PAI (Polyamide-imide)

Definition and Classification in the Sealing Context

PAI stands for polyamide-imide. The material is a high-performance thermoplastic — that is, a technically used plastic with high temperature and load resistance. Chemically, PAI combines building blocks from polyamide structures with imide structures. The imide fractions primarily increase stiffness and temperature resistance.

In sealing technology, PAI frequently appears where standard plastics creep too strongly under temperature, surface pressure (pressure on a contact face), or under mixed friction (partial lubrication, partial solid-body contact). Creep is a time-dependent deformation under load. PAI is therefore mostly used as a dimensionally stable sliding and wear material close to the actual sealing element. For classic sealing lips it is less typical, because elastic materials are usually needed there.

Typical Components and Functions in the Sealing System

PAI takes over, in the sealing system, mainly tasks that require dimensional stability and wear resistance. Frequent applications include:

• Back-up rings: they support softer sealing materials and reduce the risk of extrusion. Extrusion means that a sealing material is pushed into a gap under high pressure.
• Guide rings: they guide pistons or rods and keep components centered, so that the seal is loaded uniformly.
• Wear rings: they absorb friction and side loads and protect adjacent components.

Particularly at high pressure and tight extrusion gap, this role matters, because the seal only works reliably when guidance and support remain stable.

Key Material Properties for Hydraulics and Pneumatics

PAI is used because of its high stiffness, compressive strength, and wear resistance. For hydraulics and pneumatics, this matters because dynamic sealing points often consist of a composite of seal, back-up element, and guide. Within this composite, the guide and wear material frequently determines how stable the gap remains and how strongly friction and wear develop.

At the same time, PAI has limits that should be checked early in practice. Particularly relevant are moisture influence and the media- and temperature-dependent chemical resistance.

Temperature: Tg and Continuous Use

A central parameter is the glass transition temperature (Tg). Tg describes the range in which a plastic transitions from hard and stiff to noticeably softer. For PAI, Tg is frequently cited at approx. $275–280\hspace{0.17em}°C$.

For continuous use in air, values of approx. $250–260\hspace{0.17em}°C$ are frequently cited. Above Tg, mechanical stiffness drops noticeably, which quickly becomes relevant near the seal: guide elements then tend to lose their dimensional stability, and friction contacts can become more unstable.

Mechanical Loading: Surface Pressure and Creep

PAI is often chosen when components have to carry high surface pressures over long times. Compared with many standard plastics, PAI typically deforms less under load through creep. This helps when the extrusion gap is supposed to stay small or when the guide length is tightly dimensioned.

In dynamic applications, this dimensional stability directly affects the sealing function. Stable guidance reduces edge pressure, lowers wear at the sealing edge and the mating surface, and stabilizes the friction behavior.

Friction and Wear: Influence of Fillers

In sliding contacts, tribology plays a role — that is, the interaction of friction, lubrication, and wear. PAI is also available as a filled grade. Typical fillers are PTFE or graphite, which can lower the friction coefficient and reduce wear.

These advantages, however, are application-dependent. Friction and wear behavior changes with temperature, surface roughness, mating material, and lubrication state, which is why testing under real conditions is often sensible.

Moisture/Water: Dimensional Change and Processing Sensitivity

PAI can absorb moisture. As a result, dimensional changes can occur, which is critical with tight fits and small gaps. In sealing technology, this affects in particular components that define extrusion gaps or take over the guidance function.

Moisture is also relevant during processing, because uncontrolled moisture can worsen component quality and properties. In the application, it should therefore be clarified early whether the component permanently operates in a humid environment or whether the media contain water fractions.

Distinction from PA, PI, PTFE, and PEEK (Material Selection Aid)

PAI is often classified in a spectrum between very low-friction, chemically robust materials and very temperature-resistant, stiff high-performance polymers. For pre-selection, a compact comparison helps:

In sealing technology, PAI shows particular strength where temperature, load, and wear are jointly high, while PTFE rather comes through friction and chemistry advantages, and PEEK frequently covers the robust middle ground.

Suitability in Practice: Typical Use Cases and Clear Limits

PAI often fits when the application sees permanently high temperatures and at the same time high side loads or surface pressures occur. This is typical, for example, for dynamic hydraulic guides where lubrication is at times short and the contact runs in mixed friction. Tight extrusion gaps also speak for a dimensionally stable back-up or guide material.

In practice, the combination of operating data decides. The first question is which temperature applies continuously and whether there are only short peaks. Equally decisive is which medium actually circulates in the system — for example mineral oil, water-glycol, or ester. From this, it follows whether the chemical resistance must be checked at the real temperature. After this come the mechanical operating conditions: how high is the pressure, and how large is the extrusion gap that the back-up or guide ring must master. Finally, it is clarified whether dry running or mixed friction is possible and whether tight tolerances are required that could become critical through moisture absorption.

Limits become apparent particularly when humid environments meet very tight dimensional specifications and when no design compensation is possible. With unclear media resistance, PAI should also not be selected on suspicion, because the combination of medium and temperature can influence the outcome strongly.

In the end, PAI is a very capable material for back-up, guide, and wear tasks in the sealing environment. For critical media, moisture conditions, or boundary-friction cases, a specialized material and application design is sensible.