PEEK (Polyetheretherketone)

Definition and Classification

PEEK stands for polyetheretherketone and is a high-performance, semi-crystalline thermoplastic. Semi-crystalline means that the plastic has both ordered (crystalline) and disordered (amorphous) regions. This structure is a central reason why PEEK remains comparatively dimensionally stable under heat.

In terms of material systematics, PEEK belongs to the family of PAEK (polyaryletherketones). In sealing technology, PEEK is chosen when a component must retain its shape and strength under high temperature, high pressure, and in aggressive media — for example in contact with oils, fuels, or many chemicals. Precisely there, many standard plastics reach their limits, because they creep too strongly, soften, or age chemically.

Material Properties Relevant for Seals

For seals and seal-related components, PEEK is less about a single characteristic value and more about the interplay of dimensional stability, low creep, and media resistance. These properties help keep sealing gaps controllable and stabilize the function over longer periods, particularly under sustained load and elevated temperatures.

As a rough orientation value, PEEK is frequently cited with a continuous-use temperature of around $250\hspace{0.17em}°C$. In practice, however, the actually permissible temperature depends strongly on the specific PEEK grade, the mechanical loading, the installation situation, and the medium.

Semi-Crystallinity and Creep Resistance (Practical Reference)

In sealing systems, the gap size (the remaining gap between components) often decides on safety and service life. A relevant mechanism is creep — that is, a slow, permanent yielding of a material under constant load. When a back-up or guide element creeps too strongly, fits change, sealing gaps grow larger, and the loading of the actual seal increases.

Compared with many engineering plastics, PEEK shows a lower creep tendency, particularly at higher temperatures. As a result, geometries and gap conditions remain stable for longer in many applications, which raises the functional reserve of a sealing system.

Temperature and Media Resistance (Orientation Framework)

PEEK is frequently used when seals or back-up elements must simultaneously withstand temperature, pressure, and chemicals. As a medium, in sealing technology oils, fuels, and numerous chemically aggressive substances often play a role. In addition, PEEK is attributed in many sources with good hydrolysis and steam resistance, which can be relevant for applications with hot water or steam.

The following compact orientation helps classify suitability:

Role of PEEK in Hydraulic and Pneumatic Sealing Systems

In hydraulics and pneumatics, PEEK is rarely the classic primary sealing material like an elastomer. Frequently, it takes over structural functions in the sealing assembly, especially where pressure and gap size are critical. A typical failure mechanism is extrusion: a soft sealing element is pushed under pressure into a gap. In the process, nibbling can occur — that is, breakouts or shearing in the area of the sealing edge.

PEEK is therefore frequently used as a back-up, guide, or wear element. Which role is sensible depends on pressure, temperature, extrusion gap, type of motion (static, sliding, or rotating), and the surface quality of the mating surface.

PEEK as a Back-Up Ring (Anti-Extrusion Ring)

A back-up ring (also called an anti-extrusion ring) typically sits beside an O-ring or another soft sealing element. Its task is simple but decisive: it blocks the extrusion gap, so that the O-ring does not flow into the gap under pressure and become damaged.

PEEK is suitable for this function because it is stiff and creeps comparatively little under load. Both help to effectively “support” the gap, even at elevated temperatures and over long periods. Design, however, remains decisive: even a good back-up ring does not replace suitable gap and tolerance design.

Limits, Tribology, and Alternatives (PTFE, Polyamide)

PEEK has clear strengths, but in practice limits also count. The material is noticeably more expensive than standard plastics and is considered demanding in processing, because high processing temperatures and tighter process windows are typical. For selection, this means: PEEK pays off in particular when the application actually needs the performance reserves.

In dynamic sealing points, tribology is also central — that is, the interaction of friction and wear. Tribological behavior depends strongly on the mating surface, the lubrication, and the material grade. Therefore, PEEK compounds are frequently used — for example with carbon-fiber or PTFE content — to influence friction or wear in a targeted way. Here too: the best material choice remains ineffective when gap size, surfaces, and installation space do not match the loading.

When PTFE or Polyamide Can Be More Suitable

In many sealing applications, PEEK is compared with other plastics, particularly with PTFE and polyamide. A brief distinction helps to sharpen the selection parameters:

For the decision in sealing technology, the following questions usually take the lead: which temperature and which pressure apply, how large is the extrusion gap, which medium acts, and which motion or speed occurs. Under complex operating conditions, specialized design and material consultation can be sensible.