PTFE (Polytetrafluoroethylene)

Definition and Chemical Structure

PTFE stands for polytetrafluoroethylene. It is a fluoropolymer (also called a fluoroplastic) — that is, a plastic whose structure contains many fluorine atoms. In sealing technology, PTFE is chosen above all when sealing points are chemically heavily loaded or when low friction matters.

Chemically, PTFE consists of many repeated building blocks of the monomer tetrafluoroethylene. The material is semi-crystalline — that is, part of the polymer chains is ordered (crystalline) and part remains disordered (amorphous). This mixture influences, among other things, stiffness, deformation, and temperature behavior.

The decisive point for the high resistance lies in the structure of the chain. The carbon chain is practically “shielded” by fluorine atoms. The C–F bond (bond between carbon and fluorine) is very stable. As a result, PTFE reacts only with great difficulty with many chemicals. Precisely this lack of reactivity makes PTFE so attractive for seals in aggressive media.

Material Properties That Matter for Seals

PTFE combines several properties that often occur together at sealing points: low friction, high chemical resistance, and a broad temperature range. In practice, however, almost always the combination of medium, pressure, temperature, gap size, and motion decides whether PTFE “fits” as a sealing material.

A core effect is the very low friction coefficient. This helps when seals run dynamically — that is, with motion between seal and mating surface. Less friction often means less frictional heat and a lower tendency to stick-slip. Stick-slip describes jerky sliding through alternating static and sliding friction, which can lead to noise, uneven motion, and increased wear.

At the same time, PTFE is not a universal solution: it has only low recovery force and tends, under sustained load, to time-dependent deformation. These points become particularly important in seal design, because tightness always depends on contact pressure and gap control.

Temperature and Media Resistance

PTFE has a melting point of around $327\hspace{0.17em}°C$. For practical continuous use, a range up to approximately $260\hspace{0.17em}°C$ is frequently cited, although this limit depends on loading, installation situation, seal geometry, and medium. At high temperatures, strength typically drops and deformation processes run faster. Therefore, temperature values should always be understood as a design value, not as a blanket approval.

With media, PTFE is often of interest when other plastics or elastomers reach their limits. The chemical inertness is in many cases the reason why PTFE is used in valves, pumps, chemical plants, or with problematic lubricants.

Friction and Dynamic Sealing Behavior

In dynamic seals (e.g., piston or rod seals), friction directly affects function and service life. PTFE can bring advantages here, because it runs very low in friction with a suitable mating surface and matching design. This often has a positive effect on the break-away torque and can support more uniform motion.

Nevertheless, the surface of the mating partner remains a central influencing factor. Roughness, hardness, and any coatings co-determine how wear and the sealing edge develop. The specific PTFE variant (pure or filled) also noticeably changes the friction and wear behavior.

PTFE in Sealing Technology: Principles, Designs, and Limits

PTFE seals in many applications differently than is known from elastomers. Elastomers live from their elastic recovery: they press back against the mating surface after deformation. PTFE has this recovery force only to a small extent. For a PTFE sealing edge to lie reliably, a structural preload or an additional energy source is usually needed.

In addition, PTFE shows creep (also called cold flow) under sustained loading. Creep is a slow, time-dependent deformation under load. For seals, this means: the contact force can drop over time, and the material can “migrate” into gaps. Both can limit tightness and service life when gap sizes and support elements do not fit.

The following table classifies typical PTFE roles in sealing systems:

Creep (Cold Flow) and Gap Extrusion

With PTFE, creep becomes particularly relevant when pressure is permanently applied or when sealing elements remain compressed over long times. As a result, the geometry can slowly change. In a groove, this may show up as “settling”; at gaps, it can appear as gap extrusion: material is pushed into the sealing gap and can shear off or fray there.

The design consequence is usually gap management. In practice, this means: keep gaps small, provide support elements, and choose the geometry so that pressure does not act on soft regions in an uncontrolled way. With O-rings, a PTFE back-up ring frequently takes over this task; with pure PTFE seals, this is often done through a suitable profile shape and material selection (e.g., filled compounds).

Why PTFE Often Needs an Energizer

When pressure is low or when the seal must also be tight without pressure, PTFE’s low recovery force is often insufficient. Therefore, an energizer is frequently used. An energizer is a preload element that presses the sealing lip against the mating surface, so that a defined contact pressure already arises during installation.

Typical energizers used are:

• Elastic ring (e.g., O-ring) as simple preload
• Spring (metal spring) when temperature, media, or very constant preload are critical

Which variant fits depends strongly on temperature, chemistry, and the required leakage class.

Typical PTFE Components in Hydraulics and Pneumatics

In hydraulics and pneumatics, PTFE frequently appears where motion and pressure come together. As a back-up ring, PTFE protects elastomer seals against extrusion, particularly at higher pressures and larger gaps. In piston and rod seals, PTFE is often used as a low-friction sealing lip, combined with an energizer so that the seal also seats stably under varying pressures. As a guide ring, PTFE helps to absorb side loads and avoid contact between metal surfaces, which in turn reduces friction and wear in the system.

A special topic is gas permeation (gas passage through the material). It can become relevant with certain gases and at high pressures and temperatures. In many cases, passage decreases with larger wall thickness, while temperature and pressure tend to increase permeation. Whether this is critical for a specific sealing point should be assessed in the context of leakage requirement and media data.

Pure vs. Filled PTFE (Compounds): Selection Criteria

Pure PTFE is chosen when maximum chemical resistance and very low friction are in the foreground and mechanical requirements are moderate. As soon as pressure, gap, temperature, or service-life requirements rise, filled PTFE compounds are frequently used. “Filled” means: solids are added to PTFE in order to change properties in a targeted way.

Typical goals of fillers are lower creep, better dimensional stability, higher wear resistance, and partly better thermal conduction. Frequently used fillers are glass fiber, carbon, graphite, or bronze. These additives can also have side effects, however. Glass fiber, for example, can act more abrasively and stress the mating surface more strongly when design and surface quality do not match.

A brief decision aid summarizes the practical view:

For the design of PTFE seals, material data, gap sizes, and the real load profile are decisive. When medium, temperature, or leakage requirement are critical, specialized consultation is sensible.