersion. This provides highly useful compatibility-screening data but does not, by itself, demonstrate leak-tight service performance.

For demanding applications such as those under discussion, it is wise to evaluate media-aged specimens using the properties that connect exposure to function:

  • Compression stress relaxation or retained counterforce
  • Dimensional change
  • Visible damage
  • Leakage performance in representative gland hardware. 

Where pressure, thermal cycling, or dynamic motion is relevant, it is also prudent to conduct exposure and functional testing under those combined conditions rather than aging the material separately in a static immersion vessel. The objective is to establish that the seal retains adequate contact pressure, dimensional stability, and structural integrity for the required service life—not merely that it survives a short, dry compatibility test.

Gland-Representative Geometry 

Material properties do not directly translate into installed seal performance unless the test configuration reflects the actual gland. Gland geometry controls the installed squeeze, stress distribution, contact width, available void volume, and deformation under pressure.

 Relevant factors include seal cross-section, groove depth and width, corner radii, installed stretch, squeeze, gland fill, clearance gap, pressure direction, and any backup-ring configuration. 

Excessive stretch can reduce effective cross-section and squeeze, while excessive gland fill can leave insufficient room for thermal expansion, media-induced swell, and pressure-driven deformation.

Representative testing should include nominal dimensions and tolerance extremes for the seal and hardware, including 

  • Cross-section variation
  • Groove dimensions
  • Concentricity
  • Runout
  • Thermal expansion
  • Pressure-induced housing deflection. 

A nominal-only test can miss low-squeeze conditions that reduce contact-force margin or high-fill conditions that increase assembly damage and extrusion risk.

Surface condition and pressure loading should also reflect the real assembly. Surface roughness, waviness, machining lay, scratches, edge breaks, pressure direction, and worst-case clearance gap can all affect leakage and extrusion. For high-pressure service, test the actual anti-extrusion configuration and verify functional leakage performance in representative hardware—not only material behavior in a parallel-plate fixture.

Focus on the Application

High-performance seals require more attention to the actual operating conditions than the compressive set. The methods we covered in this article can assist engineers in choosing the right sealing solution for high-performance applications.

Back-Up Rings
by Sara McCaslin Sara McCaslin No Comments

The Case for Polymer BURs in High-Pressure Seals

Engineers are intensely frustrated when an O-ring is specified for a design and it still fails. This is likely because the O-ring needed a backup ring (BUR), not because the O-ring was not specified correctly. In fact, polymer BURs remain an underrated but critical component in many systems, in industries ranging from food to oil & gas. 

This article looks at the main problem BURs solve, the benefits of opting for polymer BURs, and commonly used polymers. 

O-Rings and Extrusion

The clearance gap is the space between mating parts where the O-ring sits. Under high pressure, the O-ring is going to behave more like a fluid than a solid. It will get forced, or flow, into the clearance gap. When the O-ring extrudes into the clearance gap, it can lead to two different failure modes: nibbling and spiral failure.  Nibbling looks like something has taken multiple small bites from the O-ring. Spiral failure refers to tearing and cracking around the circumference of the O-ring.

This type of failure is irreversible and inevitably leads to weeping or a catastrophic leak. For hydraulic systems operating at high pressure, it will either manifest as a slow leak (weeping) or a catastrophic failure.

How BURs Solve O-Ring Extrusion (and More)

A backup ring (BUR) is installed against the O-ring, downstream of the pressure, to ensure it stays within the clearance gap. In short, a BUR physically closes the clearance gap, giving the seal nowhere to migrate. This not only prevents unwanted leaks but can extend the life of a seal from cycles to years.

BURs are often used in high-temperature or high-pressure applications. BURs can also help to evenly distribute pressure in applications involving varying clearance gaps or pressure fluctuations. Additionally, BURs help to reduce the effects of wear and friction on O-rings and serve as a thermal barrier to protect them from excessive heat.

Benefits of Using Polymer BURs

There are several benefits to using polymer BURs. 

