PA6.6 (Polyamide 6.6)

Definition and Classification in Sealing Technology

PA6.6 (polyamide 6.6, PA66) is a semi-crystalline thermoplastic from the polyamide material family. “Semi-crystalline” means that the plastic consists of crystalline and amorphous regions; this usually results in good strength and a defined melting temperature. In general engineering design, PA6.6 is often used when components must be mechanically robust and wear-resistant.

In sealing technology, PA6.6 appears primarily where a hard, dimensionally stable material is needed. Typical roles are back-up and guide elements in hydraulic and pneumatic systems. These components support softer sealing materials and help reduce gap extrusion. Gap extrusion is the “pushing” of a softer material into a gap under pressure, which can lead to damage and leakage. As a result, PA6.6 acts functionally as a mechanical component in the sealing system, while the actual sealing function is frequently taken over by elastomers or PTFE.

What Does “6.6” Mean in PA6.6?

The numbers “6.6” describe the chemical building blocks of the polymer. PA6.6 is typically produced by polycondensation of hexamethylenediamine and adipic acid. Both starting materials have a chain length of six carbon atoms, which is reflected in the shorthand notation “6” and “6”. In American usage, PA6.6 is also known as nylon 66.

Key Properties for Guide and Back-Up Elements

PA6.6 is frequently chosen in sealing systems because it combines strength and stiffness with wear behavior that is favorable in many applications. This matters when guide elements absorb side loads or when back-up elements need to stabilize the sealing gap under pressure. In practice, not only the material decides but also the operating condition: pressure level, gap size, sliding speed, and surface roughness determine how high the mechanical loading actually is.

The material condition also matters. Polyamides can absorb moisture in service, which changes mechanical characteristic values. Many parameters are therefore stated separately in data sheets for “dry” and “conditioned” (moisture-equilibrated). For the design of guidance and extrusion protection, this distinction is often decisive, because stiffness and dimensional stability can shift accordingly.

Variants: Unreinforced vs. Glass-Fiber-Reinforced

PA6.6 is available in many recipes. Frequently, unreinforced grades and glass-fiber-reinforced grades are used, depending on which combination of stiffness, temperature behavior, and tribological properties is needed. Glass-fiber reinforcement typically increases stiffness and reduces thermal expansion. At the same time, it can influence toughness as well as friction and mating behavior, which is relevant for sliding pairs in the sealing environment. Specific characteristic values are strongly data-sheet- and manufacturer-dependent.

Temperature and Moisture Behavior (Practical Relevance for Dimensional Stability)

PA6.6 is semi-crystalline and has a relatively clearly defined melting temperature. In the literature and in data sheets, the melting range is often cited around $255–265\hspace{0.17em}°C$; the glass transition lies roughly in the range $50–70\hspace{0.17em}°C$, with measurement method and conditioning playing a role. For sealing technology, the decisive question is less the melting temperature than how stiffness and dimensional stability develop across the application temperature. The permissible continuous-use temperature is moreover recipe- and application-dependent and should be derived from the data sheet and the load profile.

A central practical feature is water absorption. Polyamides absorb moisture from air or media. As a result, measurable dimensional changes can occur and the mechanical properties — particularly stiffness — can shift noticeably. For guide and back-up elements, this directly affects gap dimensions, preload, and the distribution of contact forces. In tight tolerance chains, this can decide whether a component runs freely or binds.

Chemical Resistance, Limits, and Selection Guidance (Hydraulics/Pneumatics)

PA6.6 is generally considered to have good resistance to many oils, greases, fuels, and numerous organic media — in particular when these are not strongly polar. As a result, the material fits many classic environments of hydraulic and pneumatic components in which lubricants and oil-based media occur. Nevertheless, the real resistance depends strongly on temperature, exposure time, and recipe (stabilizers, fillers, reinforcement).

Frequent limits are strong acids and strong bases. Hot water and hot steam can also be critical, because they can promote hydrolysis and thereby influence long-term stability. For selection, it is therefore important to clarify which medium is present, which temperature is reached continuously, and how long the loading acts. In sealing technology, suitability is usually confirmed via media lists, material data sheets, and application-near tests.

Standards, Data Sheets, and Test Methods as the Basis for Selection

“PA6.6” describes a material family, not a single, unambiguous recipe. Therefore, specified data sheet values and standards-based tests are the reliable basis for decisions. Many parameters are reported according to ISO or ASTM methods, and often a distinction is explicitly drawn between dry and conditioned state. For guide and back-up elements, the following parameter groups are typically relevant:

• Mechanical parameters (e.g., strength, stiffness, toughness),
• Thermal parameters (e.g., heat distortion resistance, expansion),
• Moisture parameters (water absorption, dimensional change) and their effect on mechanics.

When operating conditions are complex or tolerances become tight, a brief, specialized material and application consultation is sensible.