Elastomers

Definition and Distinction from Other Plastics Groups

Elastomers are plastics that deform highly elastically under load and largely return to their original shape once the load is removed. In sealing technology, this recovery behavior is decisive, because it allows a seal to maintain contact pressure and to compensate for roughness on the mating surface. So what is an elastomer? A rubber-like, elastic material whose molecular chains are tied into a three-dimensional crosslinked polymer network.

The crosslinking is what clearly distinguishes elastomers from other plastics groups. Thermoplastics soften when heated and can therefore be melted and reshaped, because their chains are not permanently linked. By contrast, thermosets are heavily crosslinked, which makes them dimensionally stable and heat-resistant but usually not very elastic. The third group, thermoplastic elastomers (TPE), feel elastic at room temperature, yet they can be processed like thermoplastics, since their “crosslinking” is physical or reversible and gives way under heat.

Why Crosslinking Enables Elastic Recovery

The permanently elastic behavior arises because long polymer chains are tied into a network through crosslinks. Under deformation, the chains within this network are stretched and aligned. After the load is removed, the entropy of the chains drives the return to the disordered state, while the crosslinks prevent the chains from sliding permanently past one another.

How rubbery or hard an elastomer feels often depends on the crosslinking density. A higher crosslinking density typically increases hardness and dimensional stability, and it can reduce swelling — but, depending on the system, it also lowers extensibility. For seals, this balance matters: too little crosslinking favors creep and permanent deformation, whereas too much crosslinking can limit conformability and sealing performance at low contact pressures.

Important Properties for Seals: Tg, Swelling, Aging and CS

In hydraulics and pneumatics, what counts is not just initial tightness but stability over time. Why do seals leak? Often because elasticity is lost, dimensions change through media absorption, or the material ages chemically. Four terms come up especially often in this context: glass transition temperature (Tg), swelling, aging and compression set (CS).

The Tg is the temperature at which a polymer transitions from glass-like and hard to rubbery and elastic. Swelling describes the absorption of media (for example oil, water or fuel) along with the resulting changes in volume and properties. Aging covers time-dependent changes caused by heat, oxygen, ozone or chemicals. The CS is a measure of how much a seal stays “flattened” after long compression, and therefore how much contact pressure it loses.

Glass Transition Temperature (Tg) as the Cold-Side Limit

Above the Tg, an elastomer is soft and elastic. Below the Tg, it becomes hard and brittle, because chain mobility drops sharply. When does this become critical in practice? Whenever the minimum operating temperature comes too close to the Tg. The restoring force then decreases, the seal follows movement and tolerances less well, and leakage becomes more likely.

For design purposes, the rule is therefore: the Tg should sit clearly below the lowest operating temperature. In dynamic applications, this margin matters even more, because additional friction heat and brief cold spikes can occur together.

Swelling and Media Resistance

Swelling occurs when a medium diffuses into the elastomer and expands the polymer matrix. What happens at the seal then? The component can grow larger, become softer, or lose strength. In the short term, this can even improve the sealing effect, because the contact pressure rises. In the long term, however, the risk of extrusion (being squeezed into the sealing gap), abrasion and permanent deformation grows.

How strongly a seal swells depends mainly on the medium, the temperature and the degree of crosslinking. In hydraulics, the specific fluid formulation also plays a role, especially when additive packages are involved. For this reason, media resistance is usually specified for defined media classes and temperature ranges in datasheets and approvals.

Manufacturing and Crosslinking: Vulcanization (Sulfur, Peroxide)

Elastomers acquire their typical performance properties only through vulcanization — the chemical crosslinking of raw rubber. How does a moldable compound become a permanently elastic material? Through reaction systems that create crosslinks between the polymer chains. In sealing technology, sulfur crosslinking and peroxide crosslinking are the two most widely used routes.

Sulfur crosslinking forms sulfur bridges between chains and is commonly used for many classical rubbers. Peroxide crosslinking creates different bond types and can offer advantages in heat and media resistance, depending on the elastomer. Which crosslinking system makes sense depends on the base polymer, the temperature profile and the medium. The recipe details remain manufacturer know-how, yet the principle is central to material selection: the type of crosslinking influences hardness, swelling behavior, fatigue strength and CS.

Elastomers in Hydraulics and Pneumatics: Types, Applications and Selection Criteria

Elastomers are so widespread in hydraulics and pneumatics because they conform to micro-roughness under contact pressure and compensate for tolerances. Where are they used? Frequently in O-rings, profile seals, wipers and diaphragms. In dynamic systems, they additionally have to manage friction and wear, while in static sealing they often need to remain stable against CS and aging over long periods.

Typical failure risks usually trace back to a small set of causes: swelling in the medium, extrusion into excessive gaps under high pressure, abrasion under motion, low-temperature embrittlement near the Tg, and oxidative or chemical aging.

Typical Elastomer Types (Codes) and Rough Application Profiles

The codes follow common standards systems (e.g. ISO 1629). For sealing technology, the following families are particularly common:

This classification does not replace testing against the specific medium and temperature window, but it helps to set the direction of material selection.

Practical Checklist for Material Selection

In practice, a short sequence of questions often leads more quickly to the right elastomer, because it avoids the most common false assumptions:

1. Which medium is in contact? Hydraulic oil, water-glycol, cleaning chemistry or gas determine media resistance and swelling tendency.
2. Which temperatures actually occur? Both the minimum (Tg margin) and the maximum (aging, CS rise) are critical.
3. Which type of motion is involved? Static, oscillating or rotating motion influences friction, abrasion and heat development.
4. Which risks dominate? Swelling, extrusion (sealing gap/pressure), abrasion, low-temperature embrittlement and aging should be assessed deliberately.

Once these points are clarified, the elastomer family can usually be narrowed down with confidence, and the fine design over hardness, crosslinking and geometry follows. For demanding media combinations or high safety requirements, specialized material and application consulting is advisable.