by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Why PTFE Keeps Winning: The Material Science Behind High-Reliability Seal Solutions

Some materials survive extreme environments while others redefine what “extreme” means. From cryogenic hydrogen at -459°F to continuous operating temperatures of 500°F, from high-speed rotary shafts approaching 100,000 RPM to chemically aggressive process media, modern sealing applications demand more than incremental improvement. They demand stability, predictability, and performance under conditions where traditional elastomers begin to fail.

PTFE seals occupy that space.

This article examines why PTFE continues to be specified in aerospace, chemical processing, food and pharmaceutical systems, and high-speed rotary equipment. This article explores its extreme temperature stability, low-friction tribological behavior, chemical inertness, high-speed capability, long-term reliability, and the ability to tailor performance through fillers and spring energization. The goal is not simply to list advantages, but to understand how those properties translate into measurable engineering performance.

Extreme Temperature Stability

One of the properties that sets PTFE seals apart is their ability to maintain mechanical integrity across a wide range of temperatures, from -459°F (-273°C) to +500°F (+260°C). In addition, they resist melting and degradation at continuous operating temperatures of 500°F, including spikes up to 600°F. High temperatures are not the only area where PTFE excels. PTFE seals remain flexible and retain sealing force even in the presence of cryogenic fluids (e.g., liquid hydrogen, deep space). They successfully prevent the embrittlement problems that are so common in traditional elastomeric seal solutions. In addition, PTFE has a very low coefficient of thermal expansion that supports dimensional stability even during rapid temperature cycling.

Superior Tribological Properties

PTFE also offers an extremely low coefficient of friction, as low as 0.04. This property reduces torque requirements and power loss. PTFE also deposits a microscopic film on the mating surface, creating a PTFE-on-PTFE interface that minimizes wear and enables dry running. Dry running is an extremely useful feature of PTFE, meaning it can run without external lubrication and eliminate stick-slip even after long periods of inactivity. 

Chemical and Environmental Resistance

PTFE is one of the most chemically non-reactive polymers. It is non-reactive with almost all industrial chemicals, including aggressive acids (hydrochloric and sulfuric), solvents, and hydrocarbon fuels. Furthermore, it does not become brittle, swell, or degrade when it is exposed to corrosive media. Virgin PTFE grades are FDA, USDA, and 3-A Sanitary approved for food, dairy, and pharmaceutical use. PTFE is hydrophobic: it repels water and resists moisture buildup, making cleaning and sterilization much easier. Finally, PTFE is ideal for aerospace and high-vacuum environments because of its minimal outgassing, which prevents the contamination of sensitive optics or electronics.

High-Speed and Pressure Performance

PTFE rotary shaft seals can handle shaft surface speeds around 6,900 rpm and rotational speeds on the order of 100,000 RPM, which exceeds the limits seen in traditional elastomeric seals. They can also handle pressures up to 100-150 psi, and when BUR or sprig-energized options are used,, they can withstand significantly higher pressures. 

Operational Longevity and Reliability

PTFE seals have an unlimited shelf life, unlike traditional rubber seals, which age and degrade during storage. In addition, the self-lubrication and wear resistance of PTFE seals reduce unexpected downtime and maintenance downtime.  In addition, non-abrasive grades of PTFE can protect mating shafts from grooving and other surface damage that compromise seal performance. Finally, the low friction of PTFE reduces energy consumption. 

Customizability via Fillers and Design

Another key benefit of PTFE is that it can be compounded with fillers to target specific material properties. 

  • Glass Fiber: Increases wear resistance and compressive strength
  • Carbon/Graphite: Improves thermal conductivity and allows for use in steam/water, and is safer for softer shafts
  • Molybdenum Disulfide (MoS₂): Increases hardness and lubricity for high-pressure dry running
  • Polyimide: Ultra-low friction for use against soft mating surfaces like aluminum

In addition, metal springs can be added to provide a constant sealing force and compensate for seal wear, hardware misalignment, or thermal contraction.

Conclusion

In high-performance applications, failure rarely stems from a single variable. It emerges when materials, load, temperature, motion, and environment are not aligned. PTFE provides the stability needed to bring those variables into balance.

To learn more about PTFE options for sealing solutions, contact the PTFE seal experts here at Advanced EMC. We are here to help you from the initial design stages through to testing and implementation.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Creep and Stress Relaxation in High-Performance Polymer Seals

Creep and stress relaxation are types of time-dependent deformation that matter in sealing as too many engineers in the field typically see “assembled dry, passed leak test, then seeps later.” Sealing force is not a fixed number: it decays over time. And polymer seals can be affected by factors such as viscoelasticity, temperature sensitivity, and constraint effects. 

This article explores core definitions and concepts related to creep and stress relaxation, then covers how different polymer sealing materials behave and tips for the design and installation of seals to minimize these issues.

Definitions and Concepts for Creep and Stress Relaxation

Creep is defined as the increase in strain under constant applied stress. The constant stress can be, for example,  contact stress from interference, bolt load transferred through a gasket, or differential pressure loading. The results of creep are dimensional change, extrusion growth, reduced interference, and/or a contact pattern shift.

Cold flow refers to creep at moderate or ambient temperature and is controlled by a combination of stress and constraints. As a type of creep, cold flow is dominated by a combination of viscoelastic and viscoplastic deformation under a sustained compressive load.

Stress relaxation is decreasing stress under constant strain as the result of fixed gland volume, captured seal, or fixed squeeze. This can be a problem for static seals, where the gland maintains constant displacement, not constant stress. The results of stress relaxation include clamp-load loss, loss of sealing force, and an increased possibility of leakage.

