Thermoplastics

Definition and Classification

Thermoplastics are plastics that soften or melt when heated and become solid again when cooled. This process is repeatable because their polymer chains are not permanently cross-linked. Cross-linking here means: chains are chemically connected into a rigid network and can then no longer slide freely against each other.

For sealing technology, this is relevant because thermoplastics can be processed well into precise components and, depending on the type, deliver favorable properties for sealing points. These include low friction, good wear resistance, or high resistance to media such as oils, fuels, or chemicals. Which property is in focus depends on the application — for example whether a seal is moved dynamically or seals statically.

Compared with other plastic classes, the classification becomes clear through temperature behavior:

Thermoplastics are frequently used in seals where dimensional stability, friction behavior, and wear are more important than a very high elastic recovery force.

Structure and Temperature Limits: Amorphous/Semi-Crystalline as well as T_g and T_m

Thermoplastics can be structured as amorphous or semi-crystalline. Amorphous means: the polymer chains are arranged in a disordered way, similar to glass. Semi-crystalline means: in addition to disordered regions, there are also ordered crystalline regions.

For design, two temperature parameters are decisive. The glass transition temperature (T_g) describes the range in which the amorphous fraction transitions from glassy and rather brittle to tough and noticeably more mobile. T_g is not melting but a structural transition that noticeably changes stiffness and damping.

Semi-crystalline thermoplastics additionally have a melting temperature (T_m). At T_m, the crystalline regions melt; only then does the material become genuinely flowable and processable like a melt. Amorphous thermoplastics have no sharply defined melting point but soften over a temperature range above T_g.

For seals, a simple principle follows from this: below T_g, the risk of brittle behavior rises — for example under impact loading or during installation. Above T_g, deformability increases, which can ease sealing adaptation but influences dimensional stability and gap bridging. Semi-crystalline materials often remain stiff longer in practice and frequently show good chemical resistance, while amorphous types often soften more uniformly across temperature ranges.

Significance in Hydraulic and Pneumatic Seals: Selection Criteria and Typical Materials

In hydraulic and pneumatic seals, thermoplastics are used when a sealing point requires low friction and controlled wear. This concerns particularly dynamic applications — that is, piston and rod seals — where motion, pressure, and temperature act together. Thermoplastics frequently take on a defined function in the system design there, for example as a sliding sealing body or as a support element against gap extrusion.

Frequent Functions

• Guide rings for guidance and load transfer, so that metal contact is avoided.
• Back-up rings for gap protection, so that a softer sealing element is not pressed into the sealing gap.
• Sealing elements in combination with elastic elements, when a defined sealing edge and low friction are required.

Which thermoplastics are typically chosen? In practice, PTFE, PA, PEEK, and thermoplastic PU, among others, are widespread. The selection usually follows a few core questions: which medium is present — e.g., hydraulic oil, water-glycol, or air? Which temperatures occur continuously and short-term? How high are pressure and gap dimension, and how critical is friction in the system (stick-slip, energy loss, heating)?

A compact orientation is provided by this table:

Specific limit values depend strongly on type, fillers, geometry, and installation situation. Therefore, the material choice is almost always made together with the sealing geometry.

Practice: Creep/Cold Flow, Manufacturing, and Typical Failure Patterns

Thermoplastics show creep under continuous load — that is, a time-dependent permanent deformation. In seals, this is particularly relevant under continuous pressure and elevated temperature, because gaps can widen or sealing edges can press in without fully returning afterward. With very slippery thermoplastics, this effect is often described as cold flow. This does not mean that the material flows cold like a liquid, but that it yields over time under load.

In design, creep is usually not argued away but managed deliberately. Customary measures are suitable gap dimensions (sealing gap), support elements against extrusion, and material modifications — for example through fillers or reinforcements that increase stiffness and shape stability. Particularly with PTFE, cold flow is in practice frequently addressed via filled variants and suitable support geometries.

Thermoplastics are furthermore flexible in manufacturing. Frequent processes are injection molding and extrusion (melt processing for series and profiles) as well as machining from semi-finished material, when very precise geometries or smaller quantities are required. For seals, this is important because geometry, surface quality, and dimensional stability directly influence leakage, friction, and service life.

Typical failure patterns can often be traced back to a few causes:

In many cases, the cause can be narrowed down systematically through the combination of operating data (pressure, temperature, speed), component geometry, and material data. With safety-critical applications, a specialized design and consultation is sensible.