Glossary

Terms as they are used on this site. A number of them are used inconsistently in practice; where that is true the entry says so rather than presenting one usage as settled.

Abrasion resistance

The ability of a surface to withstand material loss by rubbing. Used inconsistently: several distinct test methods exist, they load and move the specimen differently, and their results rank compounds in different orders. An abrasion figure is only meaningful alongside the method that produced it, and it does not predict fatigue wear against a smooth counterface.

Ageing

Change in a material’s properties over time as its polymer network is chemically altered — by oxygen, heat, ultraviolet light, ozone or a fluid. Distinct from fatigue and from wear, both of which need mechanical work. An unloaded part sitting still still ages.

Antiozonant

A compounding additive that protects against ozone attack, generally by migrating to the surface and reacting there in place of the polymer. The reservoir is finite and can be consumed, extracted by a fluid or washed off, so the protection has a duration rather than being permanent.

Back-up ring

A stiffer ring fitted alongside a seal on the low-pressure side to bridge the clearance gap the seal would otherwise be forced into. It addresses extrusion geometrically, which is the alternative to addressing it by making the seal harder.

Bloom

A film of a compounding ingredient that has migrated to the surface and become visible. Sometimes a fault, sometimes the intended behaviour of a protective additive doing its job — the appearance alone does not distinguish the two.

Chain scission

Breaking of the polymer backbone or of crosslinks. Softens the material, lowers strength and, taken far enough, produces a tacky surface. Runs concurrently with additional crosslinking during ageing, so the net change in a bulk property can be small while the network has changed substantially.

Compound

The actual moulded material: polymer plus reinforcing filler, plasticiser, cure system and protective additives. Two compounds sharing a polymer family name can differ from each other more than either differs from a neighbouring family, which is why a family name is not a specification.

Compression set

The portion of an applied deflection that does not recover after the load is removed, expressed as a percentage of the deflection applied. Measured after release, on a defined geometry, at one temperature, for one duration. Frequently conflated with stress relaxation; see that entry.

Cord diameter

The diameter of an O-ring’s circular cross-section in its free state. Also called the cross-section or the section. Because rubber is nearly incompressible, stretching a ring reduces its cord diameter, so the installed value is smaller than the free one.

Crosslink density

How many connections tie the polymer chains into a network. Raising it increases stiffness and elastic recovery and reduces elongation and permanent deformation. It is the underlying variable that hardness partially reflects, and it changes during service as the material ages.

Durometer hardness

Resistance to indentation by a standardised indenter under a standardised force, reported on a bounded scale. Several scales exist with different indenter geometries; published conversions between them are approximations rather than unit changes. Readings depend on specimen thickness and curvature, so a thin part reads softer than the same compound in a block.

Elastomer

A polymer that deforms substantially under load and returns close to its original shape when released, because its chains are crosslinked into a network. Used interchangeably with “rubber” in most contexts; where a distinction is drawn, it is usually that “rubber” implies a vulcanised material and “elastomer” includes thermoplastic ones.

Elongation at break

How far a specimen stretches before it ruptures, as a percentage of its original length. Often the first property to collapse during ageing, and therefore a more sensitive indicator of network damage than hardness or tensile strength.

Explosive decompression

Failure caused by gas that has dissolved into an elastomer under pressure expanding faster than it can diffuse out when the pressure is released. Damage appears as internal blisters and splits and can be invisible from outside. A genuinely abrupt failure mode, unlike most elastomer failures.

Extrusion

Material forced by pressure into the clearance gap between the parts a seal sits between, where it is then nibbled away on each pressure cycle. Resisted by a smaller gap, a stiffer compound or a back-up ring; the first of those costs the least elsewhere.

Filler

Particulate material — carbon black, silica and others — added in quantity to reinforce a compound. Raises stiffness, strength and abrasion resistance, and also raises hysteresis, which means more self-heating in dynamic service. Carbon black additionally acts as an ultraviolet screen, which is one reason so much rubber is black.

Glass transition

The temperature range over which an elastomer stops behaving as a rubber and becomes stiff and glassy. It is a range rather than a point, it depends on how fast the material is being deformed, and its approach — not its arrival — is what makes a seal leak on a cold start.

Gland

The cavity a seal is installed in, formed by the groove and the mating surface. The gland, not the seal, determines squeeze, so two products using the same part number can give the seal quite different service lives.

Gland fill

The proportion of the gland’s cross-sectional area occupied by the seal. Too little free volume leaves nowhere for the seal to go when it is squeezed, heated or swollen by a fluid, which raises contact stress far above the design intent and drives material into the clearance gap.

Hysteresis

Energy lost as heat when a material is deformed and released, seen as the gap between the loading and unloading curves. Wanted in a damping mount, unwanted in a fast dynamic seal, where the heat is generated inside a poor conductor and accelerates every temperature-driven degradation mechanism.

Modulus

Used inconsistently, and the inconsistency matters. In general engineering it means Young’s modulus. In rubber technology it conventionally means the tensile stress at a stated elongation — a point on the curve, not its slope at the origin. Two compounds can match on hardness, which reflects small-strain behaviour, while differing substantially on modulus in the rubber sense.

Ozone cracking

Surface cracking caused by ozone attacking double bonds in the polymer backbone. Requires tensile strain above a threshold: without it the surface reacts and becomes inert without cracking. Cracks run perpendicular to the strain direction, which makes the damage pattern diagnostic of installed strain.

Plasticiser

An additive that softens a compound and improves its low-temperature flexibility. Being a small, mobile molecule, it can be extracted by the fluid the part is in contact with, so a compound may stiffen and shrink in service in a way no short test reveals.

Squeeze

The compression applied to a seal’s cross-section by the gland. Used inconsistently: sometimes as a percentage of the free section, sometimes as an absolute dimension, and occasionally on the stretched section rather than the free one. Always worth confirming which is meant before comparing two figures.

Stress relaxation

The decay of force in a material held at constant deformation. Measured under load, unlike compression set, which is measured after release. Relaxation is the quantity that determines whether a seal still seals, since a seal in service is never released; the two are related but not interchangeable.

Swell

Volume increase caused by a fluid dissolving into the polymer network. Physical rather than chemical, and in principle reversible — a seal that has swollen and then dried out will shrink, sometimes below its original size. Distinct from chemical attack, which breaks bonds and is not reversible, and which can proceed with very little swelling.

Thermoplastic elastomer

A material with rubber-like behaviour achieved through phase separation rather than chemical crosslinking, so it can be melted and reprocessed. Buys processability and recyclability; gives up the upper end of the temperature range and, generally, compression-set performance.

Vulcanisation

Forming crosslinks between polymer chains to turn a plastic mass into an elastic network. Used interchangeably with “curing”, though “curing” is the broader word and covers systems that are not sulphur-based. The cure system determines much of how the network behaves under heat, which is why two compounds with identical polymer and filler can age very differently.