Rubber

A rubber is a material that can be stretched to several times its length and return, and that behaviour has a specific structural cause: long tangled chains, free to move past one another, tied together at intervals by crosslinks that stop them sliding apart permanently.

Stretching one uncoils the chains, which is entropically unfavourable, and releasing it lets them recoil. That is why a stretched rubber band warms slightly and a relaxing one cools, and why rubber contracts rather than expands when heated under load — a genuinely counterintuitive property that follows directly from the mechanism.

The crosslinks are what vulcanisation puts in. Unvulcanised natural rubber is sticky in heat and brittle in cold and is close to useless; Goodyear's discovery in 1839 is the single change that made the material industrial.

As a class it spans natural rubber and at least half a dozen synthetics with quite different chemistries and no shared property beyond elasticity, so this entry records no measured value.

Compounded rather than simply used: raw polymer is mixed with fillers, oils, sulfur or peroxide, accelerators and antioxidants, and the compound is as much of the engineering as the polymer choice. Carbon black is not an adulterant — unfilled rubber has almost no abrasion resistance, and it raises it by an order of magnitude.

Then formed by moulding, extrusion or calendering, and vulcanised under heat and pressure, usually in the mould.

Thermoplastic elastomers are the exception to all of this. They are physically rather than chemically crosslinked, so they can be injection moulded and remelted like a plastic, and they have taken a large share of automotive sealing for that reason.

Tyres take roughly seventy per cent of all rubber produced, and everything else is the remainder: seals and gaskets, hoses and belting, footwear, adhesives, gloves, vibration mounts and roofing membrane.

Selection within the class is almost entirely a chemical-compatibility question rather than a mechanical one. Nitrile for oil and fuel, EPDM for water, steam and brake fluid and emphatically not oil, silicone for temperature range, butyl where gas must not pass. Fitting the wrong one does not fail immediately, which is what makes the error expensive.

Mesoamerican cultures were compounding latex with morning glory juice to make balls and figures by 1600 BC, which is vulcanisation of a sort three and a half thousand years before Goodyear.

Europe treated it as a curiosity until Charles Macintosh's rubberised cloth in 1823 and Goodyear's vulcanisation in 1839. What followed was among the more brutal episodes in materials history: the rubber boom of the late nineteenth century, and in particular the Congo Free State and the Putumayo, where quotas were enforced by mutilation and killing on a scale that is documented and enormous.

Brazil's monopoly ended when seeds taken to Kew in 1876 established plantations in South-East Asia. Synthetic rubber followed the Second World War cutting Allied supply off from those plantations, which is why SBR exists.

Vulcanised rubber is crosslinked and cannot be remelted, so end-of-life tyres are shredded into crumb for surfaces, ground into rubberised asphalt, burned in cement kilns, or pyrolysed. None returns the material to a tyre. Devulcanisation exists and remains marginal.

Tyre wear is now recognised as a major source of microplastic in the environment, plausibly the largest single one, entering water courses through road runoff. It is an emission from use rather than from disposal, and no recycling arrangement addresses it.

Natural rubber's issue is land: it is a plantation crop and its expansion is a driver of deforestation in South-East Asia. It is also, unlike the synthetics, a renewable material whose carbon comes out of the air, and the plantations sequester while they grow.