Extrusion prevention – They effectively fill the clearance gap between mating parts, stopping the primary elastomer seal from being forced (extruded) into that gap under pressure, which would otherwise cause premature seal failure.

Extended pressure and temperature range – By protecting the O-ring from extrusion, polymer BURs allow the overall seal assembly to withstand higher pressures and wider temperature swings than an O-ring alone could handle.

Reduced wear on the primary seal – BURs absorb mechanical stress and abrasion so that the O-ring experiences fewer issues with friction and deformation, thus extending its service life.

Design flexibility with looser tolerances – Because the BUR compensates for extrusion risk, designers can use larger clearance gaps. The use of larger clearance gaps, in turn, simplifies the machining process and reduces manufacturing cost.

Compatibility across dynamic and static applications – Different polymers (PTFE, PEEK, nylon, polyurethane, acetal) let engineers tailor the backup ring’s flexibility, hardness, and chemical resistance to match whether the seal is static, reciprocating, or rotating.

Corrosion and chemical resistance – Unlike metal backup rings, polymers can be chosen that will not corrode or react with process fluids. This facet is particularly important in chemical, oilfield, or food-grade applications.

Lightweight and cost-effective – Polymers are generally cheaper and lighter than metal without sacrificing performance in most standard pressure ranges needed for BURs.

Ease of installation – Many polymer backup rings (especially PTFE) are flexible enough to be installed without requiring the use of special tools. This is in contrast to rigid metal rings that may require more complex assembly procedures.

System reliability – By preventing extrusion-related blowouts, polymer BURs reduce the risk of unplanned downtime, leaks, or catastrophic seal failure in critical systems like hydraulics, pneumatics, and downhole tools.

In short, polymer BURs let engineers push seal assemblies harder in terms of more pressure, wider gaps, tougher environments. And they do so while protecting the seal from the system.

Backup Ring Materials

When choosing a BUR material, ensure it is harder than the seal material to effectively prevent migration. They must also have lower coefficients of expansion than the seal and are engineered to be both rigid and tough. 

Polymer BURs are often used in place of elastomeric versions because they offer significantly better resistance to extrusion, deformation, and chemical exposure, as well as reduced long-term creep under high pressure.

Commonly used polymers include ….

Material Temp Range Chemical Resistance Friction Extrusion Resistance Cost Best Use Case
PTFE (virgin/filled) -70°F to 575°F Excellent
nearly universal		 Very Low Good
better filled		 Moderate High-temp, chemically aggressive, low-friction dynamic seals
PEEK -50°F to 590°F Excellent Moderate Excellent
highest strength		 High High-pressure, large extrusion gaps, oilfield/downhole
Nylon (PA) -40°F to 180°F Fair to Good Moderate Good Low General-purpose hydraulics, cost-sensitive designs
Polyurethane (PU/AU) -40°F to 210°F Good
poor w/ water/steam		 Mod–High Good
flexible		 Low–Mod Dynamic seals needing toughness and abrasion resistance
Acetal (POM/Delrin) -40°F to 180°F Good Low–Mod Very Good
rigid		 Low–Mod Static or low-speed dynamic, tight-tolerance applications

In general, PEEK wins over PTFE based on strength and extrusion resistance at high pressure, but PTFE wins on friction and chemical universality. Nylon is more rigid and dimensionally stable than polyurethane, but polyurethane flexes better in dynamic strokes. However, polyurethane degrades faster than Nylon in wet/steam environments. Acetal is often the “in-between” choice because it is more rigid than nylon or Polyurethane, but not as chemically or thermally capable as PTFE or PEEK.

Conclusion

Polymer BURs serve as cheap insurance against otherwise expensive failures. In fact, they are almost indispensable in chemical processing, hydraulics, oil & gas, and aerospace applications. And remember: it is best to decide on the inclusion of a BUR before failure, not after. But whether you need BURs before or after design, contact Advanced EMC for help. Our sealing engineers are ready to put their experience to work for you.