PET (Polyethylene Terephthalate)

Definition and Material Classification

PET (polyethylene terephthalate) is a thermoplastic polyester. Thermoplastic means: the material can be deformed by heat and becomes solid again on cooling. This property matters for manufacturing precise plastic parts — for example through injection molding or machining from semi-finished products.

Chemically, PET consists of long chain molecules linked via ester bonds. These bonds explain a central limit in operation: PET can be degraded under heat and moisture by hydrolysis. Hydrolysis is the cleavage of bonds by water, with the result that strength and toughness can decrease over time. In sealing technology, this form of aging is particularly relevant when components operate permanently in warm-moist media or in hot water/steam.

Why PET Is Used for Guidance, Support, and Sliding

In hydraulic and pneumatic systems, PET is frequently used where dimensional stability counts. This includes guide rings, back-up rings, and sliding elements. These parts maintain geometries, support seals against gap extrusion, and guide pistons or rods, so that the actual seal is loaded uniformly.

For highly elastic sealing lips, by contrast, PET is usually less suitable, because materials with noticeably higher elastic deformability are needed there. In practice, the materials complement each other: PET frequently takes over the load-bearing or sliding function, while elastic materials take over the sealing.

Structure Variants (Amorphous vs. Semi-Crystalline) and Consequences for Component Properties

PET can be amorphous or semi-crystalline. Amorphous means: the polymer chains are disordered, which frequently leads to higher transparency and different mechanical parameters. Semi-crystalline means: regions in the material are ordered, which in many cases influences stiffness, heat distortion resistance, and wear behavior.

For sealing and guide components, it is decisive how stably a part keeps its shape and how it behaves under temperature changes. After processing, PET can still post-crystallize when later exposed to heat. This is a structural reordering that can change dimensions and properties when the application temperature lies close to relevant transitions. Therefore, for tight tolerances, it is important to consider the structural state through material choice, manufacturing process, and component design.

Technical Properties in Service: Dimensional Stability, Creep, Friction/Wear, Temperature Limits

PET is often chosen as an engineering plastic because it combines high stiffness and strength with low water absorption. Low water absorption means that the material takes up little water into its volume, with the result that swelling (dimensional increase due to water) remains low. This supports the dimensional stability of guide and back-up parts, particularly in applications with varying air humidity or contact with media.

Under sustained load, creep plays a role. Creep is the slow, time-dependent deformation under constant loading. PET is frequently described as a material with good creep properties, which supports back-up functions over longer periods — for example under permanent system pressure or preload.

Tribologically, PET can show low friction and good wear resistance in many pairings — for example against metallic mating surfaces. Tribology describes the interaction of friction, lubrication, and wear. For dynamic sealing points, this is relevant because friction generates heat and wear can release particles that, in turn, stress seals.

On the temperature side, the glass transition temperature (Tg) is a practical limit point. Tg is the range in which a polymer transitions from rather “glassy and stiff” to noticeably “rubber-like and softer”. Above Tg, stiffness and strength typically drop noticeably, which can lead to higher deformation of guide and back-up parts and thus to changed gap and sealing behavior.

Dimensional Stability and Water Absorption

PET often remains geometrically stable even when the environment is moist, because it absorbs little water compared with other plastics. This helps when guide rings or sliding elements have to maintain tight fits and even small swelling would change the clearance.

Creep Behavior Under Sustained Load

In back-up applications, a constant load often acts over a long time. PET frequently shows a relatively low long-term deformation here. As a result, the risk drops that a back-up ring loses its function and the seal is pushed more strongly into a gap.

Friction and Wear in Sliding Pairs

For dynamic motion, what counts is how stable a sliding pair behaves over runtime. PET can deliver good wear values in suitable pairings, but the result depends strongly on the mating material, surface roughness, lubrication, and pressure-velocity loading. In practice, therefore, not only the material but the entire contact zone needs to be assessed.

Temperature Behavior: Glass Transition as a Practical Limit

When the application temperature approaches Tg or lies above it, PET becomes noticeably more compliant. In guide systems, this can lead to greater deflection, a larger contact area, and thus more frictional heat. For design, this means: temperature peaks and continuous use must be considered together, not only a short-term maximum value.

Chemical Resistance and Typical Failure Mechanisms (Limits in Sealing Operation)

PET is well resistant in many technical media — for example against oils, greases, hydrocarbons, and fuels, as well as against many diluted acids. This resistance makes PET interesting for numerous environments in which guidance and sliding take place alongside classic sealing materials.

Critical are media that attack the ester bonds or that accelerate hydrolysis. These include in particular strong bases, which can promote alkaline hydrolysis. Hot water, steam, and continuous warm-moist loading also noticeably increase the hydrolysis risk. Halogenated solvents are frequently problematic as well, depending on the specific substance and temperature.

Typical failure mechanisms in sealing technology are:

• Hydrolysis: chain breakdown, decreasing strength, possible embrittlement.
• Stress cracking and embrittlement: promoted by media attack and mechanical stresses.
• Wear: with unfavorable pairing, missing lubrication, or excessive surface pressure.

Quick Scheme for Media Selection (What Usually Works, What Is Critical)

A practice-oriented classification helps for quick pre-selection but does not replace detailed checking via data sheets and media lists:

For safety-relevant or borderline applications, material- and media-specific testing is sensible — ideally based on current manufacturer data and real operating conditions.