Polymers can still look like they kept their shape, but they may not be pushing as hard against the metal anymore. In elastomers, “compression set” is mainly about the rubber not springing back. In polymers, the bigger issue is that the internal stress slowly bleeds off over time, so sealing force drops even if the part does not look significantly deformed.

When a polymer is compressed, part of the squeezed portion will spring back right away, but part of it returns slowly, and another part never returns because the material has permanently shifted shape. The longer a seal is under compression, the more the polymer begins to relax and flow, so even after the load is removed, it may not be able to rebound to restore the original sealing force. 

And if you compress a polymer seal and then release it, the force on the way back will usually be lower than on the way in because some energy is lost inside the material. That’s why repeated squeeze-and-release cycles will not bring the seal back to the original force level.

Material Behavior in High-Performance Seal Polymers

PTFE (unfilled): PTFE has extremely low friction and is very chemically resistant, but it gives up the sealing load over time. Virgin PTFE tends to creep and relax under sustained compression, therefore requiring a strong gland support, tight extrusion-gap control, or spring energization.

PTFE (filled): Filled PTFE holds up better because fillers increase stiffness and reduce cold-flow behavior. Filled PTFE can usually retain its sealing force longer than virgin PTFE, but the filler used can also increase friction and may affect counterface wear.

PEEK: PEEK is typically chosen when long-term load retention matters greatly. PEEK’s higher stiffness means better resistance to creep and stress relaxation, though solid gland design and surface control still matter. PEEK is also available in filled variants that can impact its properties.

UHMW-PE: UHMW-PE is excellent for abrasion and low friction, but it can still relax under long compressive dwell, especially if stresses are high or support is limited. It performs best when the design itself minimizes sustained stress and prevents extrusion.

PAI (Torlon): PAI offers the strongest resistance to time-dependent deformation in this group. It retains shape and sealing load well, making it a strong fit for high loads and elevated temperatures where other polymers may drift.

MaterialCreep ResistanceStress Relaxation ResistanceRebound After Long DwellExtrusion Risk (if poorly supported)
PTFE (unfilled)LowLowLowHigh
PTFE (filled)ModerateModerateLow–ModerateModerate
PEEKHighHighModerate–HighLow–Moderate
UHMW-PELow–ModerateLow–ModerateModerateModerate–High
PAI (Torlon)Very HighHigh–Very HighHighLow

Design Variables That Control Creep and Relaxation

Gland constraint is the first major factor. A fully confined gland gives the seal fewer places to move, which cuts down creep flow and helps prevent extrusion. If the gland is only partially confined, any clearance becomes an escape route for the seal, and support has to be both radial and axial. Radial support keeps the polymer from pushing into the extrusion gap under pressure. Axial support helps prevent shifting and uneven edge loading. The small geometry details count as well; add corner radii and lead-in chamfers, and avoid sharp edges that create stress concentrations. Also, remember tolerance stack-up: as the seal relaxes, the “effective” clearance and contact conditions can change even if the metal parts do not.

More squeeze is not going to automatically be safer. Higher initial stress can accelerate creep and stress relaxation, especially with heat. The goal is to start with enough contact stress to seal, then still have enough after the material settles. That means designing around the minimum required contact stress at end-of-life, not just at assembly.

Extrusion gap control is about finding where pressure can escape and blocking it. The gap changes with temperature, pressure-driven hardware deflection, and assembly variation. Backup rings help by mechanically closing off that path. Their details matter, though.

Surface finish can make or break long-term performance. Roughness peaks concentrate stress and encourage localized flow, and surface lay can create leak paths. With filled polymers, counterface hardness matters because wear risk can increase with the wrong pairing. Aim for a finish that reduces stress peaks without creating new friction or lubrication issues.

Hardware stiffness also impacts load retention. Flexible joints can magnify clamp-load loss as polymers relax, so stiffer flanges, spacers, and bolt patterns will significantly assist with stability. For demanding duty cycles, spring-energized seals are an excellent option as they add an additional force to compensate for potential issues, such as relaxation, wear, thermal cycling, and small misalignment. 

Installation Tips for Mitigating Creep and Stress Relaxation

Many issues with creep start at installation, where a small nick, a cut, or a twisted seal can leak early, then get blamed on cold flow. Over-compressing the seal during assembly also makes it worse by driving high stress that speeds up relaxation and can leave permanent deformation. A simple fix is better handling and proper lubrication during installation to reduce the potential for surface damage and help the seal seat without problems due to uneven stress.

Load management matters just as much after assembly. Polymer gaskets and seats often benefit from controlled retorque protocols (when the application allows it) because the initial load can drop quickly during the first dwell. A common approach is initial torque, a short wait, then a retorque and verification check. Keep in mind that if over-torque pushes stress too high, it can accelerate creep and shorten the sealing life.

Finally, storage can quietly pre-load your failure. If a seal sits compressed on the shelf, it may relax before it sees service, starting life with serious issues related to sealing force. Temperature history matters as well, especially if parts are stored near heat sources or in hot warehouses. When possible, ship and store seals uncompressed, and for critical applications, controlled conditioning and careful packaging can protect long-term load retention.

Conclusion

Creep, cold flow, and stress relaxation are not mysterious defects, but rather predictable behaviors that appear whenever polymers sit under load for long periods. For this reason, treat them as design inputs and build a sealing system around them by choosing the right material, controlling deformation with proper gland constraint, relying on stiff hardware to maintain load, and validating the design with tests that match real pressure, temperature, and dwell-time conditions.

Advanced EMC is here to help with all your sealing needs, and our engineers are happy to help you navigate your way through potential creep and stress relaxation issues. Contact us today!