How Does Rubber Work? Why It Stretches, Snaps Back and Once Melted in Summer

Natural rubber is a polymer, a molecule made of a single small unit, isoprene, repeated several thousand times into a chain, and a piece of rubber is billions of such chains coiled and tangled like cooked spaghetti. In its resting state each chain is crumpled into a random ball, because a chain jiggled by heat can adopt vastly more crumpled shapes than straight ones and so is nearly always crumpled. Pull on the rubber and the chains are dragged out straight along the line of the pull, which they resist, not because the bonds are being stretched but because a straightened chain has been forced into one of its few improbable shapes. Let go, and the thermal jiggling returns the chains to their crumpled state and the rubber snaps back. The force is entropic, a matter of counting shapes, and it has an odd consequence: a stretched rubber band warms up, and one held stretched contracts when heated, the opposite of almost every other material.

The Amazon tree Hevea brasiliensis bleeds a milky latex when its bark is cut, and the peoples of Central and South America were making balls, shoes and waterproof cloth from it centuries before Columbus. Europeans brought samples home from the 1730s, and the English chemist Joseph Priestley noted in 1770 that a lump would rub out pencil marks, which is how it got its name. As a material it was hopeless. Raw rubber's chains slide past each other, so it flows slowly under load, softens to a sticky mass in warm weather and freezes hard in cold, and the first rubber raincoats, made in Glasgow by Charles Macintosh in the 1820s, stank and melted on their wearers in summer.

Charles Goodyear, who had spent years and his family's money trying to make rubber stable, dropped a mixture of rubber and sulphur on a hot stove in 1839 and found that instead of melting it charred at the edges and, in the middle, became a firm, dry, elastic material that stayed that way in heat and cold. Sulphur, at temperature, forms short bridges between the polymer chains, cross-linking them into a single network, so that the chains can still straighten and crumple but can no longer slide past each other permanently. Goodyear patented the process in 1844, though the name vulcanisation, after the Roman god of fire, was coined in England by a friend of his rival Thomas Hancock; he spent the rest of his life in litigation and died in debt; the tyre company named after him was founded forty years after his death by people who had never met him. The properties that vulcanisation and later additives tune:

• •Cross-link density: a little sulphur gives a soft rubber band, a lot gives hard ebonite, once used for bowling balls and fountain pens
• •Carbon black: fine soot mixed in at up to a third of the weight, which reinforces the network and multiplies a tyre's wear resistance tenfold; it is the reason tyres are black rather than the natural pale amber
• •Antioxidants: rubber's double bonds are attacked by oxygen and ozone, which is why old bands crack and perish
• •Oils and fillers: for softness, cost and grip

By 1900 the bicycle and the car had made rubber a strategic material, and the boom in the Amazon and then in the plantations of Malaya and Ceylon, grown from seeds smuggled out of Brazil in 1876, made and broke fortunes and cost tens of thousands of lives in the Congo, where Leopold II's regime extracted wild rubber by terror. The search for a synthetic began in Germany before the First World War and succeeded in the 1930s; when Japan seized Malaya in 1942 and cut off ninety percent of the Allied supply, the United States built an industry from nothing and was making 800,000 tonnes a year of styrene-butadiene rubber by 1945. Synthetics are now about half of world production, each tailored to a use: butyl for inner tubes because it holds air, nitrile for gloves and fuel hoses because it resists oil, silicone for anything that must stay flexible from minus 60 to 200 degrees, neoprene for wetsuits. Natural rubber keeps the jobs where its strength and heat resistance matter most, which is why aircraft tyres and the sidewalls of truck tyres are still made from a tree.

About 70 percent of all rubber goes into tyres, where its ability to deform against the road and recover thousands of times a second is what gives grip and where the small fraction of energy lost in each cycle, as heat, is a tyre's rolling resistance and a car's fuel bill. The rest seals engines, damps vibration in every machine and bridge, insulates cables, keeps water out of buildings and blood in surgical gloves, and provides the elastic in clothing. The latex from which gloves and condoms are dipped is the raw tree sap, concentrated and vulcanised at low temperature, and the proteins left in it are what some people are allergic to. A material that a Connecticut hardware dealer accidentally cooked on a stove holds up every car on the road.

Rubber is only rubbery above a certain temperature. Cool it enough and the chains lose the thermal jiggling that lets them crumple and straighten, and the material stiffens and eventually turns glassy and brittle, a change called the glass transition. Well before that point, rubber recovers its shape more slowly than it does at room temperature. For most everyday rubbers this happens far below freezing, but the rate of recovery can already be sluggish on a cold winter day, which matters wherever a seal must spring back quickly to keep something in.

This property was at the centre of the loss of the space shuttle Challenger in January 1986. The joints of its solid rocket boosters were sealed by rubber O-rings, and on the unusually cold launch morning the rings were too stiff to expand quickly enough to seal a joint as it flexed at ignition. Hot gas escaped, and the vehicle broke apart shortly after launch. During the inquiry, the physicist Richard Feynman dropped a piece of O-ring material into a glass of iced water on live television and squeezed it with a clamp to show that it did not spring back, making the point in seconds.

A useful way to see what cross-links do is to look at polymers that lack permanent ones. Silly Putty came out of wartime research in the 1940s into a substitute for natural rubber, and it is a silicone polymer mixed with boric acid. The boron forms weak links between the chains that constantly break and re-form. Roll it into a ball and drop it, and it bounces, because the impact is too quick for the links to let go. Leave the same ball on a table overnight and it slumps into a puddle, because over hours the links have shuffled and the chains have slid past one another.

Chewing gum makes the same point with your teeth. Its base was originally chicle, a latex tapped from the sapodilla tree of Central America in much the way rubber is tapped from Hevea, and today it is mostly synthetic polymers such as polyisobutylene, a close relative of the butyl rubber in inner tubes. Because nothing ties its chains together, gum stretches and stays stretched rather than snapping back. Behaviour that depends on how fast you deform a material is called viscoelasticity, and every rubber has some of it; vulcanisation simply pushes the sliding out to timescales too long to notice.

Natural rubber is a pale, creamy colour, yet almost every tyre is black. The colour comes from carbon black, a fine powder of carbon particles mixed into the rubber in large amounts. It was found in the early twentieth century to make rubber far tougher and more resistant to wear, greatly increasing how far a tyre could travel before wearing out.

Carbon black also absorbs ultraviolet light, protecting the rubber from sunlight, which would otherwise make it crack and perish, and it helps conduct heat away from parts of the tyre that warm up as it flexes. Silica has since become an important additional filler in many tyres, improving grip in the wet and reducing rolling resistance, but carbon black remains the reason a tyre looks the way it does.

Rubber stretches and recovers because it is made of long, tangled polymer chains that are dragged straight under load and pulled back into their random coils by thermal motion, an entropic spring that warms when stretched. Raw tree latex flowed and melted until Goodyear's vulcanisation of 1839 cross-linked the chains with sulphur, carbon black made it wear-resistant and black, and synthetic rubbers, developed under wartime pressure, now supply about half of a material that mostly goes into tyres.