by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Why Engineers Are Choosing PTFE O-Rings

PTFE O-rings combine the robustness of standard O-rings with the powerful properties of PTFE. PTFE (Polytetrafluoroethylene), commonly known as Teflon, is among the most chemically inert and low-friction polymers used in sealing solutions.  

PTFE is well known for its low friction, extreme temperature, and pressure resistance. When used to manufacture O-rings, they excel at maintaining their integrity and performance even in demanding conditions where other materials might fail.

In this blog post, we’ll review the key features of PTFE,  discuss the benefits of PTFE O-rings, and under what circumstances their use is recommended.

Key Features of PTFE

Here are three specific features of PTFE that make it an excellent choice for O-rings.

Friction

Unfilled PTFE has a coefficient of friction around 0.09 static and 0.05 dynamic, with slightly higher values depending on the filler type used. PTFE is also self-lubricating, making it an ideal option for applications where lubrication is discouraged or impossible (e.g., cryogenic environments), and it does not exhibit stick-slip behavior.

Chemical Compatibility

PTFE is compatible with a wide variety of chemicals, including strong mineral acids (sulfuric, nitric, hydrochloric), organic acids and solvents, bases and alkalis (including concentrated caustics), and oxidizing agents (with temperature limits considered). It is also resistant to fuels, oils, and hydraulic fluids as well as alcohols, ketones, and esters.

Engineers take advantage of this broad resistance by choosing it for applications such as seals, valve seats, bearings, and liners in chemical processing, as well as semiconductor manufacturing and pharmaceutical systems.

Temperature Range

PTFE maintains flexibility and sealing capability even at cryogenic temperatures while resisting thermal degradation, oxidation, and embrittlement at elevated temperatures. Such low-temperature toughness and high-temperature stability make PTFE an excellent choice in operating conditions with thermal cycling, vacuum service, and extreme environments where other polymers simply cannot maintain their performance.

Fillers

PTFE is available in a variety of grades, with some grades filled. Carbon-filled PTFE includes carbon as a natural lubricant while also increasing the longevity of PTFE O-rings. Glass-filled to improve strength and longevity at the cost of abrasion and a higher coefficient of friction. MoS2 (Molybdenum Disulfide) filled PTFE achieves results similar to glass-filled, without issues with abrasiveness.

There are several benefits to using PTFE O-rings. They exhibit excellent chemical resistance, making them an excellent option for the most aggressive operating environments. They also possess an extensive operating temperature range, which means they can be used in many different applications, from cryogenic to extremely high temperatures. Unlike elastomers, PTFE O-rings do not crack, harden, or degrade due to UV exposure, ozone, or long-term aging.

The low surface energy of PTFE reduces issues with stick-slip, wear, and torque for O-rings, making them a low-friction, efficient choice. PTFE O-rings are non-leaching, non-reactive, and FDA-compliant in many grades, making them suitable for industries with strict cleanliness requirements (e.g., food, dairy, and pharmaceutical). They also exhibit near-zero water uptake, which can ensure dimensional stability and consistent sealing performance even in humid or submerged environments.

Their broad pressure capability (with proper design) means that PTFE O-rings can withstand high pressure when supported by appropriate gland geometry or backup rings, especially in static sealing applications. Finally, PTFE O-rings perform well with cryogenic fluids, corrosive gases, and aggressive process chemicals where elastomer O-rings fail quickly.

When to Use PTFE O-Rings

PTFE O-ring pressure ratings vary greatly: standard PTFE O-rings handle lower pressures (around 3,000 psi without backups), but with backup rings, they can reach 10,000 psi. Specialized spring-energized PTFE seals can handle extreme pressures up to 60,000 psi, depending on design, temperature, and extrusion gap, making them excellent for high-pressure, wide-temp-range applications where other elastomers fail due to extrusion.

Conclusion

PTFE O-rings address sealing challenges that push conventional elastomers beyond their limits. Their low friction minimizes wear and torque, while their chemical inertness allows reliable performance in aggressive media. A wide operating temperature range further enables use in applications that experience cryogenic conditions, high heat, or frequent thermal cycling.

These characteristics make PTFE O-rings well-suited for chemical processing, semiconductor, pharmaceutical, and food and dairy applications. With proper gland design and support elements such as backup rings or spring energization, they can also perform effectively in high-pressure environments.

When friction, chemical resistance, temperature extremes, or cleanliness are critical design factors, PTFE O-rings offer a durable and proven sealing solution. Contact Advanced EMC today for help in selecting the appropriate material grade and configuration to ensure long-term reliability in your application.

by Sara McCaslin, PhD Sara McCaslin, PhD No Comments

Startup and Running Friction in Polymer Bearings

Startup and running friction can vary based on several different factors, and in this article we focus on what they are, why their values differ, and typical coefficients, as well as a detailed look at why these differences occur. It ends with a discussion of what impact startup and running friction have on bearing design.

