What Rubber Is

Polymers, monomers, and chains

Rubber is a polymer — a very large molecule built by linking many small molecules into a long chain. The small starting molecule is the monomer, and the section of chain that repeats is the repeat unit.

A polymer built from one kind of monomer is a homopolymer. From two, a copolymer. From three, a terpolymer. Most commercial rubbers are not homopolymers: SBR is a copolymer of styrene and butadiene, and EPDM is a terpolymer of ethylene, propylene, and a diene. Knowing which one you have tells you immediately that there is a ratio involved, and that the ratio is a grade variable somebody chose.

The number of times the repeat unit recurs is the degree of polymerization. Rubber chains are long — tens of thousands of repeat units is normal, giving molecular weights in the hundreds of thousands. That length is not incidental. Almost everything that makes rubber behave like rubber traces back to the chains being very long and very flexible.

Why rubber stretches and snaps back

A rubber molecule at rest is not a straight line. It is a long chain coiled on itself in a random tangle, wriggling constantly from thermal motion. Picture a length of cooked spaghetti dropped on a plate rather than a stretched string.

Stretching pulls those coils straighter. Releasing lets them retangle. The reason they return is statistical: there are enormously more ways for a long chain to be randomly coiled than there are ways for it to be pulled straight. The coiled state is overwhelmingly more probable, so the material is driven back toward it.

This makes rubber fundamentally different from a metal spring. A spring stores energy by stretching atomic bonds — a small deflection, a large force. Rubber stores energy by reducing disorder — a large deflection, a modest force. That single difference explains why rubber can stretch to several times its length and return, while a steel spring stretched that far simply fails.

Two consequences follow that surprise people, and both are worth knowing because they show the mechanism is real. Stretched rubber warms slightly, because ordering the chains releases energy. And a stretched rubber band under constant load contracts when heated rather than expanding, because more thermal motion means a stronger drive back toward disorder.

What being long and flexible actually buys

Two chain properties do most of the work, and they are independent of each other.

Length gives entanglement. Long chains loop around their neighbours, and those entanglements act as temporary anchor points. This is why raw rubber has any strength at all before it is cured, and why very short chains behave like a sticky liquid rather than a solid.

Flexibility gives mobility. The bonds along the backbone can rotate, so the chain can change shape rapidly. A stiff backbone cannot coil and uncoil, and a polymer built from one is a plastic rather than a rubber. Polystyrene and natural rubber are both long-chain polymers; the difference in how they behave at room temperature comes down to backbone mobility.

Everything downstream depends on these two. Molecular weight controls entanglement. Backbone chemistry controls flexibility. When you later see that molecular weight governs processing and that glass transition temperature governs the low-temperature limit, those are these same two properties reappearing with names attached.

Raw rubber is not useful rubber

Uncured rubber does not fully recover from deformation. The chains uncoil when stretched, but they also slide past one another, and that sliding is permanent. Pull raw gum and it stays longer. Leave it under load and it creeps. Warm it and it flows.

Vulcanization fixes this by chemically bonding chains together at intervals along their length. Those bonds are crosslinks. Once crosslinked, the chains can still uncoil and recoil freely — the material is still elastic — but they can no longer slip past each other permanently.

That single change is what converts a soft, tacky, temperature-sensitive mass into an engineering material with defined strength, elasticity, and dimensional stability. It also makes the rubber insoluble: a cured compound placed in a good solvent will swell, sometimes dramatically, but it will not dissolve. Small molecules — oils, waxes, unreacted curatives, antidegradants — can still be extracted by solvent, and that is the basis of standard compositional analysis. What cannot be removed is the crosslinked polymer itself, because the chains are chemically joined into one continuous network.

It is also irreversible. A cured part cannot be melted down and remoulded. This is the single largest practical difference between rubber and plastics manufacturing, and it drives everything from scrap handling to how mistakes get dealt with.

Three material families you will hear named constantly

A cured rubber part is permanent, and its scrap cannot be reground and remoulded. A TPE part can be, and it runs on plastics equipment at far shorter cycle times — but it flows once past the softening point of its hard phase, so its upper service temperature is lower. That trade is the whole reason both families exist.

What is actually in a compound

Very little rubber is used as a pure polymer. What goes into a mould is a compound: the base elastomer plus everything else needed to make it processable, durable, and affordable. A typical compound has ingredients from seven functional groups.

Each of these gets a full treatment later. What matters now is the shape of the thing: a compound is a deliberate assembly where every ingredient is solving a specific problem, and changing one often disturbs another.

Reading a recipe

Rubber recipes are written in parts per hundred rubber, abbreviated phr. The elastomer totals 100 by convention. Oil-extended polymers are conventionally counted at their rubber content with the extender oil listed separately, so check which basis a supplied recipe uses, and every other ingredient is expressed relative to that. A recipe therefore totals well over 100 — often 180 to 250.

This convention exists because it makes recipes directly comparable regardless of batch size, and because it keeps the polymer as the reference point. Saying a compound has 50 phr of black tells you something immediately; saying it is 24 percent black by weight does not, because that number shifts every time anything else changes.

Notice the asymmetry. Filler and oil are measured in tens of parts; the entire cure system is often under 8 parts combined. A weighing error of half a part means almost nothing in the filler and a great deal in the accelerator, which is why cure ingredients are weighed on precision scales and added at a separate stage.

Where rubber comes from

Natural rubber is harvested as a latex from the Hevea tree, coagulated, and dried into sheets or bales. It is effectively the only elastomer grown rather than synthesised at commercial scale — guayule and Russian dandelion sources exist but remain marginal in volume, and it remains close to half of world rubber consumption — around 47 to 48 percent in recent years.

Everything else is made from petrochemical monomers by one of several polymerization routes. Synthetic rubbers reached commercial production in the 1930s and wartime demand expanded output enormously in the 1940s, but synthetic did not overtake natural rubber in world volume until the early 1960s. The split has stayed near half and half since, moving with tyre demand and natural rubber supply, so any single figure is a snapshot. Note also that production and consumption shares differ.

This matters more than it sounds. Natural rubber is an agricultural product, so it varies with clone, season, region, and handling, and its price moves with weather and politics. Synthetic grades are manufactured to specification and are far more consistent batch to batch. When you see a compound blending both, part of what is being bought is the natural rubber's mechanical performance and part is the synthetic's consistency.