Thermosets

Definition and Context

Thermosets (also: thermosetting plastics, duroplastics; in German: Duroplaste, Duromere, Thermodure) are plastics that, during curing, form a three-dimensionally crosslinked polymer network through a chemical reaction. “Crosslinked” here means: polymer chains are connected to one another by permanent covalent bonds. These bonds remain in place after the reaction.

What does this mean in practice? After curing, a thermoset is not meltable and not reformable. When strongly heated, it usually does not flow but rather decomposes, because the network cannot “release” itself like meltable plastics.

For the classification of the three plastic classes, the degree of crosslinking serves as the guiding principle: thermoplastics are (largely) uncrosslinked and can be melted and reformed repeatedly through heat. Elastomers are lightly crosslinked, therefore elastic, and likewise non-meltable. Thermosets are heavily crosslinked, which makes them dimensionally stable, yet permanently “fixed”.

Structural Principle: Crosslinking and Consequences Under Heat

At the molecular level, a thermoplastic consists of long chains that can move against each other when heated. With thermosets, by contrast, curing creates a connected 3D network. This network has no state in which the chains are free enough to flow like a melt.

Why does this matter for thermal loading? When heated, motion within the material is excited, yet the network prevents rearrangement into a flowable structure. Above a certain temperature, breakdown processes dominate, so that thermal decomposition sets in instead of “melting”. This explains the high dimensional stability up to the material-specific limit, and at the same time the end of usability under overtemperature.

Distinction from Thermoplastics and Elastomers

The differences can be summarized compactly through crosslinking and behavior under heat:

For sealing technology, this classification is practical because it shows early on whether a material is suitable as a flexible sealing element or rather as a dimensionally stable structural component.

Curing and Processing: From Resin to Component

Thermosets are processed while they are still reactive, often as a resin or precursor (prepolymer). In this state, they can be brought into a mold, for example through casting or compression molding. Curing follows, which chemically crosslinks the material and locks in the final properties.

What does this mean for production logic? Forming must be completed before crosslinking. After curing, geometry can no longer be adjusted through reforming. When subsequent adjustments are needed, only machining methods such as drilling or milling usually remain — which must be considered with more brittle thermosets (notch effects, risk of cracks).

Why Curing Is Irreversible

The irreversibility comes from chemistry: during curing, new covalent bonds are formed. As a result, a network arises that cannot be returned to a flowable state through reheating. Heat does provide energy, yet it cannot “undo” the crosslinking without simultaneously damaging the polymer skeleton. Therefore, high temperatures lead to aging or decomposition rather than to a controlled melt.

Properties, Typical Systems, and Relevance for Sealing Technology

The strong crosslinking results in typical properties: thermosets are frequently stiff (high modulus of elasticity) and dimensionally stable, even at elevated temperatures up close to their decomposition limit. Many systems also show good chemical resistance, depending on the specific resin and medium. At the same time, elongation at break is low, which in practice can lead to brittle behavior and higher notch sensitivity.

Among the well-known thermosetting systems are epoxy resins (often in adhesive, casting, and fiber-composite applications) and phenolic resins (frequently used as heat-resistant, dimensionally stable molding compounds). For sealing technology, these materials are less relevant as a “classic sealing lip”, but rather as components that hold geometry and loads stable.

Use in Hydraulics/Pneumatics: Where It Makes Sense, Where It Is Critical

At dynamic sealing points, materials often must accommodate microscopic unevenness and build up a contact-mechanically stable sealing line. The low elongation of many thermosets is critical there. In many cases, an elastomer therefore takes on the actual sealing effect, while a thermosetting material acts as a dimensionally stable support or carrier component.

Sensible roles for thermosets in the sealing system are frequently:

• Anti-extrusion support at high pressures, when dimensional stability is required.
• Carrier and structural parts, when dimensional stability and temperature resistance matter more than elastic conformability.
• Media- or temperature-critical environments, provided the specific system is chemically suitable.

It becomes critical when impact loads, notches, assembly edges, or high local stresses are expected. Then the risk of cracks rises, and the design needs larger safety margins or alternative materials.

Limits: Repair, Recycling, Damage Mechanisms

Thermosets cannot be reformed after curing. As a result, repair concepts become more difficult, because adjustments are usually only possible through machining and material reserves remain limited. Recycling is also typically harder than for thermoplastics, because simple remelting is not possible; depending on the system, options include shredding for use as a filler, or chemical or energetic recovery routes.

As damage mechanisms, brittle fracture, crack initiation at notches, and temperature- or media-driven aging appear most often in practice. In sealing technology, clean edge design, suitable material pairing, and a realistic view of load spectra therefore matter. For complex applications, specialized material and design consultation can be sensible.