Startup and Running Friction

Polymer bearings tend to exhibit higher breakaway (startup) friction than steady-state running friction due to factors such as static adhesion, microasperity interlocking, and transfer-film formation dynamics. 

The startup friction coefficient µₛ is measured at the onset of motion and represents static friction. The term “startup” does not refer solely to time zero, however. It represents the peak friction force or torque required to break the bearing free after a period of rest. Startup friction is actually the regime of dry contact when the polymer surface is still unconditioned. Unconditioned  means that the transfer film on the counterface is incomplete or patchy

The running friction coefficient µₖ represents a kinetic or dynamic measure of friction. It takes place when steady sliding motion is established and is represented by. Running friction relates to the frictional resistance that exists when two surfaces are in motion, steadily sliding against each other. 

Running friction applies after the initial breakaway event has occurred and the system has moved past issues such as static adhesion and micro-locking. As a result, the coefficient of running friction is typically lower and more stable than startup friction, especially for materials such as PTFE and UHMW-PE.

Why Startup and Running Friction Can Differ in a Polymer

There are some key factors that differentiate startup friction from running friction in polymers. For example, at rest, there is adhesion and junction growth. Polymer chains can increase the real contact load at under load rest (creep/relaxation), thereby increasing µₛ.In addition, at startup, there will be surface roughness and plowing. The roughness increases issues with mechanical interlocking and plowing. These two effects also raise the starting friction value.

In running friction conditions, materials like PTFE form a transfer film that reduces the effect of asperities and surface roughness, which reduces running friction. There is, however, a risk of stick-slip. This phenomenon is more likely to occur when the stiffness of the system is low, the speed is low, and the µₛ / µₖ ratio is high.

Typical Coefficients of Friction

The values below represent commonly used engineering polymers and are typical dry sliding vs steel values. These values can vary with pressure, speed, temperature, finish, fillers, and test method.

  • PTFE (virgin)
    • Startup friction (µₛ): ~0.05–0.10, often nearly identical to running friction
    • Running friction (µₖ): ~0.05–0.10
    • Minimal difference between startup and running friction
  • PEEK (unfilled)
    • Startup friction (µₛ): ~0.20
    • Running friction (µₖ): ~0.25
    • Exhibits a noticeable increase from startup to running friction
  • UHMW-PE
    • Startup friction (µₛ): ~0.15–0.20
    • Running friction (µₖ): ~0.10–0.20
    • Running friction can be equal to or lower than startup friction
  • Nylon 66 (PA66)
    • Startup friction (µₛ): ~0.20 (against steel)
    • Running friction (µₖ): ~0.15–0.25 (typical)
    • Moderate variability depending on surface finish and condition

What Is Behind the Difference Between Startup and Running Friction

Several factors account for the difference between startup and running friction. Pressure and dwell time, for example, mean that higher loads and long dwell times increase the real contact area and have the potential to raise µₛ. For speed, higher speeds can actually reduce friction after the polymer transfer film stabilizes, but can also raise heat generation. 

Temperatures are known to impact polymer modulus and creep, which can shift both µₛ and µₖ and alter the risk of stick-slip. In addition, the counterface material and hardness will affect the adhesion and transfer film, which is why it is important that the frictional coefficient used in design calculations represents the friction against the counterface material (e.g., PTFE vs steel, PEEK vs aluminum).

Note that PTFE-filled PEEK, MoS₂-filled nylon, and glass/bronze-filled PTFE shift friction and wear differently, often lowering friction but sometimes increasing counterface wear.

Surface finish also has a significant impact. If the surface finish is too rough, plowing will occur, increasing both friction and wear. On the other hand, if the surface finish is too smooth it can increase adhesion issues.

Impact on Bearing Design

Startup and running friction impact material selection, clearance, and surface finish in bearing design. Startup friction is dominated by static friction and adhesion at rest. This fact significantly impacts breakaway torque and can be a limiting factor in low-speed, intermittent, or precision motion systems. In such systems, stick-slip, noise, and control instability are unacceptable. 

Running friction, on the other hand, is governed by dynamic friction. Once motion is established, it controls steady-state heat generation, wear rate, and long-term dimensional stability. It directly influences PV limits and service life. 

Because many polymers exhibit higher startup friction than running friction, engineers need to balance low breakaway forces with acceptable operating temperatures and wear. This is usually accomplished through the use of self-lubricating materials, fillers, or surface texturing to manage both regimes. A successful polymer bearing design accounts for the full friction lifecycle, ensuring reliable motion at startup without sacrificing durability during continuous operation.

Conclusion

Startup and running friction have a significant impact on bearing design, as well as factors such as material fillers, pressure, temperature, and counterface material. If you are looking for a polymer bearing solution, contact the experts at Advanced EMC. Our team of bearing specialists can help you find the best bearing material for your design and can help you select the optimal material from our range of bearing-grade polymers.