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.

PTFE Rotary Shaft Seals
by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Pressure Cycling and Pulsation Issues in Polymer Seals

Pressure cycling and pulsation can lead to seal issues like extrusion, blow-by, and fatigue damage. There are, however, some design principles that can address these issues and mitigate their effects. This article takes a look at the issues related to cycling and pulsation and addresses six key design considerations related to them.

Pressure Cycling and Pulsation

Pressure cycling refers to repeated transitions between low and high pressure, including dwell time at each level. Pulsation, on the other hand, is associated with high-frequency pressure oscillations superimposed on the mean system pressure (often pump- or compressor-driven). Pressure spikes are short-duration transients that exceed nominal operating pressure.

Designing polymer seals for cyclic pressure and pulsation is actually a system-level problem. Consideration must go into the material, energization method, gland geometry, hardware stiffness, surface finish, and validation testing.

Seal Issues Related to Pressure Cycling and Pulsation

When polymer seals are subject to pressure cycling and pulsation, the primary design objective becomes the ability to maintain adequate contact stress and sealing integrity throughout the entire pressure waveform while avoiding extrusion, blow-by, and fatigue damage.

Extrusion occurs when the seal is forced into a clearance gap by pressure, like a soft solid getting pushed into a narrow crack. Blow-by takes place when a pressurized fluid or gas leaks past the seal because there is not enough contact stress. Fatigue damage is the progressive cracking or material breakdown that is caused by repeated loading cycles. Note that each individual cycle can be below the material’s one-time strength limit and still result in fatigue damage.

Signs of Pressure Cycling and Pulsation Issues

There are several signs that pressure cycling and pulsation are causing problems. One of the first is early leakage after a very short run-in period. The seal might also experience intermittent leakage that is related to the duty cycle or pump frequency. Another sign of seal problems is the extrusion of the gear lip, torn edges, or nibbling. Finally, backup ring displacement or seal rotation can also be a signal of issues. 

These problems usually show up in hydraulic actuators, pumps, and manifolds, gas compression stage and valve plates, chemical processing skids with pulsation dampeners, and high-cycle test equipment and aerospace pneumatic systems.

Design Tips for Addressing Pressure Issues

Here are some design tips for working with seals undergoing pressure cycling.

Pressure Waveform

In order to mitigate issues with pressure cycling and pulsation, it is important to look at the pressure waveform and not just the peak pressure. For example, document mean pressure, peak pressure, minimum pressure, ramp rate, frequency, and dwell times. Then identify the transient spikes separately from the steady cycles. Once this information has been gathered, map the waveform to the duty cycle and the total number of cycles.

Polymer

Remember to select the polymer family for the seal based on cyclic strength and creep resistance. Filled PTFE offers good creep resistance and extrusion margin. PEEK and PPS options can lead to a higher modulus, better load retention, and improved wear. UHMW-PE offers low friction but lower stiffness. However, keep in mind that the material choice should also be considered with regard to the temperature, media, PV, and allowable deformation.

Spring-Energized Seals

Another excellent option is to utilize spring-energized seals to maintain contact stress when system pressure drops. These seals have pressure-energized lips designed to avoid issues during pressure reversals. In addition, consider the use of dual-acting geometries for bidirectional pressure. And avoid relying solely on squeeze for long-life high-cycle conditions when relaxation is expected.

Seal Gland

When designing the gland, it is important to ensure that the seal is both well-supported and deforms in a controlled manner when subjected to pressure cycling. The compressive fa orce should provide reliable initial sealing force without being so high that excessive creep results over time. Utilize radii and lead-in chamfers to eliminate sharp edges that can result in problematic notches or tears. And when clearances cannot be held tightly, use anti-extrusion features to ensure the pressure cannot force the polymer into a gap.

Backup Ring

Another potential aspect of the design is the use of a backup ring. Its material should be fully compatible with the primary seal and can maintain strength and dimensional stability across the operating temperature range. When deciding between split or solid design backup rings, keep in mind potential issues with rotation and migration during pressure pulsation. 

Surface Finish

Under pressure cycling, the surface and interface details matter significantly. Small leak paths are the potential problems here, and can be addressed. First, the counterface roughness should result in a surface that supports film formation but does not lead to bypass channels or issues with abrasive wear. The lay direction should prevent machining grooves from behaving as micropumps during pressure fluctuations. In addition, if there is a possibility that erosion, wear, or corrosion could affect the roughness over time, use coatings or surface treatment that will stabilize the counterface.

Conclusion

Pressure cycling and pulsation can cause extrusion, blow-by, and fatigue damage. Careful design, however, can mitigate these issues.

If you are working on a seal design that must provide reliable performance when subject to pressure cycling and pulsation, let the polymer seal experts at Advanced EMC help. Contact us today.