Plastics

Definition and Distinction: Polymer vs. Plastic

Plastics are technical materials whose basis is polymers. A polymer is the chemical building block: very long molecular chains that arise from the repetition of small units. As a pure polymer, however, the substance rarely exists in a form that is directly optimal for engineering use.

A plastic, therefore, is in practice usually a material system consisting of a polymer plus additives. Additives are substances added to set properties in a targeted way. These include, for example, fillers (increase stiffness or reduce cost), plasticizers (increase flexibility), stabilizers (protect against heat, oxygen, or UV), and colorants (identification and appearance). In sealing technology, this formulation is decisive, because friction, wear, cold flow, or media resistance often depend more strongly on the recipe than on the base polymer alone.

In everyday language, the word “plastic” is often used. Technically, “plastic” is the more precise umbrella term in English-language usage, because it captures the material concept of polymer plus additives.

Main Classes by Thermal Behavior: Thermoplastics, Thermosets, Elastomers

Plastics are frequently classified according to how they behave under heat, because processing and application limits follow from this.

Thermoplastics become soft when heated and can be deformed repeatedly. This matters for processes such as injection molding or extrusion. In application, it means: thermoplastics can tend to creep under load and temperature (time-dependent deformation).

Thermosets cure irreversibly through a chemical reaction. After curing, they are strongly cross-linked and can no longer be melted or reshaped. As a result, this often leads to high dimensional stability but also to more brittle fracture behavior.

Elastomers deform in a rubber-like way and largely spring back. Many elastomers are also cross-linked, but they remain soft and elastic. Precisely this recovery capability is the reason why they so often perform the actual sealing function in seals.

Consequences for Sealing Technology

In many sealing systems, elastomers take over the sealing function, because they conform to roughness and small shape deviations. Under high pressure, however, extrusion can occur: material is pressed into a gap and can be damaged in the process. The risk rises with pressure, temperature, gap size, and insufficient support.

Thermoplastics such as PTFE (polytetrafluoroethylene) are frequently used in seals when low friction or high chemical resistance is required. They often do not seal through elasticity alone but through geometry and preload. In high-pressure applications, thermoplastic back-up rings are used to safeguard elastomers against extrusion and to control the gap better.

Technical Classification by Performance Level: Standard, Engineering, and High-Performance Plastics

Alongside thermal behavior, the performance level matters when selecting which plastic guides, supports, or serves as a sliding partner for a seal. In practice, a distinction is often drawn between standard plastics, engineering plastics, and high-performance plastics.

Standard polymers (also called commodity plastics) are widely available and cost-efficient. Their limits typically lie in temperature, fatigue strength, wear, or chemical contact. In sealing technology, they tend to appear in less demanding secondary functions or where loads are clearly limited.

Engineering plastics are designed for mechanically more demanding components. They frequently offer better strength, dimensional stability, and continuous-use properties. In a sealing context, they become interesting when a part has to transmit loads, define gaps, or work as a guide and sliding element.

High-performance plastics are developed for high temperatures, aggressive media, and demanding mechanical requirements. They are used when standard and engineering plastics no longer cover the required combination of temperature, pressure, and media resistance.

Examples from the Sealing Environment (Brief Profile)

This classification helps when asking: what for is the plastic being used? In many seals, plastic serves less as the primary sealing lip and more as a support, a guide, or a low-friction mating partner.

Material Selection and Standard Abbreviations in Hydraulics/Pneumatics

Material selection usually starts with the question of which medium is present and which temperature can be expected continuously. After this come pressure level, type of motion, and gap conditions. In hydraulics and pneumatics in particular, it is also decisive whether the seal sits statically or runs dynamically, because friction and wear then dominate.

Important selection criteria include:

• Temperature: determines aging, stiffness, and tendency to extrude.
• Medium: influences swelling, hardness change, and chemical resistance.
• Pressure and gap size: control the extrusion risk and the need for back-up rings.
• Motion (static/dynamic): determines the friction requirement and wear behavior.
• Friction/wear: matters for stick-slip, efficiency, and service life.

For everyday technical communication, standardized abbreviations are established. For elastomers, they are structured among others in ISO 1629, for plastics in ISO 1043. As a result, specification and comparison become easier — but this does not replace testing of the specific recipe and operating limits.

Abbreviations: Quick Orientation (Without a Material Database)

In sealing technology, this systematic approach quickly leads to the core question: which function should the plastic perform? Elastomers often seal through elastic adaptation, while engineering and high-performance thermoplastics frequently stabilize geometry, lower friction, or limit extrusion. For critical combinations of pressure, gap, and temperature, specialized material and application consultation is sensible.