Elastomer Families

Three service groups

There are dozens of commercial elastomers, but selection almost always begins by sorting them into three broad groups based on what they can tolerate. The grouping is a working framework rather than a formal standard, and a few polymers sit on a boundary.

The general purpose elastomers are all hydrocarbons, which is precisely why they cannot resist petroleum oils — polymer and oil are chemically too similar. Everything in the other two groups carries some polar or inorganic feature that makes it chemically unlike a hydrocarbon, and that is what buys the resistance.

Cost rises steeply from left to right. A general purpose rubber may cost a few dollars per kilogram; a fluoroelastomer can be fifty times that. Selection is therefore always partly a question of using the cheapest polymer that genuinely survives the environment.

Natural rubber (NR)

The benchmark against which other elastomers are compared for mechanical performance. Its regular chain structure lets it crystallise when stretched, which reinforces the material exactly where it is most stressed.

Typical uses are engine and body mounts, bushings, tires, and mechanical goods where strength and low heat build-up matter more than environmental resistance. Where ozone resistance is also needed, it is normally blended rather than replaced.

Synthetic polyisoprene (IR)

Chemically the same repeat unit as natural rubber, made by solution polymerisation instead of grown. High-cis grades approach natural rubber closely, though not entirely — cis content is typically a little lower and the strain crystallisation slightly weaker.

What it offers instead is consistency. It arrives at a specified Mooney viscosity with no clonal or seasonal variation, contains no proteins or fatty acids, and needs no breakdown step before mixing. Where those advantages outweigh a small loss in ultimate strength, IR replaces or partly replaces NR.

Styrene-butadiene rubber (SBR)

The highest-volume synthetic rubber and the general purpose workhorse of the industry. It trades some of natural rubber's mechanical performance for consistency, availability, and cost.

Used in tire treads, belting, hose covers, footwear, and general mechanical goods. It is frequently blended with natural rubber and polybutadiene rather than used alone.

Polybutadiene (BR)

Rarely used on its own because it processes poorly, but blended into natural rubber and SBR in very large volumes. What it contributes to the blend is what matters.

Ethylene propylene rubber (EPM and EPDM)

The standard choice wherever weather, ozone, and hot water are the dominant threats and oil is absent. Its saturated backbone is the source of everything it does well.

EPM is the plain ethylene-propylene copolymer. It has no cure sites for sulfur and is normally peroxide cured; radiation crosslinking is also used in some cable applications. EPDM adds a small amount of a diene as a third monomer, which places double bonds on side chains and restores the ability to sulfur cure without exposing the backbone.

Typical uses are weatherstripping and door seals, coolant and emission hose, brake components, roofing membrane, and wire and cable insulation.

Butyl and halobutyl (IIR, CIIR, BIIR)

Butyl is unusual in having two properties almost no other elastomer offers: it holds gas remarkably well, and it absorbs energy rather than returning it.

Halogenating butyl adds a reactive site that improves cure versatility and allows it to co-cure with high-unsaturation rubbers — which is what makes a tire innerliner possible, since the liner must bond to the diene carcass beside it. Butyl's damping behaviour makes it useful in vibration isolation where absorbing energy matters more than returning it.

Polychloroprene (CR)

Widely known by the trade name Neoprene®. Polychloroprene does not excel at any single property but offers an unusually balanced combination, which is why it has stayed in use for so long. It is cured with metal oxides rather than sulfur.

One behaviour worth remembering: heat does not soften polychloroprene the way it softens most elastomers. When CR fails from heat it hardens and loses resilience, so a stiff, brittle polychloroprene part is a heat-aged one.

Nitrile rubber (NBR and HNBR)

Nitrile is the standard choice when oil and fuel resistance is the priority. Its acrylonitrile content, typically 15 to 50 percent, governs almost everything about it — more acrylonitrile means better fluid resistance and worse cold flexibility.

Hydrogenating NBR removes most of the backbone unsaturation, giving HNBR much better heat and ozone resistance while retaining the oil resistance. That combination is why it appears in timing belts and high-temperature seals.

Putting them side by side

It is worth seeing the general purpose group compared directly, because selection between them is a routine decision.

Two things stand out. Every general purpose rubber has poor oil resistance — that is what defines the group. And no single polymer is best at everything, which is why blends are so common: NR with BR for wear, NR with EPDM for ozone, SBR with BR for treads.